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Original title: Nihon-san unagi no keitai seitai narabini yosei ni kansuru kenkyu
From: Suisan Koshu-Jo Kenkyu Gyoseki Dai 42-Go (Research Report of the Fisheries Training Institute, (Journal of the Shimonoseki University of Fisheries)), 1(42) : 130-194, 1952
Translated by the Translation Bureau(KS) Multilingual Services Division
Department of the Secretary of State of Canada
Department of the Environment Fisheries and Marine Service Marine Ecology Laboratory
Darmouth, N. S. 1
1974
99 pages typescript
1
DEPARTMI-214T OF THE SECRETARY OF STATE
TRANSLATION BUREAU
CANA.:1A
TRANSLATED FI-10M TRADUCTICN DE
japa.n AUTHOR - AUTEUR
Isao Patsui TITLE IN EtIGLISFI - TITRE ANGLAIS
Studios on the morphology, ecoloy and pond-culture of the japa-
nese eel (An' ,pillf, TEMMIN( , K & SHLEGEL). , TITLE IN FOREIGN LANGUAGE (TRANSLITERATE FOREIGN CHARACTERS) TITRE EN LANGUE ÉTRANGÉRE (TRANSCRIRE EN CARACTLRES ROMAINS)
Nihon-san unagi no keitai-seitai narabini yosei ni kansuru kenkyu
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Eneish
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REFERENCE IN FOREIGN LANGUAGE (RAME OF BOOK OR PUBLICATION) IN FULL. TRANSLITERATE ■ FOREIGN CHARACTERS.
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Suisan Koshu-jo Kenkyu Gyosoki Dai La-Go
. eFERENCE IN FWGilçu - .N..:pc...: EN ANGLAIS
(Research Report of tha Fisheries Training Institute e 4tm (journal of the Shimonoseki University of Fisheries)
) 5
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TRANSLATOR (INITIALS) TRADUCTEUR (INITIALES)
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1655U
Studies on the Morphology, Ecology and Pond-Culture.
of ths Japanse Eel (AhL;g11a_jaronica
TF,1.1.11.NoK. SCHLEGEL)*
Isao MATSUI
* Research Report of the Fisheries Training Institute
The Journal of the Shimonoseki College of Fishries
110 Vol. 1, No. 1.
S0'..-200-10-31
1
•
•
Part 3 Study on Eel. . Culture in Japan
Chanter 1 On the Standard Concernim the techniques in Eel
Culture
ast Pesill.ts and the Purpose of the Research•
In 1897 Kurajiro Hattori built a breeding pond 23,762 m2 in
area in Senda Shinden in the district of Fukagawa in Tokyo. This
is pellerally believed to be the first commercial attemrt at
eel culture in Japan. In around 1897 he built large breeding .
ponds along the shores of Lake Hamana in Shizuoka-Ken and
brought about an epochal progress in eel culture with his at-
tempts at using larvae of silkworms as feed. This sudden deve-
lopment resulted in the shortage of young eels to be used as
breeding stock and their price rose very sharply. Norihisa
(1913 a and b) (1915 b) reported that he had transplanted
ushirasun-eel found in Yahagi river in Aichi-Ken to inlets in
Ishikawa-Ken, and that he had observed extremely rapid growth--
some specimens in imae Inlet growing to 21 cm within a year and
in breeding ponds in Kawakita Inlet some specimens growing to
30 cm. Marukawa (1916 a) criticized these findings: he thought
that the growths observed were not those of the fry but those of
yearlings and second year specimens. Further experiments were
made at the breeding ponds of the Fisheries Training Institute
in Fuyuki-Machi and at the Fisheries Experimental Station in
Ibaragi-Ken, and demonstrated that Marukawa/s thesis had been
correct. In 191( Marukawa nIqq.ishnd the resfflts of his studios
on eel and concluded that the 1?shirasun-ee1 was not suitable as
s
•
•
a breeding stock in intensive eel culture; however, Mori-
hisa (1917) accepted his initial thesis and published results.
of experiments which indicated its suitability as a breeding
stock both in extensive and intensive eel cultures. The eel
culture which developed suddenly at about 1916 and 1917 suffered
from the rising costs of breeding stocks and feeds. As counter-
measures the rationalization of the business by oranizing an
associati on and the improvement in technloues by scientific
studies were planned. In 11.920 the Fresh Water Culture. Research
Laboratories (Tan-Sui Yoshoku Kenkyu-Sho) were established in
Aichi-Ken to launch full-scale studies on the culture of "shi-
rasu"-eel to selve the problem of the'breeding stock. In its
first year it was established that there were marked differences
in the deç7,ree of growth:dn "shirasu"-eel depending on the
quantity planted, that the maximum growth was approximately 19
cm, and that the gain in weight amoUnted to 193 times the weight
at the time of release. These observations were the first indi- .
cations of the success of the production of breeding stocks.
Amenomiya (1922) affirmed the theory of Norihisa concerning eels
which had shown rapid growth. In 1923 Tanaka Hatchery . in Shimo- .-
sawaki'in the village of Mitsu in Hoi-Gun, Aichi-Ken) succeeded
in ]arge-scale culture of "shirasu"-eel following the findinp,s
at the Toyohashi Experimental Fish Farm in the Fisheries Training
institute (originally Fresh Water Culture Research Laboratories).
Man commercial Irldertak.inp;s followed this success and various
fisheries research stations across the country began to experi-
ment with eel culture. And finally "shirasu"-eel culture
became a specialized industry. In the meantime Kishino (1923 -
•
•
•
?33), Nakai, Matsui (1936 c), Matsui (1937), Inaba, Yamamoto
(1938), and Inaba (11939) published their findings on "shira-
nu"-eel culture Laichi Matsui (1935 a), Takeshi Matsui (1936
a, b, and d), Yamamoto (1936, 1938) and others pub]ished their
work n- the relation between the mornholor-,, of "shirasu"-eel
and the environm ,mt,
The present paper deals with the experiments on the culti-
vation of "shlrasum-eel, which has beon conducted for 5 years
since 1933 and with attemptn to establish standards in the
techniques of eel culture during this period.
2. Method of Study
AU the studies were carried out at Yoshida Training Site
of the Fisheries Training Institute. As the eels put on flesh /131
and become heavier, there is an increase in the planting density
(number of specimens released for a unit area of pond) and a
decrease in the size of living space. Thus in order to minimi-
ze the effeets of the planting density the living space was in-
creased by increasing the number of ponds. The results were
collected by lineage and the growth, natural decrease and food
quotient were calculated and compared by year of breeding.
All the eels used in the experiments were obtained by
raising . ,"shirasu"-eels which had been caught in rivers
near Yoshida Training Site of the Fisheries Trainin institute
in Yoshida-Machi in Haibara-Gun, Shizuoka-Ken. Flesh of sar-
dines Was used as feed. During the initial . period of the cul-
ture when the fry began to eat, minced flesh of sardines was used
as feed; after the fry had matured, sardines were dried and ad-
cord - cc 7.-fith the re1lowir7 fnrmulae, lettin w, rep- e
ministered in a fixed area once in the morning between. 8 and 9.
No water was added to the ponds nor was it changed except
wben the eels had shown the so-called "u ose-lift " phenomenon or
wT, had leen transferred to other ponds for intermediate
(::), the rate e. , r natural dec-
rease (N ), an -1 the food quotient (F) were calculated in. ac-
in
resent the wel-ht at the tme or release, n o the number of sIDe-
cimens e w1 and n the weight and the number at the time of har-
vest s and f the total *feed administered during the period of the
cultr ,-n, the expressions were
—
t\- !
L)
N — no - ni
0
wi - wo
x 100 y and
Further the rate of increase in weight is influonced by the rate
of natural decrease during the culture. Thus the rate of in- .
nr.arJe in weiy:ht nt the rate of nntural decrease of zero was cal-
culated and compared i n the observed value. That is to say,
since the total weif-bt at the time of harvest (W") at zero
410
rate or rat1:-al d, ,creas can be calculated 1 ;: - the expression
• w , Wtt,. x no WI, the rate of increase in weight at
n1
zero rate of natural decrease can be calculated by the expres-
w ? wo sion
w0
3 ..._Ex-nerimenta:LResults
Resuisti.,.., of culture
A uShira-u"-eel culture - first year
The nshirasun-eels used for the experiments were caught in -
the middle of March e 1937. The results refer to observations made
at the end of the first year of culture. The names and areas
of the ponds used and the history of breeding are given below.
Experiment No. 1. The specimens were planted in pond No. III'
(299.93 m2 ) on Apr. 14, kept for 75 days e hauled in on July 21,
and measured. They were segregated by size, released in ponds
Nos. III1 and 111 2 , respectively, kept for 84 days, hauled in
on Sept. 22, released in ponds Nos. I (749.82 m 2 ) and IV2 (149.96
m2 ) after being measured, and kept for 202 days until Nov. 12.
Experiment No. 2. The specimens were planted in pond No. IV6
(149.96 m2 ) on May 1, kept for 100 days, measured, transferred
to pond No. IV' (299.93 m2 ) e and kept for 181 days from Aug. 10
to Nov. 19.
41, Experiment No. 3. The specimens were released into pond No. IV5 •
(149.96 m2 ) on May 21, kept for 125 days, hauled in on Sept. 11,
measured, segregated, planted in ponds Nos. IV 5 and 1112 (299.93
• m2 ) 5 kept for 179 days, hauled in on Nov. 4, and measured.
The experimental results during the period of breeding des-
cribed above are shown in Table 124.
ItShirq. ,, u"-nr, 1 culture - second VePr
The material used for the experiments were breeding stocks
of "shirasu"-eels obtained after a year of culture. They were
reared for a further per cd of a year. The results are shown in
Table 125. A single experiment was carried out in 1934 and four
in 1935 .
In experiment No. 1 measurements were made on Nov. 20 of the
"shirasu"-eels which had been reared since Apr. 5, 1933. They
were segregated into large, medium, and small groups and the
specimens in the middle and small groups were used for the ex- /132
perimerts. In experiments Nos. 2, 5, 4, and 5 "shirasu"-eels e
which had been kept since Apr. 10, 1934, were measured on Nov. 15,
segregated into 4 groups, large, medium, small and extra small.
Each group was then raised separately for the experiments.
C. "Shirasu"-eel culture_- . third_y_ear
Specimens used were "shirasu"-eels which had been raised
for a period of three full years, i. e., the materials of /133
experiments Nos. 1 and 4 in the second year of culture were de-
signated as Nos. 2 and 1 of the present experiments. The re-
sults are shown in Table 126.
On the basis of the observations on breedinp; described above
the rate of increase in weight (W and Wî), rate of natural de-
crease (N) and. food quotient (F) were calculated. The re-
sults are shown in Table 1270
O 1
• II. Rate of increase in weight and growth
The rates of increase in weight showed marked differences
depending on aP:e. In the one year grou-, the rate ranged 13.10-
46.43 with a mean of 32.01; it was 1.18-3.64 with a mean of 232
in the two year group and 0.97-1.47 with a mean of 1.13 in the three
year group. The variation is maximal in. the one year group and be-
comes lower with . the increase in age. It appears that the reasons
for a large variation in the one year group are weak resistance
against the environmental changes resulting in a large natural de-
crease and the extent of the inclusion of superior stocks of "shi-
rasu"-eels among the specimens. The rate of.increase in weight with
zero natural decrease ranged 29 013-61.02 with a mean of 49.26.
This .can be regarded as the limit attainable with. the present day
techniques disregarding the problems associated with the supeaor
breeding stocks. /134
The rate of weight increase is very closely related to the
amount of feed administered and the number of days the eels are
fed. Ultimately the number of days during which eels are kept
should equal the number of days during which the eels are fed.
However Kubota (1936) reports that in eel culture the amount of
food intake by eels is related exponentially to the water tempera
turc, that when the temperature drops to lower than 10 00, they
cease to take in foods, and that they either remain stationary
at the bottom of the pond or hibernate by burying themselves in
the mud. In the present experiments no food was given from Dec. .
12 to March 11 of the following year. When there were irregular
variations in water temperature over short periods of time
during this interval and when the eels became active, they
had to derend on natural foods in the pond. A similar practice
is followed by those engaged commercially in eel culture.
Monthly variations in water temperature are shown in Table
128 (cf. Fig. 43).
An examination of the table shows that the period in which
no food was given correspond to the days when the mean water
temperature was less than 10 00. The relation between the num-
ber of days of feeding and rate of weight increase is shown in
Fig. 44.
An examination of the relation between the two shows that
the rate of weight increase becomes lower with the increase in
the number of feed days, that the rate decreases suddenly during
the 200-300th day, and that the rate of decrease becomes slower
after the 400th day. If one takes the mean of the rates of
weight increase as a base for comparison, one realizes that the
rate for the one year group exceeds by approximately 10 times
the rate for the two year group. The relation between the rate
of weight increase (W) and the number of feed days (X) can be
X , 02 expressed as W = 16.648 ( ) Findings such as
200
these are the reasons that the culture of one year duration is
commercially the most profitable and form one of the scientific
bases which gave rise to the independent and snecialized stock
breeding industry and which brought about its marked progress.
The nshirasuu-eels used in the study measured 5.981; 0.02 cm in
average length and 0.18 ± 0.001 gr in average weight. The results
of measurements are shown in Table 129.
In the case of the material such as the one being used in
the present study the degree of variations becomes larger as the
breedin7, uertod becomes longer. Thus after a year of breeding
it is natural that experiments give rise to groups with extremely .
/1 3 7 ,rarl.ed derees of rr.rowth, With respect to this observation
the author wishes to make further comments in the chanter on
the relation between external morPhologv and 7rowth in eels.
Tn the pros et chr, nter ohservat*ons will be made on the mean of
all the çroups. The relation between the moan weif-ht and the
number of feed days by age is given in Table 130. It is graphi-
cally shown in Fi': 1.
Let W 0 and e :1- e rnresent the mean values of the rates - 'aver. -• -
of weight increase W and We, and Wmax and Wmax e the maximum /136 . .
values. As their numerical values and that of w n are available,
it is possible to calculate theoretically the values of We and
Wee of a single specimen on the basis of respective rate of
weight increase assuming the size of a stock of eel to be .19 g.
On the basis of this the gain in weiht of a single specimen
can be calculated we and 'Wee assumed to be eouivalent to We 'max
can be regarded as the theoretical limits attainable with the
technidues adonted in eel culture at the nresent time (Table 131,
Fig. /X)).
An examination of the table shows that in the case of W
the growth ranges 6.20-8,90 g in the first year and in the case
of We the irrowth ranges 9.44-11.65 g. As bas been mentioned pre-
viously these measurements represent the mean values. The
author (1937) wished to determine the limits of growth of leshi-
rasuleeels which had been cultured for a year. Thus at the
10
beginninfr of May, 1936 he determined by means of the otolith the
age of nshirasuu-eels which had been bred for a year, and meas-
ured the growth of specimens which had shown superior growth
and which had been determined accurately to be one year old.
Tho rosults are Fhown tr Table 132.
ey ,minatien of trlbl shows that the larest measured
41.9 cm in length and 110 gr in weight, and that a considerable
n .umber measured over 30 cm. It should be noted that the speci-
mens with superior growth, when cornpared with the average group,
correspond to 3 or 2 year group of fish. In actual practice
commercial breeders call these groups .with superior growth !ttobi"
and haul them in as required and sell them. They are also sought
after as breeding stocks. The study of groups with superior
growth will present interesting topics for the future similar to
the study of "tobi" of carps by Matsui .(19 )1 9).
Norihisa (1913 a and b) (1917) reports that there are con-
siderable differences in. the degree of growth among "shirasu"-
eels depending on nutrition and the difference in the living /137
onvirorment, that weu raised in ditcbes or ronds (?), they
7row to arrroxi ,natelv 9.9 cm, in ordinary breeding ponds to 23.1-
33.0 cm, aud in natural waters to 13.2-23.1 cm. Marukawa (1916 a)
reertc the rrrowtb to be 4.3 cm in breeding ronds and 5.6 cm in
natural waters. Inba (193Q) reports rates of weiç7ht nain of
61.6 in 1937 and of 95.42 in 1938 and a mean weight of 21.4 gr.
It appears as though these observations were made on groups with
superior growth and may not be appropriate as standards in ge-
neral eel culture.
1
•
•
1.1
III. Food Quotient
The food quotient is an indicator of the efficienty'of
feed. The smaller values indicate the greater efficiency. In the
first year of culture the food Quotient ranr;ed 5.42-7.51 with
a mean of 5.77; in the second year 7.16-20.78 with a mean of /138
8 0 27; and in the third year 10.41-10.69 with a mean of 10.55.
It is evident that it tends to be lower among younger fish
and to bqcome higher as they age. The relation between the
number of feed days and the food quotient is shown in Fig. 47.
The relation between the two is linear. Thus the least squares
equation showing the relationship is expressed as
F = 0.0126x + 3.210.
Here x and F represent the number of feed days and food quotient,
respectively.
The .food quotient is inversely proportional to the rate of
increase in weight. It appears that during the period, when the
rate of gain in weight is high, the food quotient becomes low
since the foods taken in are consumed effectively to increase
the flesh, while in older fish food quotient becomes high since
the foods are consumed to maintain the body rather than to in-
crease the flesh. E'ven amohg the fishes of the same age group
small rates of weight gain and large values of food quotient
are sometimes observed. It appears that the reasons for this
r1-11.1oellon are tbat p;rowth is inhibited by diseases and otber
causes, that foods are consumed in quantities larger than neces-
sary forsimilar reasons, or that the foods are not taken in pro-
perly on account of ”nose-lift" and other causes.
•
12
•
IV. Natural Decrease
The rates of natural decrease ranged 23.58-53.83% with
a mean of 34.32% in the first year of culture, 0.10-47.89% with
a mean of 7ore in the second 7ear, and 552-26.4% with a mean
of 5.52% in te third year ( the average rates of natural decrease
of 47.89 and 26.44% were known to have been caused by unose-liftle
artificially caused; therefore they were not included). Thus
the rate tends to be high in the first year of culture and to
decrease in the older fish.
The relation between the number of feed days and the rate
of natural decrease is shown in Fig. 48; it shows that the dec-
rease is in direct nroportion to the rate of increase in weip:ht,
r:Oere are two Fnin reasons for naturn1 decrease: natural causes
wmcL () -c , .rivironm:.7!nt and the resistance arpinst dis eases
and artificial causes associated with breeding. The former
tends to be prevalent among younger fish. The latter is observed
generally among young as well as old fish. However the arti- /139
ficial causes will be eliminated to a certain degree with the
advance in the techniques of eel culture. The exceptionally
high rates observed among the older • ish in the present experi-
ments were all due to artificial causes. The relation between
the rate of natural decrease (N) and the number of feed days (X)
•
X -2.067 N = 11.081 (-
200 is expressed by:
1 .3
4. Summaries
1. The rates of increase in weight were 13.10-40.43 with a - -, •
mean of 32.01 in the first year group, 1018-3.64 with a mean of
2.32 in th -, so-oH. rou -e, and 0.79-1.0 with a me:s'21 of 1 .13
in the third. year gro. It is maximal in young fish and mini-
mal in older fish. •
2. The rate of increase in weight corresponding to zero natural
decrease (W , ) is the limiting point attainable with the present
techniques available in eel culture.
3. The relation between the number of feed days and the rate
of inease in weii.,11t shows tbat the rate decreases with the in-
crea ,s3 in the ru,mber of feed da7s, that it suddenly decreases
at 2(Y , -300th dair, and that it gradeallv decreases after the 400th
relnt-to b • tween t'-e •wo cpn be expressed bv the
X = 16.648 (
200 following equation:
4. The food emotients were 5.42-7.51 with a mean of 5.77 in
the first year rouP, 7.16-20.78 with a mean of 8.27 in the
second year group, and 10.41-10.69 with a mean of 10.55 in the
third year group. The rates were lower in young fish and higher
in older fish.
5. The relation between the number of feed days (X) and the
c,- nt 4 cnt linPar the rd ationshtn bc ,.twenn the two
ca in x-resF.;e(J as F 3.210 n,o126x
6. The food quotient is inversely proportional to the rate of
increase in weight.
Te na es on natural doc:roaso were 23.58-53.83% with a 17 .
14
mean of 34.41% in the first year group, 0.10-47.89% with a mean
of 7.48% in the second year group, and 5.52-26.44% with a mean
of 5.52 in the third year group. The rates became lower as the
fjsh became older. The relationship between the rate of natural
(1 ,- cron (II) and the rvmh-,r of reed days (X) can be expresed as
X -2.067 N = 11.081 (----)
200
8. The rate of increase in weir:ht varies in direct propor-
tion to the rat ,,, of natnrni decrease.
•
/10
, 15
Chanter II. On the distribution of eel shoals in the breed-
ing pond
1. The nurronn of thestndy „
The investigation of the 1.:hring conditions of eels being
bred artificially in a pond is an extremely important matter not
only from the point of view of managing intensive eel culture
but also from the point of view of confining the damages to the
minimnm by adortin proper measures at the time of nnose-liftn.
The present experiments were carried out with the se objectives
in mind.
Exnerimental methods
In a pond under the identical environmental conditions the
time taken to comcletely consume foods of a given, quantity is
proportional to the number of eels which gathers to take in the
foods. Thus the author tried to administer fixed quantities of
foods concurrently at several locations in a pond, to measure
the time taken to consume the foods,.to estimate on the basis
of the periods in which foods are consumed the sizes of the
shoals of eels, and to establish the distribution of the shoals
of eels in the pond. The following methods were employed. In
experiments carried.out during.the summer fixed quantities of
feed consisting of sardines were strun?; along a strand of rore
at equal intervals by passing the rope through their eyes. Sev-
eral rows of these strands were stretched across the pond at
equal intervals in such a way as tO distribute the feed widely in
several arnas in the pond. Thon each end of the rope was hold
on both sides of the pond and at a pre-arrangod signal the feed
16
was lowered to the surface concurrently. The time interval from
the moment • hen the feed was immersed in water to the moment-
when the feed was completely consumed was measured accurately
by a stop watch.
In oxi ,eriments carried out during the winter when no food
was taken, the pond was drained to a depth of approximately
30.3 cm. Bamboo tubes 45.5 cm long wve placed at the bottom
of the pond at equal intervals and at equal distances. The
population density and the distribution were established by
examining the number of eels found in these tubes.
esults
The exPeriments were carried out in Pond No. 1 at the .
Yoshi'la Training Site in the Fish?ries Training Institute.
uShirasu"-eols were used. They weighed 187.59 kn ., (11,685 heads).
The shape and 5,d ,/,e of the pond are shown in Fle. 49.
Experiment - 1. The feeding positions are shown in Fig. 50. 600 g
of feed were administered at each position. The date of the
experiment was Aug. 30. The water temperature was 26.8 °C. The
wind velocity was somewhat strong - enough to move small branches.
The wind direction was east. It was a fine day. The experiment
was started at 9:15 am. There were shadows approximately 16 mm
on the surface of water at feeding positions st. 1-st.3 and
approximately 8 mm on the surface of water at feeding positions
st. 3-st. 5. The results of measurements of time required for
the feeding are shown in Table 133.
An examination of the results shows that the food intake
finishes in the order st. 1-st. 4, and that after the completion
of intake at st. 4, st. 8 finishes followed by st. 5. The /141'
1 7
side on which st. 3-st, 4 are located finishes earlier than the
side on which st. 7-st. 8 are located. The shoal moves along
the path described above. St. 6 is the final station. No eel
was observed at st. 9 even 30 min. after the completion of
food intake at st. 6. The tim e required for food intake on the
side of st. 6-st. 7 was approximately 3 times that of the side of
st. 6-st. 75 Further faster intake q, the side of st. 3-st. 5
in corqnarison to the side of st. 7-st. 8 appears to be eaused by
the apheliotropism in eels.
Experiment 2 The number of feeding positions was increased in
comparison to that of Experiment 1. The positions are shown in
Fig. 51. The quantity of feed administered was identical to that
of the previous investigation. The wind direction was east. The
wind velocity was somewhat stronger than in the previous experi-
ment. The cloudiness was 5. The weather was fine with occasional
cloud. The exPeriment was begun at 1:55 pm on Aug. 30. The
shadow was reverse to that of the previous experiment; a shade
approximately 16 mm in width was observed at st. 11-st. 14.
The results of measurements of time required for food intake at
each feeding position are shown in Table 134.
An examination of the results of the present experiment
shows that the time taken is the shortest at st. 1, that it is
faster at st. 1-st. 5 as in Experiment 1, that it is slightly /142
slower at st. 2 and st. 3 in comparison to st. 4 and st. 5,
however that the times taken at these stations are short being
1/5 of the times taken at the opposite side at st. 9-st. 11.
St. 14 comes next followed by st. 6 and st. 7. After these po-
sitions the side with st. 5-st. 8 and the side with st. 12-st. 14
18
follow each other at close intervals. In the present experi-
ment the side with st. 12-st. 14, despite its proximity - to the
feed depot, is apprOximately equal with resyiect to time to the
side with st. 5-st. 8. This shows that the shoal moves in a coun-
terclockwise direction startin7 at the feed depot. Further
though there was a shadow on the side with st. 12-st. 14, the
weather was fine with occasional cloud, and the shadow appeared
at discrete intervals during the exreriment and during the period
both before and after the experiment. In the periphery of the
pond st, 16 and st. 10 were last. At st. 15 and st. 16, which
were located in the central rart of the pond, several eels were
observed immediately after the completion of st. 10. After 2-
hoirs it was ohserved that only about 1/5 of the feed had been
consuraed. TI- us the exerimert was terminated.
Experiment 3. The location of the feeding positions and the
quantity of feed were identical to those of Experiment 2. The
experiment was begun at 9 am on Aug. 31. The cloudiness was 9.
The wind direction was east. The wind velocity was zero. There
was no wave on the surface of the pond. The time taken for con-
sumption of feed is shown in Table 135.
In the present experiment,in comparison to Experiments 1
and 2, because the conditions were generally poorer, the time
required for food intake was longer. The reason was that the
weather was cloudy and the eels were showing the sign of "nose-
lift". Further the marked difference was the absence of the
shadow around the edge of the pond because of the cloud. The
results of the experiment show that the time required for food
intake is shortest at the feed depot, that starting at st. 1
•
O
19
two groups, one of which proceeds in a counterclockwise direc-
tion and the other in a clockwise direction, are observed,
and that two sides with st. 1-st. 5 and st. 12-st.\14 require
approximately equal food intake time. Further st. 9, which is
located dinonally across from Ft. l e was the last. Thus the
point located furthest away along the edge of the pond from the
feed depot required the longest time. At st. 15 and st. 16 lo-
cated in the centre of the pond food intake was completed after
the completion of the feeding positions along the periphery of
the pond in the order st. 16 followed by st. 15. This reno-
meron was not oberved in the previous exreriments. The result
of observation from a 1i7her ground of the direction of the
movement of shoal towards st. 16 shows that eels in a single /143
group proceeds towards st. 16 from the feed depot in the direc-
tion shown by the black arrow in Fig. 52, that approximately
1/3 of the group turns back towards the feed depot at point A, and
that the balance of the groups weaves its way through to st. 16,
finishes the -.{. (Jd an(1 re:71c st„ 15. It arrears as thouh the ee3s
live mainly around ,the feed depot but also along the periphery
of the pond and a part of the shoal move influenced by weather
towards the centre of the pond. Two - hours after the completion
of the experiment 9600 gr of feed, which was the total weight of
the feed used in the present experiment, was administered at the
feed depot. It was consumed in 9 min. 20 sec. This shows that
a good rortion of the nonulation qonstantly live at the feed del)ot
and that . the group, which moved to the centre, is back at the
(' 2net.
20
Experiment 4. The Positions of the feeding stations were identi-
cal to those of Experiment 2 and 3. The quantit -5\ of feed adminis-
tered was 40fl gr. The experiment was started at 10:20 am on
Sent. 2, It was raining with occasional downnour. There was
no wind. The time taken for food intake at each feeding posi-
tion i_ sh ,,,Tr in Table 1 .3(). The reults of the experiment show
1, whj.nh corre -flo'Hs to the feed de-r, t, fin: shes in
the shIrtest time as in the -A-eirious eneriments s, that the side
with st. 12-st- 114 finishes slightly faster than the side with
st. 1-st. 5, that the side with st. 8-st. 12 is the slowest» A
part of the shoal proceeded towards st. 15 and st. 16 after the
consumption of feed at the neriphery of the pond. This is simi- -
lar to Exnerjmnt 3.
D. Distribution during the_winter_season
Since the eels do not feed in the winter, bamboo tubes
were used in the experiments.
7xneriment J. As in the case of experiments carried out in the
summer Pond No 1 was used.• The experiments were carried out
for three days from Dec. 4 to 7. The fish used for the experi-
ments weighed approximately 75.18 kg (estimate: approximately
4000 heads). The mean depth of water was approximately 0.378 m.
At 3 pm on Dec. i*th 20 bamboo tubes were placed at each experi-
mental position, at 8:30 am on the 5th the two open ends of each
tube were clç, sed with scoop-nets, and the number of eels found
inside the tubes was examined. Three measurements were made
following the same procedures. The results are shown in Table
137. The experimental positions were identical to the feeding
21 •
110 positions shown in Fig. 50. rIliC water was deepest at st. 3
and shallowest at st. 7.
An examination of the above table indicates that the number
of eels found is larr,,est et st. 1-st, h e that of these it is
lnr7-nt 3, rcllny.ed hit st. l e and that it decreases
adjacent to the feed depot. On the
bs55 of t'9 ru.rlbes rau- -Pt one ntes that the no-uation of
eels is not centred around the feed depot. Rather the popu-
lation density is maximal at st. 3 1. e., at the drain opening
where the water is deepest. The conditions are different from
those of summer. The population density is the least in the
centre of the pond and it is smar at st. 5-st. 9. These ob-.
servations are similar to those of the summer.
r.xpet 2_ The exnerimrts were carr5ed out twine durlrp-, the
period Dec. 7 — 11 in the same pond as in the previous experi-
ment.
The prenent experiments were carried out to establish the
relation between the population density and the distance between
the wall and the centre of the pond. The experimental positions
were similar to those of Experiment 1 0 The positions were set
up in such a way as to make st. 2 zero metre away from the wall
and st. 9, located at the central portion of the pond, 13 metres
away from the wall. Eight positions were set up between these -
the first six at 1 m apart and the balance at 2 metres apart.
A bundle of ten tubes w&s placed at each station. The bundles
were placed at the experimental stations in the evening, left
overnight. The number of eels caught in the tubes were examined
the following morning. The results are shown in Table 138.
22
The present experiments show that eels arç most numerous
to 1 m e moderately so to 4 m, and sharply decrease over 5 m.
That is to say, the number of eels found varies in proportion to
the distance from the edge and up to 5 metres it varies in a
linear fashion (cf. Fig. 53).
Mscussion
On tJ bnsi of the , xperiments described above (excludes
Experiment 2 carried out during the winter season) the sides of
the pond nrd the positions were classified into five groups,
Side I (st. 1-st. 2 in Experimnt 1 and st. 1-st ) in Experiments
2, 3, and I.) 9 Side II (st 7-st. 4 in Exneriment 1 and st. 5-st. 7
in Experiments 2, 3, and h), Side III (st. 5-st. 6 in Experiment
1 and st. 8-st. 11 in Experiments 2, 3, and 4), Side IV (st. 7-
st. 8 in Exneriment 1 and st. 12-st. ih in Experiments 2, 3, and
4) and Side V (st. 9 in Experiment 1 and st. 15 and st. 16 in
Experiments 2, 3 9 and 4), and ranked them in the order of the
number of eels found in summer and winter. The results are shown
in Table 139.
During the summer season a larger number of eels is found
in Side l e i. e., near the feeding area. The reason appears to
be the fact that the feeding is always limited to this area.
Although Side IV is located nearer the feeding area in comparison
to Side II, the distribution of the number of eels in these sides
is equal. It appears as though the number which moves in the
counterclockwise direction, is larger than the number which. moves
in the opposite direction. Further the number of eels found
in the central area is generally larger than. that in the peripheral
•
•
23
areas It appears that, since the eels are apheliotropic, they
move along the periphery of the pond. It is our opinion that
this should be noted in the management of eel culture.
During the winter season also a fairly large number of
eels is found near the feeding area. However, a still larger
number is found in Side II. It appears as though the eels hiber-
nate in the dnenst portion where the temnerature variations
are the le est.. A^7ain the fact that ever dIr- in the winter soaon
a large number is found in the peripheral areas than in the cent-
ral section seems to indicate that Mee anheliotronic tendency
is stron7 even in the winter.
) - .
There are many improvements which should be made on the
basis of the non-elation &,nsitv of eels in order to utilize
th- =face of the noed intensively. The results of the present
experiment lead one to the following conclusions. A larger pro-
portion or eels is found along the nerirhery of the pond. In
manY cr:ses the habitat is limited to the peripheral cross of
the pond. The central section seems to be utilized only when
abnormalities are present in the quality of the water. Thus
the pond should be constructed with its central portion shallow
to stimulate the oxidation and reduction of organic and inorga-
nic matters and to make it a layer for the formation of nutrients.
If the water is to be changed because of the worsenin7, in the qua-
lity of water, so-far as is possible the draining of the water
already in the pond should be prevented. In the past it has
been a practice •r) adtnister feed in o ne place- Unilever, 5,n
2 11.
the light of the knowledge concerning the habit and the distri-
bution of eels the feed will be used more efficiently if it is
r, jven in e',rel-al nroas which ar in the shade in the peripheral
areas of the pond or which are specially constructed for such
a purpose°
•
•
2 5
Chanter III. On the Productivitvof Breedinr Ponds for Eels /146
PPSt Achievennts and the ..nurpose_ of the stu . dy
psinen it s inr,ntien pt the Knwajiri district in Faibara- /
Gun, Shizuoka-Ken in 1922 eel culture has made rapid progress. In
1941 the total water surface area involved was approximately
9 1,2,146.8 se. m with an annual production of 1,000,000 yen.
This 'as an excellent result from intensively cultivated breed-
ing ponds, The productivity of eels in breeding ponds appears to
depend on such factors as physical and chemical properties of
the ponds, the techniques of eel culture, chemical properties of
the water used, or •o7)ology and age of the ponds. Inaba and
Kivoishi (19U) established the relation between the main compo-
nents of the material found at the bottom and the prnd , Irtivity,
The author made statistical studies of the breeding ponds in the
Kawajiri district assuming that the productivity in the breeding
ponds can be taken as the commercial productivity of the eel, '
studied the relationship between such factors as the area and
the age of ponds and tried to present the bases for the design
of breeding ponds.
_Method of investigation
The productivity was calculated on the basis of commercial
production of eels during one year. That is to say, the com-
mercial producers select eels of marketable sizes from their
breeding ponds without transferring them to other ponds --
the operation is referred to as "amizashi" (Translatort note:
literally "net raising" and starts around june) -- and send them
26
to market; then annually in many cases starting around November
and continuing to April of the following year they haul in all
the eels in the ponds and classify them into the finished eels
and breedin eels. The former is sold and the latter /5.s released
into the original ponds as the breeding stock. Thus surveys were
made of the amount produced at the time of namizashin and of
te amount (finished eels materia) carried over) produced at
the time of transfer and total of the finished eels was defined
as the annual co=ercial production. The aggregate total was
defined as the maximum capacity. The numerical values obtained
by dividing th-,.se figures bv the area of the water surface were
dr, fir2d ce t:kr, an-,up,1 prodtion end the maximum capacity for
nach unit n.rea of watrq-. surface (tsubo = 3.305 m). The ag,e of
a pond was expressed by the number of years since the construction
of the pond -
3. Results (If surveys (cf. Table 2 appendmed)
The number of ponds surveyed was 143. The results of sur-
veys of the areas of the ponds are shown in Table 140 and Fig. 54./1-47
The results indicate the minimum and the maximum areas to be
826.25 m2
and 999.15 m2
; respectively. The commonest sizes >
which co- -tit'Ite 50 nf the total ; are in th- r , 11^;n 3966-
5288 m2 . These are smaller thon the sizes which are observed
a)J Frkudn • - 1
J'r'eu of production por (= 3.3 ) 5 ai o
shown in Table 1/12.
The , ''nf -.; or th.- thn ran:idrquin capricit
n.rn shoTin 13.
e
27
An examination of the results indicates that the annual
production is in the range 1.16-10.75 kg per tsubo with a mean
of 5.41 kg and the maximum capacity is in the range 1.88-12.48 kg
per tsuto with a mean of 6.9/5 ki, . Thus the production consti-
tutes 77.8% of the maximum capacity and the amount of material
carried over constitutes 22.2%. The correlation between the
production. and the mati1.7m capac»ty is shown in Table 144.
:1-1 eli-, r.tinr of the table shows that tere is a cor-
relation between the two. However, it also indicates that among
the 141 cases surveyed there are 8 unusual cases. On these in
4 cases the ,-)roduction is large and the maximum carkacity is small;
and in the other 4 cases the production is small and the maximum
capacity is large. The former is the case in which the breed-
ing stock, which forms the source of supply of "amizashi" mate-
rial, is not sufficiently supplemented, and the latter is the
case in which the F,rowth of the fish released is extremely
poor althouh the capacity reaches its maximum. Neither case
can be regarded as satisfactory from the point of view of eel
culture
The relationship between the area of pond and the produc-
tivity is shown in Table 145. /148
The relation between the area of pond and the maximum capa-
city is shown in Table 146.
The mean values of the productivity and the maximum capa-
city by area are shown in Table 147 and Fig. 55. /1 1:9
An examination of the relation between the. area of the pond
and the productivity and the area of the pond and the maxirlm capa-
city shows that the maxima of the productivity lie in the ranges
2 1983-3305 m - and 7271 -8593 m2
, that the productivity decreases
28
in the range 4627-5949 m2 , and that the productivity and the
P. parallel fashion. At 9915 m2 the
drn: 1 ,a- - , -Lablished in the previous
chapter the habitats of the eels in breedin ponds ordinarily
nrc ?ini.t. ,?d to th 2 ,-.->ipberies of the ronds. AF;ain as will be
, t:71)]1 , 1 in Chr'ntr h tIlere i9 a clso bntwen th o
rearing density and V, e -rocbctivity. The r,roductivity increases
with t1-1 dec , in r).ersit-T; J20wever e there is a limit reached
at a certain point ci ter which no decrease is sinown. These
two observations show that the areas in the ran ,, e 4627-5949 m2 ,
at which there are decreases in the prosluctivity and maximum
caPacity, result frein these being the intersection of the limit-
in7 poi n t, which the rearing density exerts on 7rowth, and the the
peripheral areas utilized byAels. Although the areas of breed-
ing ponds in the Hamana-Lake and Fukuda districts exceeds 9915 m2
the productivity for one tsuho (corresponds to the maximum ca-
pacity in the present report) investigated by Inaba and Kiyoishi
(1942) shows it to be 0.79-2.97 kg with a mean of 1.62 kg in
the Hamana-Lake district and 1.01-3.23 kg with a mean of 1.65 kg
in the Fukuda district. Being approximately 1/5 of 6.95 kg,
which is the case in the Kawajiri district, these figures are
extremely small. This is due to the difference in the intensity
of utilization of the water surface and shows that there is a
constraint in the size of the area of the water surface of the
pond in order to achieve a high de7ree of efficiency in inten-
sive eel culture. Inaba and Kiyoislli (19 ) 2) attribute the
difference in the productivity in the Hamana-Lake and Kawajiri
districts to the difference in the amount of organic material
, 29
in the bottom and the content of total nitrogen (Fig. 56) .
Further since the amounts of inorganic substances and of total
nitrogen differ deending on the degrees of intensity of uti-
lization or th\ e e the difference in the qualities of
... ., . __ water surface, surfac . e
the ilaterirl et the hottnmr in tese two areas does not de .??end
on the (11f: re'rcs in the soils dur, to ,;nolov or ir the
lity of water used. It rather depends on the decom -cosition of of -
the remai ,'-, whi.ch re!7,u3ts from the manacY.ement/eel culture after
th!, construction of the pon d s, of or7anisms such as planktons /150
and benthos. It also depends on. artificial components such as
seepage of feed component out of feed adminstered or the accumu-
lation and decomposition of residual feed. Thus if one assumes
the difference in the chemical composition of the material at
the bottom to be dependent on the degree of utilization of the
pond -- the area of the pond -- one can understand the difference
in the productivities of the Hamana-Lake and Kawajiri districts.
Defining the year of the construction of the pond and of
the start of eel culture as year 1. of the pond, the relation
between the ::1 0 of the pond and the maximum capacity was de-
termined. The results ard shown in Table 148.
To age 4, i. e., to the 4th year after the construction
of a pond the maximum capacity, being 5.41-5.64 kg, is low.
Starting at the 5th year it tends to increase and gradually
increases up to the 18th year. It increases sharply especially
durins the 5th-lOth. year. Th n results of the present study
show no decrease even in the 18th year. It is said that the
ProdncLion is inferior for several years after the construc-
tion of a pond. This observation i , sunnorted by many yc, rs ,
experience on the part of commercial brenders.
30
The breeding ponds in the area under study were built in
1920, The author feels that the relation between the number of
ponds investigated and their ages indicates the variations
in the development of eel culture in the area.
The relation between the recuits of the analyses of the
materials at the bottom, carried out Irr Inaba and Kiv-oishi (19)2 ),
and the age of the pond is shown graphically in Fig. 58. Ob-
servations on ponds ever 10 years in age are limited to tl-ose
of age 18. Thus judgement is difficult. However it is.évident
that the amount (%) of organic material, which is formed after
the construction of a pond, tends to increase rapidly to the
8th year, and that the total nitrogen Content tends to decrease
to approximately the 6th . year and to increase after that. Thus
it appears as though the productivity and the relation between
the area of a pond and the productivity is influenced more by
the amount of oranie material than by the ariount of total nitro-
gen.
4. Summaries
1. The areas of ponds for eel culture in the Kawajiri
district range between 826.25 m2 to 9915 m - . The areas in
the range 3966-5288 m2 occupy
2. The annual production for each tsubo (3.305 m2 ) ranges
1.16-10.75 kg with a mean of 5.41 k7; te maximum car.acity ran7e
1.88-12.48 kg with a mean of 6.95 kg. . /151
3. Correlation between the productivity and the maximum
capacity is observed. Anomalous relationships between the two
are unfavourable to the management of eel culture.
31
4. The relation between the area of the pend and the
prodnctivity and the maximum capacity shows that maximum yields
result from the areas in the ranges 1983-3305 m 2 and 7271-8593
that the yields decrease in the range 4627-5952 m2 5 and that
the yields dePrase mrkedly when the area exceeds 9915 m2 .
5. There is a very close relationship between the are
of te pond and the maximum capacity. The productivity is low
pntil the 4th year after the construction of the pond.
•
32 /
IIIChantpr Tv, nn thn influences of the Population Density of
...
Eel on the Productivity of a Pond
1. Past stud5es and the purpose of the studY
PEARL and PARKER (1925) published their findings that, even
though plenty of food and sufficient air were cr,iven, when Dro-
sophila. were kent in a fixed volume, there was a decrease in
their rate of nropaatien, and furtller that ther mean life
became shorter. Since then Terao and Tanaka (1928) studied
using Maina_Macrocona Straus the influence on the propagation
of gregarious density and the relation between the increase in
Dopulation and the amount of the culture solution. They also
studied the influence of gregarious density on spawning in
Oryz,ias latipes, (Temminck et Schlegel) and established the
relation y = axb
between the gregarious density (x) and pro-
ntio--, W. F *orn the stnn ,,inoint of firh cuJture WILLER and
SCHNIGEWMRC (1928) established the relation between the popu-
lation density and death rate and growth in Sa3vel.inus fontinalis
(MITCHIEM, and Kawajiri (1928) conducted similar experiments /152
with Oncorhvnchus nerka f. nerka (WALBAUM) and established
the fact that the natural decrease was smaller and the weight
was lighter when the density was larger, and that the difference
in density had little effect on the rate of weight gain in the
total weight. Kawajiri, Hata and Murai (1930) and Kawajiri and Hata
(1935) confirwed these findings and Kawajiri (19 ).9) showed
with respect to Ory7das latines (TEMMINCK et SHLEGEJ2 ) that
an incrense in the i-Ter.arious denslty tended to result in the
33
decrease in the number of eggs laid and in. the rate of hatching.
T:Ilere have been no such studios on the fundamental problems on
eels and or, -) lacks hp.ses for the formulatjon of rrutdance. The
prese-t studies were undertaker in te -t-iat such guidance
is important in the culture of "shirasu"-eels.
2. F, - erimental method, material and development /156.
The specjmens of "shirasu"-eels used for the experiments
were caught between the periods Feb.-Apr., 1918 and Feb.-Apr.,
1939, They were at the stage where they would eat the foods
administered. The experimental ponds were made of concrete and
measured 8 m in length, 3 m in width with an averag e depth. of
1.5 m. Three such ponds were used for experiment A. Another
)4. ponds measuring 8 m in length, L. m in width with an average
depth of 1.5 m were used for experiment B. No drainage was
provided. A part of the water was changed crily when the pheno-
menon of "nose-lift" was noticeable.
Experiment A was carried out in 1938 with a group of 3
population densities 187.95, 375.9, and 563.85 g for each tsubo.
Experiment B was carried out with a group of 4 population den-
sities 112.77, 18795, 300.72, and 451.08 gr, for each tsubo. These
were released in the ponds of the same volume, hauled in and exa-
mined every 30 days. The population densities were corrected •
by removing the amount which had increased durin7, each period.
For contrast and comparison groups with 112.77 g for each tsubo
for A and 7502,8 g for B were provided. The eels in these were
hauled in and examined every 30 days. No correction to the po-
pulation densities of these groups was made. Thus these were
the grcu os jr will eH thn nonultlon Witb
34
The group in experiment B with the population density of
187.95 g for each tsubo was used as a contrast group for ex-
periment A.
Experiment A was carried out during the period May 18 to
Nov. 12: experiment B was carried out from May 17 to Oct. 11.
During these periods 6 examinations were made in experiment A
and 5 examinations in experiment B.
s nrçhi ' as were civen In rlishes iu exreriïcent A from
May 1. to Jiyqn 25 and. in experiment B from May 17 to june 24.
After these periods sardines str alor-,, a string throu ,,.:h their
eyes were given.. The feed was,iven once a day in the morning.
The amoi.mt of feed consumed was obtained by weighinp, unonsumed
feed two hours after it had been administered.
per.imental results
The results of observations from experiment A are given in
Table 149; those for B are given in Table 150.
On the basis of these results the rate of increase in weight
(W), food Quotient (11, rate of natural decrease (N), and mean.
weight for each eel were calculated. The results are shown in
Table 151 (experiment A) and Table 152 (experiment B).
a. Results of ex,periment A
The relation between the population density and the rate
nr e-rgn wei ,-ht is Fho-71 -Tnn .5ically in Fia:.. 59. An
tion of the diagram shows that the rates of gain in weight to
Au - ., the end of the third rerio'. 1 , are 1";94 in the first perio ,I,
0.99 in the second period, and 0.67 in the third period in the
35
-roup with the population density of 187.95 g, 1.33 in the
firt period, 0.37 in the second period, and 0.51 in the third
period in the 375.9 g group, and 0.91 in the first period, 0.29
in the second period, and 0e33 in the third period in the 563.85
7 7rolln, Tt 1.s cleer that the rate of weight gain sharply in-
creases with the decrease in the population density and that
the differences between groups are markede However in Sept. in
the fourth period the rates are 0.92 in the 187e95 g group, 0.89
in the 375.9 g, group, and 0.87 in the 563.85 g groupe The dif-
ferences between the groups are extremely small. The results
in. Oct. durinçr the fifth period show the rates to be 0.39 in
the 187.59 g group, 0.71 in the 375.9 g group, and 0.43 in the
565.85 7; p:rollpe The results in Nov. during the silm-tb period
re T e n, , Q tie 187 _• 7ro .up 0,10 5.n te [7:roun.
and 0.12 in the 563.85 g group. In the fifth period the rate is
maximum in the 375.9 g group; however, in the sixth period it
is lowest. The rates, when expressed with the average values
of each group for the whole period, are 0.68 in the 187.95 g
group, 0.65 in the 375.9 g group, and 0.49 in the 563.85 g group.
It shows that the rate of weight gain becomes higher as the
population density becomes lower. In the contrast group, whose
initial population density was 112e77 g and whose density had
not been corrected during the experiment, the variations in the
population density were 112.77 g during the first period, • 48.09
7 in the second period, 402e21 g in the third period, 620.23 g
in the fourth period, 92.4.7 g in the fifth period, and 1169e05 g
in the sixth period. The rates of weight gain were 2.45 in the
first period, which was higher than in the 187.95 g group and
36
was the highest e and 0.62 in the second period, intermediate
between the 187.95 g and 375.9 g groups. In each case it is
very closely related to the population density. While the rates
tended to rise in the other groups over the third and the fourth
periods, they tended to decrease; they were 0.54, which is
slightly better than in the 375.9 g group, in the third period
and 0.5q 5n the fourt-,b neriod in contrast to 0.87 in the 563.85
group. They docrnased rnrthor during the fifth and sixth periods
drep-7'n ,- to low velues of 0,26 and 0.01, It is evident that
the rate of weight gain falls with the rise in the population
densitir , The effect is observed in the third period when the
nonulation density is 394,70 r; it is most rrominent in the
ronrth Ty-ried wi.th. the Population density of 620 24. g. Thus it
110 an .enn.rq as tho»-b the poei -Jation dnnsities in the rann-n 3759-
620.24 g exert a great deal of influence on the rate of weight
The relation between the gain in weight of individual spe-
cimen and the population density is shown in Fig. 60. /158
The rates of weig,ht gain for the 187.95 g F7oup at each
of the survey periods were 0,49, 1.35, 2.33, 5.30 e and 7.59;
in the 375.9 g group they were 0.41, 0.79, 1.28 5 2,78, 5.33,
and 5.82; and in the 563.85 g group they were 0.38, 0.49, 0.68,
1.28, 2.29, and 2.58. The growth curves for these three groups
clearly show that the rates ers better in the r,reuns with the
smaller densities, that . there are prominent differencns among
the three groups, that while the rates tend to rise in the 187.95
C" grow) even towards the --Id of the experiment, tbey cal l i- Oct.
(the fifth period) in the 375.9 g group e and Uhat they are ex-
3'7
tremely low from the initial period in the 563.85 g group.
In the group with no correction in the population density, although
the growth is better in the initial period of the experiment when
the density is low, they fall with the increase in the density, i.
e., they are by survey period 0.56 g y 0.90 g, 1.47 g l 2.29 g,
3.04 g, and 3.11 g. The. decreases are particularly noticeable
in (the third period) and in yov. (the sixth period) and
show the same tendency as the rate or wei;ht gain. It appear s .
as thou7h the 1-opulation density 402.21 g shows the limit.
The relation between the food quotient and the popula-
tion d;r1sity is F,hown ln F17. 61.
The food anotients were in the 187.95 cr., group 6 e8, 16e9,
10.8, 5.7, 3.9, and 3.2, the 375.9 p: p:roun 7.9 ) 28.3, 11.9,
5,2, 4 ‘ 3, prid 8,9, and in the 563,85 g group 9.5, 29.1, 14.0,
6.0, 6,5, and 9.0. Althoh the quotients were high in the sec-
ond period, the mean values were 7.9 in the 187.95 group, 11.1
in the 375.9 g group, and 12.3 in the 563.85 g group. It is
clear that the food quotient falls with the decrease in the
popu1atio -1 density. Further in the group with no correction to
the population density they were at each survey period 8.5, 13.7 5
8.5, 6.6, 7.7, and 40.3. The maximum rates were observed in Sept.
(the fourth period) in all the groups; in Nov. (the sixth period)
it was deeidedly hi -her in comnarlson to the other groups. It
will be observed that in the relation between the rate of weight
in an -i me an weight and the - onulation density the reriodp in
which the rate changes to the maximlim, is delayed by a mo'nth in the
case of the food quotient, and that the population density ai;
this period is 620.23 g.
The roi on between the 1-r5,t , or i»1-11.
It is evident that the rate of natural decrease tends •
38
population density is shown in Fig. 62.
The rates of natural decrease were in the order of the
survey periods 2,8%, 17.4, 6.7%, 17.8%, -1.2%, and 2.7% in
the 187-.95 g group, 10.8%, 20.7%,.12.6%, 12.8%, 1.8%, and 0.2%
in the 375.9 g group, and 27.3%, 1.7%, 6.4%, 6.3%, 17.4%, and
1.7% irLthe 563.5 fr, group. It will be observed that in the
187,95 g frroun, with the nypeTt .ion of Sent. (the fo ,-th
wen te ra47e was the highest, the rates were -eerally low,
that they were low in all the groups in the fifth and the sixth
periods e that in the fifth period it was the highest in the
375.9 g group, and that although exceptionally high values were
observed in every groun,the average values were 6.4% in the 187.95
g grouP, 9.2% in the 375.9 g group and 10.1% in the 563.85 g
5 9 >
te fP11 with the dPerease 1 -, the por , ilation dersit'r. It ap-
pears as though the frequency of ocurrence of "nose-lift" is
related to the number of eels released when the volume of water
is fixed or when there is no residual food due to overfeeding
and the quantity of phytoplanktons is fixed. The reason is
that the phenomenon of "nose-lift" in many cases is an indica-
tion of dyspnoea caused by the lack of oxygen dissolved in
water. The relation between the population density and the
freouen:-y of ocurrence of "nose-li ft " phenomena is showr 7ranhi-
cally in Fig. 65,
It will be observed that there is a very close relation
between the population density and the frequency of ocurrem'e
of "nose-lift", and that the frequencies are greater in the
group with the population density of 563.85 g and in the contrast
39
• than in other groups.
b. Results of experiment B
The relation between the rate of weight gain and the po-
pulation density is shown graphically in Fig. 64. The popu- /160
lation densities at eacb survev period were in the group with
the re-nlation denity 112.77 T 1.6, 1.4P. 1.07, 1.15, and
■ -1. _P :177.95 a grena 1.27, 0.99, 1.21, 1.11, and. 0.16,
in the 300.72 g groun 0,93, 1.12, 1.31, 0.89 and 0.21, and in
the ).51.08 g group 1.11, 0.72, 0.9 5, (-)G92 and 0.29. It will
ho oerved that in the 112.77 group the population density is
lower Alin. (the third period) in comparlson to those of the
187.9"; 7, and 0e72 g gropas„ and thnt in other crises the cin-
sitv in thr. T12,77 g grolip ts always hi7ber. in the 187,95 p:
7rorp„ except in Oct. (the fifth period) when the figure is
lower than those- in the - other groups, the densities are exceeded
only b7 those of the 112.77 g groun. In the 300.72 g group the
the density is lowest in June (the first period), intermediate
between the 112.77 P' and 187.95 7 cmoups in July (the second
period), highest in August (the third period), lower than that of
187.95 g group in Sept. (the fourth period), and lower than that
of 451.08 g group in Oct. (the fifth period). In the 451.08 g
group they are generally lower than those of the other groups.
The mean values of the rates of weight gain for these groups
were 1.20 in tlin 1 1 2.77 n'0 1 1n i 0.95 in the 187.95 g Fmolin,
0.89 in the 300.72 g 7rour, and 0.80 in the 451.08 g group. As
ir the case or ex-crip, ent A it is irident that the rate of we , ht
gain rises with the fall in the population density. In the
/1 61
40
contrast group, in which no correctio had been made to the
population density, it was 2.36y the highest, in June (the first
period), 0.94 in July and placed below the 187.95 g group in
July (the second period), identical with the 451.08 g group in
Aug. (the third period), the lowest in Sept. (the fourth period),
and 0.33, a hi7h value though below that of the 112.77 g groupe
in Oct (the fifth period). The variations in the population.
density were 75.18 7 in the first period, 251.85 g in the second
neriorl, 477.39 g in the third period, 890.88 7 in the fourtb
period, and 1278.06 g in the fifth period. With the exception of
the fifth neriod it is possible to observe the variations in
the rate of weiTht rein with the changes in the nopulation densi-
(V1 hr.v ,rnrrt. 1 1 r", 171 '111 .T CI 1-.1)
lation density is shown in Fig. 65.
The mean weights at each survey period were in the 112.77 g
(-mow-, .60 7, :!.43 7, 2.97 g, 6.28 g, and 9.02 g, in the 187.95 g
s:roup .53 7, 1.09 7, 2.56 g, h.96 g, and 5.75 g, in the 300.72 g
rronn n,) , () 7, 1. 0 1 g , 2.7 7, 5.07 7, and 6.32 7, and in the
451.08 g group 0.49 g, 0.94 gg 2.14 ge 4.10 g, and 5.34 g. These
findings show that the 112.77/group showed the optimal growth
curve and that the the i'Towth curve was the worst for the 451.08
g group. However, although the 300.72 g group is inferior to the
187.95 g group up to the second period, it shows a good growth
curve a ft er thaL. The 187,95 g c7ronn on the other hand shows a
low rate of growth after the third period. Thus there is an op-
posite relation between these groups. Further although the con-
trast group shows a markedly good growth curve up to the third period
'Ph?
1+1
• the growth becomes retprded with the increase in the population
density in the later periods. As in the case of the rate of
weight gain the growth is better in comparison to the 451.08 g
grour. It apnears as thour,,h a favourable growth at the ini-
17J . ,j n F:tren- inr.1, 1--ine. The d'il-e'.7ces in
,rrowt5 [7oPPr, are s1iht in comoarison to
those of experiment A with the exception of 112.77 g group.
Further, the nop'llati..)n clby at whlch the growth in the
contrast groups becomes retarded is 477.39 7. /162
The relation between the food quotient and the population
density show' rmapicallv in Fig. 66.
The food quotients at each surveyriod wre in the 112.77.
7rour 1 .71, u.80, 5.29, 9.9, and 12.12 e in the 187.95 g group
17.5, 11.6 e 8.3 e and 12.4, in the 300.72 g croup 9.5 e 13,
7.5, 6.6, and 16.0, and in the 451.08 g group 6.7 e 16.3, 8.7, 6.6,
and 10.3. It will be observed that the differences in food
quotient between the groups are small. A Comparison of the mean
values of the food quotient - being 9.35 in the 112.77 g group,
11.57 in the 187.95 g groun, 10.79 in the 300.72 g group, and -
9.71 in the 451.08 g group - shows that it is the lowest in the
112.77 g group and the highest in the 187.95 g group, and that
those of the 300.72 g mroup and h51.08 g group are intermediate
between the two. One further observes an'unusual phenomenon of
the rate for the 451.08 g group being lower than that of the
300.72 g group. It appears ahot this is due to the effects of
the rate of weight gain. Further as will be explained in a
later section the rate of natural decrease is favourable in the
contrast group. Thus the anomaly may be due to inadequate
4 ^71 , Me mean iias 9.71. This is
42
at the time of the transfer toanother pond. In the
contrast group ln 7Thlch nm cn-rtion had been made to the popu-
for, d n ntient u at each Furvev ner5od
t y. ont,1 -■ -■ t,
unl-, npl in tha:t the value is identical to that of the 451.08 g
group .
The relation between the rate of natural decrease and the
porulation den5, tty is Fhown in Ficr„ 67.
The rates of natural decrease at each survey -period were
in the 112.77 agroun 10.2%, 1.6%, 4.8%, 14.7%, and 0%, in the
187.95 5 p,roup 11.1%, 4.8%, 7.9%, 6.0%,. and 3.5%, intho 300.72 g
groun 14.2%, 0.5%, 23.2%, (.5%, and 0%, and in the /51.08 g
gronn 16.1%, 11.4%, 20.0%, 2.9%, and 2.9%. The mean values of
the rate of natural decrease were in the 112.77 g group 6.3%,
in the 187.95 g group 7.7%, in the 300.72 g group 8.9%, and in
the 451.08 g group 10.7%. It is evident that the rate of natural
decrease is related to the population density, i. e., the rate
of natural decrease becomes lower as the population density
decreases. Further in the contrast group the rates were at
each survey period 7.4%, 7.4%, 7.5%, 4.8%, and 0%. The mean
value was 5.4%. This is lower than that of the 112.77 g group
and is the best amon- all the FmourS,
c ollnqrine n? vd and B
An exfflination of the findings with respect to the 187.95 g
group which was used as a contrast group (187.95 g group was cOm-
mon to the two oxnnrimonts) shows thnt the rates of
were 6.8 in experiment A and 9.5 in experiment B, the food quo-
43
tients were 7.8 in experiment A and 11.5 in exneriment B e the
rates of natural increase were 6.3% in exreriment A and 7.7% /163
in experiment B e and the average weights were 8.98 g in exper-
iment A and 5.75 f; in exneriment P; thus there are differences
1-etmeen tn. EY'''r.lrimnntS PUrt /3 were carried out in dif-
ferent years. Thus it appears as though the differences are due
to dief--e— ar, the fechniell-, s of eel r-71tvre,
The relation betyeen the averae ,ralue of the rates of
1,ret-1- t 7aln r).nd the ro -eulaion drinit' , is 7rar11ica11y shown in
Fig. 68
tion of fe shows that the relationship
'.-;araboir,. Thus if o, !e plots t',.-te
11, of wet r) -
obtains the graph shown in Fig. 69. The curve is approximately
a straight line. Thus it can be represented by the relationship
The calc.J7n17,1en of t1. -In coefficients, a and b e results
:In th e follnwirc
•
y =
for experint A
for experiment B
for the average values of the two
Y = 1.080x-0.145
Y = 1.486x-°-149 and
Y = 1.150x-n11 '7
371. 1r5.1fIr 4 :1 " relatiorshi- between the avera7e weir:71"f,
'ood quotient, and rate of natural decrease and the no-Illation
density is shown graphically in Fig. 70.
An examination of the rraph77 nlinils that each of the err-yes
ho repre,arted bv a strai7ht 1jni. Thus the relr!tinnnhin ca ri
44
be represented by a straight line y = ax ± b. If one calcu- ,;.
lates the coefficients, a and b, for the average weight W,
food nnotient F e and rate of natural decrease N with X rer•
resenting the population density, one obtains the following
egrations:
W 0.097x - 2.594
F = 1.042x ± 2.50 and
= 0.075X +
L. Discussion
The relation between the population density and the coef-
ficient of weight nain is expresed by the expression y = axb
thn same exrression ht 5 :i. p( h.r Tnrao an Tanaka (192',q in their
ny - r ih yl t with lptlre rnmniucK scur(lL) and Ma en.
Macrocepa_Straus and by Kawajiri (1949) in his exneriment with
0Eyzias latipfs (TEMM1NCK et SCHLEGEL) t'a ascertain the effects
of the population density on spawning. A similar relationship
exists note only between the population density and the rate
of propagation but also between the population density and the
rate of weight gain. The rate of weight gain falls with the
increase in the population density. Further in view of the fact
that in the prescrit exrnriments, in exreriment A While the limit-
noints of the effects of the population ,ensity on the rate
of weight gain are similar -- 0.68 and 0.65 -- in the 187.95 g
ronn erd the 3?', .9 g pl-eur, it is extremely lo -- 0.49 -- in
the 563.85 g group, that in the contrast 7,ronn with no correction
to the population density there are marked decreases in the
average weight and the rate of weight gain during the period
when the population density changes from 402.21 g to 620.24 ge
and tat in experiment B there is a marked decrease during the
nr->rjod when the po -nulation. density is 477.39 ,,, the limiting point
of the -nonulation density apnears to be in the range 375.9-
•
The fren .'l-r.-cv 2f a-nearnnce of the u nos'-lifte nhe-
•
nomenon is related to the population density; the frequency of
"nose-liftu increses with the ircree in the nnpulation dsr-
rrnp7n,, to be the censnmtien of dissolved
oxvyen. Thronhout exTerimnnts A end B the relationships bet-
ween the population density and the food quotient, rate of
naturel decres e and the avera7,e ', elfht are enressed by the
expression y = ax, and the relationships between the number of
feed days and the fond quotient and the average weight are ex-
pressed by the equation y = axb
The reasons for the difference in the results of the 187.95
g group, whjch had been used as the contrast r::roup between ex-
periments A and P, appears to have been caused b-.7 the difference
in the environment and the breeding stock due to the difference
in the years of the exeriments. The abnormalitv in the 187.95 g
group and the 300.72 g group in experiment B appears to be due
to inadequacy in the experimental manipulations made at the
time when corrections were made to the population density.
This observation is based on the fact that the rate of natural
decrease is better in the contrast group in which no correction
was r-ade to the quantity of eels released. It will he noted
that the clifferertces among the population densitles is larger
in experiment A and smaller in experiment B. It appears as
though the reason for these differences is the fact that the
46
unit of population density was 187.95 g in the former and 75.18-
150.36 g in the latter.
Kawajiri (1928), Kawajiri, Hata, and Murai (1930), and
Kawaijri and Hata (1925) investigated the effects of the popu-
latjen density on 7rowth, natural decrease, spawning, and hatch-
ing of Oncorhvnchus_nerka_f. nerka (YALBAUM) and Salvelinus.
FTPTETL) and. ob-e-ved th-t when the nol-ult'on den-
si. Lv -as I erie the nati .iral. decrease was less and the averag-3
we'r:Lrfr was M.-Mar, and thc.t the ropulation density had little
effect on production. The finding on the average weight is in
a7reement wi th that of te ant,hor with his ex ,-, eriments; however,
with respect to the rate of natural decrease and production
tHeir res1 ,1 t-, riir entirely counter to those of the author's.
110 While the former is reared in running water, the latter is
reared in still water. Thus difference in the effects of the
population density is due to the difference in the methods of
cfflture.
5. Summaries
1. The relation between the population density (x) and
the rate of weight gain (y) is expressed by the equation
0. y = 1.150x 147 -
2, The limitj point of the effect of the population
density on the rate of weight gain lies in the range 375.9- -)
563.85 g per tsubo (Translator's note: tsubo = 3.305 m - ); it
is rather biased towards 375.9 g.
3 . The relation between the porulation density (x) and
•
47
the avera g e for a single speciment (w) is expressed
by the equation w 0.097x - 2.594.
4. The relation between the population density (x) and the
food ouctient(F1 is exnressed by the equation F = 1.042x + 2.50.
The relntior between the population denrdty (x) and the
rate of natural decrease is expressed by the equation N
0.075x + 1.982. /166
6, Y'-'re 1nr7 close 1-e]ationchlp 1 , rq::wne .-1 the nnpu-
lation density end the freclur,rey of appearance of "nose-liftn
phenomenon. As the density increases the frequency becomes large.
•
•
Chapter V. Relation between the degree of variations in size
of frit and the productivity.
1. Pest achievements and the purpose of the study
111 - f."1. 0 7,6 ;rr t'-nt tliere was P close relation-
- the techniques of eel culture and the degree of
variations in the eize of fish at the time of harvest. Thus the
01 1 2FtiOn to wl- at n-tent the deç-ree of variations'in the ‘;0 of
fish influences the production is an important problem in eel
culture. In the method presently adopted by commercial breed-
ers the fishes are transferred to different ponds at least once
a year, eelected by r.ize, and those, which have grown to marketable
eizee, are sold as mature eels. However, for operational reasons
the selection is not perfect and the eels, which remain in the
-ed-, ran-n in size from mature specimens to fry; thus the vpri-
tion in size covers a wide ran-,e. Further, in the place of the
large proportion of mature eels selected and removed, the stock
is supplemented with the so-ealled osashin material, fry rangine,
in size from 37.5 g to 75.2 7. Thus at the present time eel
culture is carried out under conditions which allow extremely
large degrees of variations in size. The present experiments
were carried out to test if such methods are favourable to the .
production of eels.
Experimental method and development
The experiments were carried out in 11/4 and IV5 ponds
in the Yoshida Training S 4 te of the Fisheries Research Institu-
te (at the mouth of the Oi River in Shizuoka-Ken). Both ponds
49
• had an area of 45.375 Tsubo (20 m in length, 7.5 m in width,
and 1.5 m in depth). The water from the river was used in the
pond":1, The water remained in the ponds for tl-ie duratlon of thp
exnerlmerts.
rp1 srer..i7!ens of "shirasun-eel used were collected in
rivers rear the Yoshida Tr3inir7 Site in Arr. 1937 and raised
under condition as intical as possible for a period of one
year after the anadromous migration. The experiments more be-
gun on Air. 5 0 1938 and concluded on Aug. 22 of the same year.
The experiments were carried out by dividing all the spe-
cimens into A-group with a small degree of variations in size
and F-gronn with a lar^;er de7;ree of variations. The size of
the degree of variations was determined by dividing all the
specimens into large, medium, and small size classes. Specimens
for inclusion in A-2: -'op were selected from the lar7e size
class; B-group was made of eels from all the size classes. The
composition of the two groups, number of eels released, names of
the ponds, and si se rer eel are shown in Table 153.
Fresh sardines were used as feed. The feeding was carried
out once a day at approximately 9 am. The quantity of food taken
in WaS determined weic'hirg the sardines before feeding and
by weighing the amount left unconsumed after leaving the sardines
for twenty four bonrs. The intermediate surveys were carried /167
out once a month to determine such factors as the total weight,
amount of feed admininr,,te ,-ed, and number of fish. The eels then
were released into the original ponds and the experiments were
carried out without an y correction to the population density.
There were not any narticularly noteworthy differencs with res-
50
pect to the rearing. The onose-lift" phenomena were observed
in more or less different degrees. In many cases it was more
marked in B-group. There was no case in which the phenomenon
was observed in one group but not in the other. After an exami-
naton on jui ,T 22, 0n account of unavoidable circumstances, the
eels in both ponds were kent in n].ive-basketsn in running water
with the consequence that they were attacked by Saprolegnia and
suffered from "tail-rotn. Some dead specimens were picked
up over a period of 7 days. The food intake after the survey
was favourable in both ponds.
Experimental results
The results of the experiments are shown in Table 154.
On the basis of the findings shown above the rate of weight
gain (W, food quotient, and the rate of natural decrease (N)
were calculated in accordance with the following formulae:
wi
w0 w1 -
nO - n1 x 100, no
, and N =
where w0 -- the weight at the time of release, .
w1 -- the weight at the.time. of harvest,
no -- the number of fish released,
n1 -- the number of fish at the time of harvest, and
f -- The total quantity of feed administered.
It will be observed that with respect to the rate of weight
gain B-group was sunerior to A-97roup up to June 19 and after
this date A-group became superior. At the time of the termina-
tion of the experiments the rates were 1.59 in A 7groun and 1.49
51
• in -B group. Thus A group was larger by approximately 6% in
comparison to B-group. The average weight for each specimen was
by far the larger in A-group throughout the whole of the experi-
mental -eeriod (cf. Fig. 71)
Wth rent to tie fro', crnotient, with th_e, ,,xceptinn or
Pnv 20-21 the ',fi cinc was bntter in B-grov-, , the o"otient
was 1-, ttrn- I- Ai t1-i time o termiPation of the exori-
"n"P 7.9 iu A-uroiln Fold inn,
Thu- U- o food efficie ., cv is blter bv an-roDdmately 30% in the /168
group with lesser degree of variations.
The rate of decrease of natural decrease is less in A-group
in com-arison to B-imoup throughollb the leri.ori of the surveys.
In short with respect to each of factors such as the rate
of weight gain, food quotient, rate of natural decrease, or
averag-, weight nor indii!idual s ,)ecimen the groin with lessor deg-
ree of variations is far superior to the group with larger degree
of variations.
Discussion
The proportion of eels of smaller size exceeds 20% of the
number of eels in B-gronD which is made of sPecimens of varying
si s es. Generally in eels of smaller size the rate of weight
gain is better, the food quotient is smaller, and the rate of
natural decrease is larger in comparison to those of eels of
larger si-n (cF. (ha -,-'er 1). Thus B group ceritains, in com-a
rison to A-L7roup e factors which show these LechnieueT of e;'1
culture and rouPs which should exhibit suneriority in 7nwl. 1, ;
howev-r, the experimental results run counter to these expecLa-
tion. It i th- ;,uhhorts onininn that VP-; investigation of the
• 52
cause will furnish the key to the solution. Observatrs made
on the manner of food intake of the ,wour) with the lar?:er degree
of variations in size show that the fish of larger sizes take
advantae of their size and not only takein greater quantities of
food b72t r0 , c tal‹- e in food more freouentiv. Thus with a. fixed
amount of food the fi!-h of smaller sizes, which should show
better growth, cannot take in food in sufficient quantities for
7nowth resulting in better growth only for fish of larer sizes.
Tn other words it anilears as thouh the group with larger sizes
attains growth by obstructing -Q--, 7rowth of the group with
smoller sizes resulting in still larger degree of variations
in s 7-,e.. Further, ti-le rate of natural decrease increases with
the decrease in the size of the fish (cf. Chapter 1). Thus it
is only natural that the natural decrease is larger in the group
of fish of smaller sizes when groups of fish of larger and
smaller sizes are raised together. Further eels food on one
another and one can imagine this factor contributing to the
natural decrease.
In order to reexamine this difference in the rates of
natural decrease between the larger and smaller groups the
author carried out the following experiments (Table 156).
A group of fish of lar7r and smaller sizes wore raised
together in a pond 40 m2 in area and a comparison was made of
the natural decrease in the two groups. The natural decrease /169
wa.s very mucll langer in the smailer groin thereby supr'ortin7
the reasons outlined above (Fig. 72).
The food quotient is greater in the group with a larger
dree of variation than in the group with a smaller degree
•
•
53
of variatio') jP(iicating that the feed js not utilized efficien
tly. This is due to the fact that the group with larger sizes .
consumes a greater portion of the feed with the consequence that
the feed has little chance of bning utilized by the group with
nr):1 1.nr nizes, 1:!hjch sboiHd baYe a srlallr food quot.îrt, and
that th-.›. 7roup with lnr7er sizes has a larer food eudtiento
This along with the relation to the rate of weight gain is an
interesting problem,
5. c crls • . . _
The group with a larger degree of variations in the size
Of the fr ,, in i-ferior in comparison to the group with less
degree of variations in size with respect to factors such as the
rate of weight gain, food quotient, or the rate of natural dec-
rease which controls the techniques in eel culture. Thus one
should give careful consideration to the Question of improving
the usasile material (Trar,slato ,-ts note see Section e (Thaptr
Y. for an explanation of the term) in the techniques of eel
culture nresetly adopted. Thus to bring about improvements
in various olements which centrol the present technices of eel
culture, it will be necessary to frequently rearran7e the place-
ment of eels in ponds to consolidate them by size and to lessen
the dec'ree of variation as much as possible. It will be possible
to increase te Production to a certain extent by the adoption
of such methods. /170
■7,..1 r1
C, apter VT On the influence of the quantity of food on the
ifrowth ratn, rate oor nat7Ira] (Jerre-1 ,1e fond cointiPnt and
degree of variations in size of frV.
or the stu _
Matsui (1939) reports that the cost of production of
mature eels can be classified roughly into the cost of fry and
cnst n -. 17 feed and tbat the eot or fend, 39.5% of the total
cost of prodction, is an important item. Thus the quntity of
feed administered greatly influences the quantity and cost of
nroduction of eels . Consequently from the point of view of the
ma -r7er2e-, t of oel culture it is necessary to determine the amount
of feed to be administered to maintain the weight of eels. How-
ever there has not-been any study of this kind. At the present
time the commercial breeders generally adopt the system of feed-
ing the maximum amount which the eels demand. The present ex-
periments were carried nut to determine the suitable and effec-
tive amount of feed to be administered to lower the cost of pro-
duction to the minimum and to contribute to the rational mana-
gement of eel culture.
3Experimental method
The snecimens used for the experiments were ushirasun-
eels just at the feeding stage. Specimens of average weight
0.15 m; were divided into 3 groups and each group was fed with
the amo nt of feed equivalent to 15, 10, and 5%, .respectively,
of the total quantity of eels released. The total quantity of
the eels released in each group was identical. The experiments
were carried out for 138 days from May 20th to Oct. 8th. The
, i
e • . L
. 55 k
), 0 9 6 •
. opulation density resulting from growth.
The feed used was ground fresh sardines. During the last
30 days of the exneriment the eels were fed with sardines strung
along a strand of string through their eyes. The eels were
fed once a day in the morning. The food. intake was determined
by weighing the unconsumed quantity of feed.
The degree of variations in sige was shown by the coeffi-
cient of variation. The latter was calculated by collecting
100 heads of eels at random from each group e by measuring their
length and weight, and by calculating the mean and standard
deviation.
3. Experimental results
The results of the present experiments are shown in Table
157.
No temperature difference in the waters of the ponds was
obc;erved d.uri the norlod of th e owrnripent. The ,tnose-liftft
phenomenon was observed on June 12th in the 15% feed groupe
On June 13th the so-called nmizukawari” (literally nwator-chan , ")
rol , 'Init-'()' Iran obs ,,rvod in 1.1‘n 5(,, feed group: the colour of the
water changed from bluish green to dark brown and recovered in
period of 48 days from May 20th to July 8th was defined as the
first stage, the period of 45 days from July 9th to Aug. 23rd
as the second stage, av'd te -eriod of 45 days from AvuT., 23rd
to Oct<, 8th as tho third stage. The quantity of eels released
ijc correoted at each sta to make the poulation density
identical which was set in the range 375.9 g to 15.0 g per tsubo
3.305 m2 ) to remove the error due to the difference in the
56 . ,
, 1 a week.
: 11, The results of measurements of the degree of variations ,
in size in the first stage are shown in Table 158.
On the basis of the results of measurements shown in Table
157 the rate of weiP.ht gain, food ouotient, and rate of natural
deureae were calculated. Th.ese are shown in Table 159.
An exe?,mination of the results of the exnerimr,nts in the
ftrst stae shows th.at there were decreases in the yield nt the
time of the harvest in comparisôn to the amount of eels released
in the 10% and 5% feed -croups. They were 180.43 g in the 10%
feed group and 932.23 g in the 5% feed group. An examination /171
of the rate of raturai decrease shows that they were 44.5% in
the 10% feed group and 21.41 in the 5% feed group. In the light
0
of the fact that the rate is twice as large in the former as it is in the latter, one realizes that the relation with res-
pect to the amount of weight gain is reverse. Thus the
findings indicate that the amount of weight gain of the 10%
feed grour is btter than that of the 5% feed group. Further
the rate of weiht [7:du in the 15% feed grour e beino., 0.09, is
- very much lower than that of the latter. However the low rate
was a phenomenon common to al the feed groups in the first
stage. It appears as though this is due to the fact that the
environmental changes were too sudden at the beginning of the
experiment and that the fry were not mature enol.mh for feeding.
The reasons for the fact that the rate of natural decrease is
very much larger than the other two groups are not known. The
11›
food quotient of the 15% feed group is 3.71 which bears no
comparison to those of the other two groups whose quotients
57
exceed 100. Durir- the cond stage the rates of weiP:ht gain
were 2,60 ip the 15% feed grorn, 1.83 in the 10% feed -Toni), and
0.51 in the 5% feed group: the relationship is directly pro-
portional to the amount of feed administered. It will be ob-
served that the rate in the 5% feed group is approximately 1/5
th[3t, of the 15% feed [Tonn. The rates of natural (:ecrease
were 20 .5'', ln PTonn e 22.1% in the ln% fe-, d (Ton-,
a-d '7 8,3% in the 5% feed -rour, The rates rise with the dec-
rease 1', the amont of feed adrinibered and their effectr on
the rate of weight gain is large. The food quotients were 3.99
in the 15% feed group, 3 0 57 in the 10% feed group, and 7.20
in t1-1 ,- 5% food i,rou-. The differnnce between the 15% food /172
group and the 10% feed group is small. It is best in the 10%
feed group. Further although the quotients in these two feed
grouns are anprnximately twice thab of the 5% grown, it appears
as thrv , h bhis in rnlated to the rrtrkedlvhigh rP.1:': of natural
decrease in the 5% feed group.
In the third stage the rates of weight gain were 1.60 in
the 15% feed 7rouP, 1.11 in the 30% feed u.oup, and 0.61 in the
5% feed group; they show a relationship directly proportional
to the amount of feed administered. Further the rate in the
15% feed group is approximately three times that of the 5% feed
group. The rates of natural decrease are 4.3% in the 15% feed
group, 13.3% in the 10% feed group, and 35.7% in the 5% feed
gronp and are inversely nroportional to the amount of feed ad-
ministered. The rate in the 15% feed group is approximately
1/5 that of the 5% feed p-oup. The food quotients are 5.62 in
the 15% feed group, 5.73 in the 10% feed group, and 5.68 in th e
re!, :rrsed grou. Tbere is no ma -r'ked difference between the varions
58
feed groups.
With respect to the degree of variations in size one
finds the body length and weight in the 15% feed groun to be
178 and M.() in 1 .,1:,5 15(;) foed group, 16.3 and 40.4 in the 10%
feed group, and 10.3 and 38.6 in the 5% feed group . They
are in -direct proportion to the amount of feed administered. The
varjatio-2. decrees with the decrease in the anont of feed ad-
rinintprod,
An examination of the results on growth with .respect to
the average weight shows that they are 0.26 g in the first
stage, 1.16 g in the second stage, and 3.01 g in the third stage
in the 15% feed group, 0.30 7 in the firSt stage, 1.16 g in the
second stage, and 2.74 g in the third stage in the 10% feed group,
and 0.15 g in the first stage, 1.16 g in the second stage, and
3.0 in the third sta7e. In short in the first stage the 7rowths
are titrce as lare in the 15% and 10% feed croups as in the e5;-
feed ,group, in the second stage they become equal in the varions
groups, and in the third stage the growth in the 5% feed exceeds
those of other groups followed in turn by those of the 15% and
10% groups. Thus the growth in the 5% feed group is extremely
inferior at the beginning but becomes optimal with the passage
of time.
/:., DI.s.--1_
The group fed with a lesser ouantity of feed has a higher
rate of natural decrease in comparison to groups,fed with greater
euantities of feed. This fact greatly influences the rate of
weight gain, food quotient, and the growth measured in terms of
the average weight. As reasons for a larger rate of natural
decrease in the group fed with a lesser quantity of feed, one
may suggest that eels are voracious and that when the amount
of f'ood is restricted, some eels become le5:s resistive on
account of th" shorta7e of food and bec ,7,me rrevn to stron7er
eels or .(-7-ow , !e., . 1(or and die, TI-us while ther-, n
decrease in the 5% fned p:roup in comrarison to other groups, and
the de,c;ree of variations in size of the groun is small, there
is a gradual increase in its average weight, and its rate of
weight gain and food quotient become high. In short, specimens
of uniform size and weight are gradually selected out by a natural
process resulting from a struggle for eXistence: this point
can be explained on the basis of the high rate of natural dec-
Ilk rease. Thus a restriction on the amount of feed at the
be ,7innin of cnitivtior of "shiraF, uu-ools unnecessarily ar7ra1n-
tes the natural decrease and influences the production. There-
fore at the period when nshirasun-eels reach the feeding stage,
they should be fed with an excess amount of food.
The fact that in the third stage there is no difference
with respect to the food quotient nmon the various fedd grouns
and the frict that e while the rats of weiht 7ain in the other
feed groups show a decline, in the 5% feed group it tends to
increase through the second to the third stages suggest that
it is not necessnrir to always ad)7iinlster an excess quantity of
feed, that it is possible to determine at various stages of growth
the amount necessary for the maintenance of weight, and in
the ligt of the i'7formntion so obtainerl, usin7, it as t:
lowest to dec=1:3 ,2 the amonnt of feed adinistred- Fur-
•
60
ther 't nnnears that the nroner lies helew the re,P;ion
corr-, n-e-, di to 5% of te total Quantity of eels released.
Con.clusions
Tt i.nossible to rnre the eon-- e feed h'T adjustin7, the
quantity of feed to be administered in accordance with the /173
growth in fish A method wherein a rearing period of a year is
dividc, d into three stages p.nd wherein eels are fed progressively
at 10 to 5% at successive stages does not result in a decrense
in the production. The quantity of feed to maintain the weight of
eels is less than 5% of the weight of the whole fish popula-
tion
•
Chapter VII. On the growth of "shirasun-eels differing in
the reriod of uostream migration
1 Past achievements and the purroqe of the studv
The selection of the fry is an important item in the cul-
ture of nshirasun-eels. Nakai and Matsui (1936) established
the fact that there were superior varieties among the fry. This
was later confirmd bv Inaba (1959).
The nrosert e7rorinents were earr 4 ed nt-, to study and
compare the degree of 7.rowth of fry differing in the period of
upstream migration.
2. , o.*,. od . foi rints
and the ntorv of reari1'7.
The specimens used were collected from Katsumata.River in
Kawasaki-Cho in Aibara-Gun, Shizuoka-Ken. Three groups were
provided in accordance with the three periods &wing which the
were C ollections made. The first groun was collected at
the enrly s1;3 ,:es of tl, e iirstroam migration (Jan. 24-Feb. lie
the second group at the height of the upstream migration (Mar.
18-Anr. 1), and the third p.roup towards the later stages of
the migration (Apr. 2-May 2) (Table 160).
The experiments were begun on May 13, 1938 and terminated
on Nov. 16 of the same year. The experiments were carried out
at the Yoshida Training Site of the Fisheries Research Institute.
Each of te ex-erion,t1 - ponds an nroa of ]11.996 m2 Is . Throe
pon,'w with Uho iv,n e and caT.sneitv were used.
As the srecimens were collected, they were placed in the
62
rearing ponds by the period of migration, kept until the start
of the experiment,feedinc thul with sufficient quantity of food.
At the start of the experiment they were collected in required
quantities frcyn the respective ponds and placed in the experi-
mental ponds. Fresh sardines were used as feed. The residual
feeds wm-o weighed and the amounts consumed were determined.
They were hauled in once a month and the increase in weight,
number lost and quantities of feed administered were investi-
gated. At each survey the population densities were corrected
to make them equal in order to eliminate the effects of the
differences in population density.
Experimental results
The results of the experiments are shown in Table 161.
The minimum, maximum and the average monthly variations
in water temperature during the period of the experiment are
shown in Table 162.
An examination of the results show that the average water tempera-
turc exceeded 11 ° C, that the average water temperature from May
to Oct. exceeded 20 ° C, that the maximum temperatures were reached
in July and August, and that the maximum water temperature during
the day exceeded 30 ° C. It also shows that thcre was no difference
in water temperature among the various ponds. /174
On the basis of the results of the experiments calculations
wl w0 were made of the rate of weight gain, W , food quotient,
w0
F = and the rate of natural decrease,
wl WO
63
12, nl N
.0
The relationships between the number of rearing days and
the rate of weight gain, food quotient, the rate of natural
decrease and the average weight are shown in Fig. .73.
The rate of weight gain decreases with the number of
rearing days, it decreases sharnly at the beginning during the
first month and after that it decreases gradually. That is to
say, while it ranges between 2.06-4.03 during the first month, /175
it lies in the range 0.81-1.18 in the second month, a decrease
of approximately 32%. There is a slight increase in the third
month throughout all the grouns. A comparison among the three shows
groups /that during the first month the rate in the second
group, being 4.03, is the maximum and the rate in the first group,
being 2.06, is the minimum, and that the differences in rates
among the various groups are large. From the second month to
the end of the experiment the differences in rates among the
groups are small. During the fifth month the differences are
the least. Although the rate is the maximum in the second
group from the first to the fourth months of rearing followe d .
by the third and the first groups, starting at the fifth.and
in the sixth months the rate becomes the highest in the first
group and the lowest.in the second group. In short, the rate of
weight gain in the first group tends to increase gradually with
the increase in the number of rearing days. However, the dif-
ferences in the rates of weight gain observed at the beginning
persist till the end of the experiments. This iS clearly seen
by examining the average values. That is to say, they are 0.93
x 100. These are shown in Table 163.
6/1.
- in the first group, 1.44 in the second group, and 1.14 in the
third group: it is maximal in the second group collected at
the height of the upstream migration and minimal in the first
group collected at the beginning of the migration.
In each group the growth in the average weight per speci-
men increases in a sigmoid-lilm curve with an increase in the
number of rearing days,and the growth is largest from the third .
to the fifth month from the start of the experiment. The average
weights atthe end of the first month were 0.53g in the first
group, .94 g in the second group e and 0.64 g in the third group e
and at the time of the termination of the experiment they were
10.82 g in the first group, 24.24 g in the second group, and
13.72 in the third group. Thus the growth was the best in the
second group collected at the height of upstream migration, the
worst in the first group collected at the beginning of migra-
tion; that of the third group collected at the end of migration
was intermediate between the two. Further the difference•bet-
ween the first and the third groups was slight while the dif-
ferences between the second and the other two groups were large,
Thus the growth in the seCond group was by far the best in compari-
son to those of the other two. As was observed previously, while
the rate of weight gain in the second group is low at the fifth
or the sixth months, its influence on the average weight is
minor. Thus it appears as though the influence of the rate /176
of weight gain on the growth of eels is large.
The food cuotient is closely related to the rate of weight
gain; an inverse relation is observed (cf. Chapter I). Up to
the fifth . month of rearing the food quotients lie in the range
•
65
6.94-11.51 in the first group, 3.24-7.58 in the second group,
and 4.26-9.83 in the third group; thus it was the best in the
second group, followed by the third and the first groups.
However at the fifth month they were 3.18 in the first group,
3.84 in the second group, and 3.48 in the third month. Thus
the order was the first, third and the second groups although
the difference between the first two groups was small. In
the sixth month they were 3.37 in the first group, 6.05 in the
second group, and 2.98 in the third group. Thus the quotient in
the second group is decidedly high. A comparison of the mean
values of the food quotients shows them to be 7.04 in the first
group, 4.94 in the second group, and 5.34 in the third group.
Thus it is lowest in the second group and highest in the first
group. As in the case of the rate of weight gain the feed is
most effectively used in the .second group collected at the
height of upstream migration and least effectively used in
the first group collected at the beginning of migration.
The rate of natural decrease tends to decrease with the
increase in the number of rearing days. Throughout all the
groups the quotient reached the maximum at the third month
of rearing. Up to the fourth month they were in the range 7 , 71-
2l.7 in the first group, 0.93-4.80 in the second group, and
3.62-13.46 in the third group; thus the rate was lowest in the
second group and highest in the first group and the differences
among the groups were large. However in the fifth month while
they were 2.80 in the second group and 2.01 in the third group,
110 the rate in the first group, being 9.03, was very high. In the sixth month the differences among the three groups were
3 • 66
• very small. They were.0.80 in the first group e 0.h8 in the
second group, and 0 0 78 in the third group. The average values
were 10.69 in the first group, 2.38 in the second group, and
5.24 in the third group. Thus it was lowest in the second
group and highest in the first group.
4 , Conclusions
A comparison of the rate of weight gain, food ,quotient ,
rate of natural decrease and average weig,ht per eel through
the duration of the experiment by the period of upstream mig-
ration is shown in Table .164-
An examination of the table indicates that the group col-
lected at the height of upstream migration is the most superior
with respect to the rate of weight gain, food quotient, and
the rate of natural decrease, and those collected at the begin-
ning of migration is the most inferior; somewhat biased towards
the group collected at the beginning of migration lies the group
collected at the end of migration. The average weight per eel
clearly indicates the growth of the three groups and the find-
ing indicates that the fry collected at the height of upstream
migration is the most valuable in the selection of the fry.
The reasons for the difference in the quality of the fry which
differ with respect to the period of migration are that a large
number is lost through natural decrease as clearly indicated by
the high rate of natural decrease among the fry collected at the
start of the upstream migration, and that because of the long
interval of time involved between the migration and the time when
the fry reach the feeding stage, they are placed in an unnatural
• environment during this period resulting in a marked loss in
vitality. The difference between the specimens collected at
the end of the upstream migration and at the height of the
upstream migration appears to be due to the difference in the
rate of appearance of the superior growth group(Pakai.and Matsui
(1936)) and in the female-male ratio (cf. Part Two, Chapter IX).
'68
Chanter VIII. On the natural decrease in eels in the eel culture
/177 nond d•rinp- the winter season
1„ --1--n of the study
Whe., the water temnerature falls below 15 0 C 9 the food
intake of eels decreases markedly and their movement becomes
sluggish. As the water temperature falls further, the food
itake eeasee comnletelv and the eels enter into the so•called
Further the iefluence of the length
of this hibernation period on the annual production is signi-
ficant, This is taken as one of the conditions for determining
tho snitability of an arPa fo- eel culture. One mav regard the
district where eel culture develops as meeting this require-
ment. In the Kawajiri eel breeding zone at Yoshida-Cho in
Inabara-Gu -e, Shizuoka-Ken the water temperature rises from
t. i'e to time durin7; the wi-ter and active movoment by eels are
observed, This phenome-on occurs often aroud Pnleruary and
March. On such occasions does one administer feed to give
the eels food neoessary to ma 4 ntain their weiht as a .-eans of
r)reventinr: losses? Should one actively make an attempt to put
flesh on the eels? Or should one not feed them at all even when
the water temperature rises oecasionally during the winter
and start feeding them all at once when the water temperature
rises to the point suitable for food intake and thereby attempt
to economi7e ou the oost oduction? All these nuestions
are closely related to the natural decrease in the number of
eels durinr. the winter. On the other hand these topics are
related to such questions as the following from the standpoint
69
• of the manar;ement of eel culture should the eels be trans-
ferred to diUerent ponds beflore hibernation and an attempt made
to market adult specimens or should they be consolidated into
difHrent ponds when the price of eels rises? The present study
was carried out to answer these °vest:10 -1s.
2,._Exp,erimental material and method and the history of rearim
The sreclwen,s 11d for the ex,)erin-rts consigterl of two
n 2 been collteJ in rive-s
r ,7 1-• -; n ̀111, n ' 7 -1 --"-•
vqnr (1-Pix—tric)nt Uos. 5, and 6), and the otber t'rn so-called
Pn2
These
"Yochve-e-1,
to three y-ars,
;) 3) wn.s renr2d for two
tuo zroups were fed with sardines up
110 to the start of the experiment. They were fed from the first part or the latter nart of Anr. to the latter part of Nov. or
the first np. -1-t , r DP^„ Tb oc were fod for annroxim1 .- 200
(-r;-- the 1-,-, -1n , q of
The experiment was carried out in the followinn manner:
tL n, cols CnlinCtnd 7 dq-s after tl'n feodiru; wns discentin-
ued, weighed and counted, released in various ponds, and kept
for 81 to 143 days without feeding; thon the number lost duc to
nntural decrense and the rate of natural decrease were cplculated.
3. Experimental results
The vm-iation in the water tem -fleratvre durinr the period
of the experiment are shown in Fig. 7/4..
As no dlfference was observed in the water temperatures
-,:ra7'h is shown with
70
the average values. During the period of the experiment
the maximum water temnerature was 15.8 ° C and the minimum
1.0 ° C. The water temperature he7an to fall on Dec. 12 and
reached 4.8 ° C within 2 to 3 days. Thereafter the water
temperature re7istered less. than 10 ° C till March 15. D13.rn5,-
this period the water temperature varied and the movement
amon the eels was observed. As the eels were not fed at all
cl.r.in the rerind e the e-,:neriment, the object of foragin
by the eels must have been planktons. However, during the
period of the experiment the occurrence of Microcystis ,spp.
peculiar to eel breeding ponds was few and the water appeared
blulsh browr in color. The main zooplankton was Rotatoria;
Cyclops spe was found in relatively small quantities. The
amount of planktons collected by towing a plankton net (No. 25)
with an opening of 20 cm through the surface layer over a dis-
tance of 20 m was 0.-1.6 cc.
The results of the experiment are shown in Table 165.
An examination of the results shows that the rates of
natural decrease were in the range 0.92%-12.8% with the average
of 5.6% with respect to weight, and 0%-2.1% with respect to the
number of eels. Further in Experiment Nos. 1-3, in the specimens
in the third year of rearin7 the rates were in the range 0.9%-
/ with i»e •,v- -'re & 2,1 wi.t 1:' resect to wnight,r0-!d the rate
wr; fl.n - with r.'snct .,-) ti. Yll'mber of cc s, ffiTloP7 the sne-
cimens in the second year of rearing (Experiment Nos. h-6) they
were in the range h.3%-12.8% with the average of .9.0% with
rnr:rnct to weio-Ilt ard p.3%-2.1% 19-5.th the avera7e of 1.9% with
respect to the number of eels. Thus the rate of natural dec-
/177
•
O
71
rease can be regarded as almost nil. It is also very small
in the case of the specimens in the second year of roaring.
Various conjectures have been made concerning the phenomenon
of the natural decrease during the winter and it has been
thonght that the rate of natural decrease was fairly high.
However the present experiment clearly establishes the fact
that the rate is very. small. It will be observed that a compa-
rison of the rate of natnral decrease with respect to the
weigbt and the number of eels shows . that the loss of indivi-
dual eels is alwas smaller than that of weight. It appears
therefore that the factor which controls the rate of natural
decrease is not the loss of individual eels but the loss of
body components due to fasting.
The relation between the rate of natural decrease and
the size of the fish (weight) is graphically shown in Fig. 75.
The derease is small in eels with larger size and lare in
eels with smaller size. The relationship between the two can
be represented by a parabola.
Oshima .(1933) investigated the chemical changes in the
components of the body of the eel during the period of the fast
and concluded that the decrease in weight was mainly due to the
decrease in the dry matters and that much of the decrease in
dry mPtters uns (1 , u) to the consumrtion of exudd substances of
ether. The feeds administered prior to the experiment were
sardines and dried purae. The decreases in weight due to 50 days'
fastino: from Oct. 22 to Dec. il were in the range (:.%-14.2% with
the average of 11.1%. The maximum decrease occurred on the
fifth day aftr the start of the fast -- the rate of decrease
experiment and those of Oshimats experiment (1933) That is
72
was slightlY over 14 with the daily average exceeding 0.8%.
There were differences between the conditions of the present
te sa y in the exneriment carried out by Oshima the water temz,«
-eernture wns over 1l ° C. There were differences between the
two experiments with respect to the water temperature and envi-
ronmental factors which were very closely related to the natural
Te-r-ese-t experiment was cnrried nut in ,breedin
Thun the en7s hibernated 11,1 ,, r natural conddtions.
Hence there were organic and inorganic substances to serve as
nutrients and the conditions in the ponds were such as to sti-
mulate 4, 1,1 occurrcPcr, of rla -lUers. Thus while in the present
experiment even during the period of hibernation there was a
supply of nutrients and the decrease in weight was prevented,
in the exneriment carried out by Oshima eels were kept in baskets
which were r,:uspended in ponds - each with an area of 33.05 m2 ,
Th i- the ïe,tcYn th^ nf 7- trients was difficult.
A comparison of the rates of natural decrease of the two there-
fore is not ap -nrorriate,
As a reason for the decrease during the winter season,
when the eels differ greatly in size, cannibalism can be
mentioned. This nhenomenon occurs less frequently during the
period of falling water temperature especially if natural
foods are present. It is rather a phenomenon which occurs .
t1-.0 per cd of risinr': ater. temperature esrecially when
the watel- riîlns pl.-nirP. 10 ° C, whn the eels start
to forn•e acti':e] ,, fo -- food, iln_(1 b!"52. no food has hnn -drli-
tered over a long period of time. Among the eels less than
/i7q
73
20 cm in length the decrease occurs because they are preyed
upon by birds especially by sea--ulls. The damage is consid-
erable since the eels/ movement is sluu.ish during the winter
season (cf. Charter IX).
The relation between the rate of natural decrease and
the size of fish was already described (cf. Chapter I) the dec-
rease being larger for smaller fish. The decrease during the
wirter season is due to deaths from nred-fin" disease and
the deaths caused by the parasite of the class Sporozoa. Al-
though the loss is related to the size of the fish, in view of the
fact that the loss occi-s in a large number in Feb., it appears
as thour,h the causes are the fall in the resistance of the
body during the winter season, the increase in pathogenic bac-
teria and a sudden rise in the metabolic activities due to the
rise in temperature.
1. Conclusions
In the case of rearing of adult eels during the winter
the loss in weight, being less than 1,,.1%, is extremely small.
Further the loss of individual eels is almost nil except through
diseases and through damages by birds. The loss during the
winter season is caused mainly by a loss in weight. Thus one
should not always consolidate the ponds in the fall and market
adult eels. Rather he should select a period of a sharp price
rlsn.
The fedin should be discontinued when the water tem-
perature falls below 10 ° C. It is difficult to recognize the
necessity for feeding at the time of irregular rises during
• the winter season. In the light of the above findins one rea-
lizes the necessity- for improvin the techniques of the mana-
gement of eel culture. -
•
•
• 75
Chapter IX, On Iniurious Birds which are observed around
1- el Culture Pond(' Culture -
Ps et n c n T.Ient s d tb n purflose of the study
From the standpoint of fish farming the damages due to
birds cannot be ignored. Since the natural and man-made damages
I 1 5e'). to a certain extent directly,
e ,fo], e ftention, The birds on the other hand invade
breeding ponds nt will devour or carry away fishes. Thus in
many cases the actual nxtent of the damages is not known and
is apt to be disre.p;arded.
Positive prntnctivn mnasurns for fry of trout, ^arns, and
goldfish are being taken. However almost no thought is given
11, to the problem in connection with the rearing of the fry of
eels in eel eniture. Tndirect damaes by waterfowls a-e the
possibilities that they propagates parasites of such classes
as Trematoda and Cestoidea and that they may act as vectors of
infectious pathogens. Thus sufficient care must be paid to
this problem. It appears as though in general fish farms near
coastal areas are invaded by varied species and a larger number
of injurious birds. However, there are many unknown areas with
respect to the relation between injurious birds and fish culture.
The author finds that the number of studies on such matters as
the effects of injurious birds on the propagation of fish, the
rel_ation between birds and disea-, es in fish, or the protective
measures is very small.
The present charter deals with the damages and protective
measur es by species of injurious birds which invade eel breedin
i s «h-, t.m so- c a11e(1
• ponds in the Kawajiri district (near the mouthi of to River Ci in
Yoshida-Cho in Haibara-Gun, Shizuokan).
2. Species*
The Kawajiri district is located in the north-western
r, f t 3-, e Pn7 of Sn-n ,ra and because it is near the
- arc,
areas, seas, and rivers. Thus there are more snociec there
than in the mountain areas. The specimens which have been
observed up to date number 16 species in 9 families.
Of those thoce vhich are reco7,nized as injurious are
migratory birds, birds of passage, and drift birds and are
numerous. In comparison to the damages wrought by these, the
damnes by local birds are slight.
I. LARIDAF,
The species which belong to this family are all migratory.
As injurious birds they chould be r;iven serious considera-
tion. They come in groups consisting of a large number of birds.
There are tl.ro species.
I) Sterna_alhifrons sinensis Gmelin (Japanese name - Koajisashi;
■
* YThe zoological names and the Japanese names follow Revised
Jaranese Wird Catalo-;ne (1932, TX) (Kaitei Cho-Rui Mokuroku)
tL1 J- cr, 01- -15tbolo;ir ,11 Soriet- ,
Caku-Kai). The popular names are those used in the Kawajiri
district.
77
• This species arrives annually at about the end of Apr., and
lays egp:s in the sands of the River 01. Towards the end of Aug.
f1ed1ings come flying accompanied by parent birds. They are
the birds of the summer which mirrate to southern areas during
the period from the end of Sept. to the be,-;inning of Oct. Two
to th' e' rr' laid at one timo. This js a common species
tb- 1- 1.f 0P Janrrn, southorn
•
rI n ri. • T1-.„ onie "F". rr> 1 s
tn tt n:(7 mP.57-,.? -7 to silo y son, lnkos mar7hos,
rivers, fish preserves and fish farms, and preys upon such
croat, :res fish, crw-taccans, fros, carps, 7oldfish,
'rv of eeln e -;-Pa.?os, or "r=7 1)ama ,rs are con.r.:idernhle.
It iF a great enemy of fjsh farms. The birds of this species
come incessantly from dawn to dusk. Matsui in his investigation
( 1 9:1 6) fo-nd 10 boadJ: of fish 5-6 cri in s!?_e P -0 Inaba in his
investigations (1936) found 12 heads of small carps 3 -4 cm in
si7e. Tt is f.ajrly voracious. The sound of guns doe() not
frighten it off; further, it becomes easily accustomed to white
strinL-,s or wire strun7, ncrocs the pond. The damages to fish farm-
ing is considerable. /181
2) Larus_canus ma2:2ryiddend2rff (Japanese name - Kamome; Popu-
lar name - Charo) •
Tho hirrls of this sY)ecies are of the winter variety and
"*"r* t1, o -oriod tn mUilo of Dec. to the
to on rare occasions did they.
invade the eel farming ponds in the'Ken:5ri district; how,vor
rr:- 1 1 0 7 tbr% b- al , ./illsrn.111.7- largc
r'nri'ic to the fishil fishinr; in th..7) e.w)n. it nr 1Yil-dS. A
• 78
was particularly bad. It appears as though groups of sea-gulls
invadel the eel farming ponds for foraging.
The present species is found in flocks in bays and har-
bours, at the mouths of rivers, and along the coastal areas and
is the mot common of the sea-7111ln in Japan. The breeding
to -0-! t,yn Siberia,
Kamchatka, and the northern part of Japan.
They cor from ear17 mor—inl to si2Iset 5n large nuribers;
however the damages are wrop,.; 1-t pecially in early morning and
near sunset. During the day they rest in ponds or at sea. At
nigLt tbey ely Lack to the sea. It appears as thoP7h they come
on the da- , r1len ln win-) is stron7: than en the d!2ys ‘•then it is
calmi Since the water depth becomes shàllow at the time of trans-
fer and the movement of eels is seen from the air, the ponds
are invado' hy large grours. Further immediately after the fry
are released, they tend to swim in the upper part of the pond.
Thus damages occur on these occasions. Again on rare occasions
when the uwater -transformation" phenomenon takes place and the
water becomes transparent, or when the water 4.:.mperature rises
suddenly and the eels begin to move, they come in large numbers.
On these occasions the damage to the fry occurs at the rate of
7-30 head for every bird. Thus the extent of the loss cannot
be regarded lightly. The eels lost consist mainly of fry less
than 25 cm in length. Healthy large specimens in many cases get
Slrk -3-1!"7 4.nb eels, or fry of cPrn,
7oldfish, rnd other fish are also canc.ht and eaten. It appear..11
as though the birds of the present species act as carriers of
such parasitic nematodes as Anguillicola globiceps Yamaguti
ç. I \
O
79
which are most commonly observed in the air-bladders of eels
or relico'Plin --- 1 21 1:!:-.o'e .. ".emncuti which are found in their
stomachs.
As the present species is relatively less frirhtened
by man, the damages are caused even in the presence of many rum
nearby.
TT. P:TC7F.TiTTTU
The following sin7ie scies bclon7s to the nr?.sent fami)v
of injuri.ono
3) .1i7r2_71oilremolr,.s (Tomminck) Papannoe name Oomizunaed- _ dor5 ,• Po1j ■ .7 1, 4 rn
Thn birds of the present species come with the sea-gulls.
Tn 71-imber wc11. r1 7-5n7 thr, wintc,r
season is very much fewer in comparison to those of the previous
snecies. qm-”s the dames are re',arded as sl17ht. Thoy are found
in Hokkaido, the mainland of Japan, coastal areas of Shikoku,
Islands of Izu, Korea, the Philipnines, Australia, and other
areas.
III. APDETDAE
Many of the species belonging to the present family are
found in flocko near the mr1u 4.h of the River Oi. There are sevPral
species. The injurious birds which invade the eel farming ponds
belon7 to the fol]owing two species.
-1,r„_ (Jarn" me - Aosa 4 ) _ _ Uvrtir-orax nveticor7.1 ,3 (Linrnello) (J;Imnr?oe _
8o
• Name - Koisa ,r,i)
They are observed throughout the year; however it seems
as though they come in greater numbers in the srring and fall.
Dirr o the day they are observed in flocks at the mouth of the River
01 their in7ade the nonals ori d-rinc, the ni. chi Tho obserira-
tH an hv 11 o author seen Ln indicte that they arpear oftener
before sunrise than at sunseb. The present species prey upon
fish hy wading into ponds or bv the edge of nonds. Thus damages
are wronTht constantly in shallow ponds or in the case where
the distance between the edge and the water surface is short.
Again care must be exercised when the water depth becomes shallow
at the time of transfer. The species are voracious. They prey
nPon oe's of rIn-er !-1-e (e -en,-oxmate]Y Jr) c'n in len ,,th), acinit
2' cs) caya, orflinn-d
IV. PODICIPIDAE
6) Podicers ruficollisjaronicus_Hartert (Japanese name - Kai-
Pon, qar name - Iccho'iugnri) /l82
They are observed throughout the year. The birds of the
nresent species inhabit lakes and marshes throng,hout Japan in
the area south of Hokkaido. They are small in number. But they
invade the fish farming ponds often. The instep spreads out
flatly to right and left. Althou?;h the webs are absent the
toes are ecuirped with flat membranes. The birds can skilfully
swim underwater and prey upon fish. Although the extent of the
damages is not known, the crops of Such species as carps, gold-
fish or eels nr-ear to bo harmed.
re-licena holbellii nni .,11-dt (Japanese rame -
81
Akaerikaitspburi)
This is the larest of ulçaitsuburin. Its appearance.is
less in comparison to the former. There was a specimen which
had Preyed upon 3 heads of fry of eels approximately 10 cm in
len-t , . Tt no-enr--1 that the birds of this species are fairJy
They anpear to be observed throughout the vear.
V. ALCITIITJU
) Al e o.fln n. -1-/c 1 1 i in -nosri niirt Paparr,,, o n.rno Kawn erri • . _ . •
_; ■
The birds of this species live near breeding ponds through-
out the yer. flPrin7 the da ,r. nerch on tree branches in
buhes or posts in water and prey upon fish which approaeh them.
They prefer relatively inconspicuous areas and are extremely
voracious. They are small in size. The damages are done to
the fry of such species of fish as carps and ushirasule-eels.
The npmber which invade fish farms is small.
VT. FALCMIne
9) Milvns mirmans lineatsu (Gray) (Japanese name - Tobi)
The birds of this species hover over breeding ponds and
forage. They gather in flocks in ponds with eels infected with
Lernea elegans (Leigh-Sharpe) and other diseases or in pond
vîhere the eels are in the state of "nose-liftn. Thus the pre-
sence of falcons is taken as an indicator of troubles so much
so that it is said "Where the falcons gather, there are weak
In fnct one observes a rdationship similr to the one
82
which exists between sardines and bonitos and Colymbus stellatus
and C. articus viridigularis. The damage is done even to the
health -y specimens at the time of the "pond-transfer" or at the
time when the fry are released. However generall y the damage
to the healthy specimens appears to be slight, The damage is
done rather to the diseased eels, Thus for ponds with diseases
the birds cari be a means of eliminating diseased eels . On the
other hand since the pathogens are propagated, indirect damages
may be significant. Thus it is a species which requires care so
far as the infectious pathogens are concerned.
VII ANAT1pA,17:,.
There are many species which belong to the present family.
All of them are observed during the winter. A greater proportion .
of the injurious birds which damages the crop consists of sea
ducks. The birds live mainly on fish. Because they come during
the night, there are some species whose names are unknown. The .
following three species have been collected.
10) Nirroca . marila_mariloides_Pigprs) (Japanese name - Suzukamo;
Popular name - Kamo)
11) Mergus merganser merganser Linnaeus (Japanese name - Kawa-
aisa; Popular name - Kama)
12) Mareca penelope Linnaeus (Japanese name - Hidorigamo; Popu-
lar name - Kamo)
The birds of these species appear at the end of Sept. How-
ever a greater proportion of the flock appears from the middle
of Oct. to the beginning of May. They visit the ponds early in
8 3
the morning or during the night. They seldom appear during the
day. They dive into the water and prey upon fish such as caps
and eels. They do not come to areas near houses or busy streets.
They invade fish farming ponds relatively less frequently and
the loss due to these species does not appear to be signifi-
canto
VIII.PITALACROCORACIDAF).
There are two species which belong to this family. Both
of these species are often observed near the sea or near the
mouth of the River Oi. /183
13) Phalacrocorax capillatuà (Temminck et Schliel) (Japanese
naine - Umiu; Popular name - U)
14) Phalacroc .ora nelaq:icus_pelarr,idus Pallas (japanese name -
Himeu; Popular name - U)
The birds of these species are rarely observed. They come
in very small numbers. Many of them appear during the night or
early in the mornlng. They skillfully dive into the water and
prey upon fish. They are extremely voracious.
IX. MOTACILIDAE
The following two species belong to this family. As the
birds of these species do not prey upon fish directly, it is
questionable if they should be classified as injurious birds.
However, they live on small creatures which are natural foods
for fish. Thus they are mentioned in the present paper as in-
jurions birds. It should be mentioned that in agriculture they
EL
are regarded as beneficial birds for stamping out noxious
insects.
15) MotacilliLalba_grandis . Sharpe (Japanese name - Seguroseki-
rei)
16) Motacilla einerea G.Gmelin) (Japanese name -
Kisekirei)
They live throwe,hout the year in fields near ponds and
prey upon small creature found in the ponds or at the bottom
of the ponds or upon insects flying around in the air. These
species come flying often to areas near ponds.
3. Protective measures
Many species of injurious birds which come flying to fish
farming ponds are voracious; Some species corne in flocks. Thus
it is difficult to mention records of experiments on losses due
to these species. However detailed observations may show the
losses to be considerable.
In eel farming ponds the feeding is discontinued during
the winter season and the eels go into hibernation. However
even during the winter season the npond-transfer" is carried
out and the fry are released. At the time of "water-transfor-
mationn or when the temperature rises the eels swim in the upper
layer, damages often result. Thus it is important to take
measures against injurious birds throughout the year. Many para-
sites have been discovered in the intestines of birds. Shaw
and two other investigators (1934) reported that there were
hardly any important species of game birds in the state of
Oregon which had not been infected with parasites, and that
these infected species acted as the carriers of these para- .
sites to fish thereby causing damages to the fish. Some species
of birds are vectors of such parasites as nematodes, trematodes e
or tapeworms found in fish. Approximately 50-90% of the air-
bladders of the cultured eels are infected with Anguillicpla
(.1obiee -r■p Yemarfuti However this does not seem to lead to
their deaths directly. An extremely large number of Heliconema
anguillae Yamagpti is occasionally observed in the intestines
of natural eels. On account of the infestation the growth in some
specimens become stunted. Their bodies turn yellowish brown
in colour and extremely thinner with only their heads remaining
large. It appears as though the parasites have been trans- - •
mitted by birds. There is a good possibility that infectious
pathogens, for instance, the parasite which causes the nred-fin'
disease in eels or microscopic and mucous parasites of the class
. Sporozoa which cause swelling in cultured "shirasun-eels, are
transmitted by birds.
There is much work to be done with respect to the protec-
tive measures taken against birds. SUMMER. (1935) carried out
experiments on the value of the protection provided to fish by
change of colour and established that the background of the
cama colour as that of the body of fish lessened the damages --
by diving and wading birds. MCATEE (1936) found that the in-
stallation of wires and screens was effective. The protective
measures can be roughly classified into 1) methods of conceal-
ing fish, 2) methods to intimidate or to provide intimidating
devices, 3) methods to capture birds, and 4) methods to prevent
approach. Since any suitable methods for a given species of
•
86
fish and a given species of injurious bird must take into
account the economics involved, it is difficult to discuss it
in general terms. Against the birds of such species as Sterna
albifrons sinensis Gmelin, Larus canus major Middendorf, or
migrans 112, eatus which prey upon fish by taking aim at
them from the air, the most suitable measues are parallel
strands of cotton string or wire stretched across the pond
along with 3) and 4). In tbis case it will be necessary to have
the snaces between the strands small so that the birds cannot
dive into the pond. On the other hand the spaces may be
widened to lessen the cost and white or red pieces of cloth
or bird feather may be attached to the Cotton string or wire to
heighten the intimidating effects. In the case of the Itwater-
transformation" in eel culture ponds method 1) is suitable, i. e.,
straw-mats or doors can be placed over the water of the pond.
The method is effective as the eels tend to gather under these
mats and doors.
The herons and ducks come during the night. Against these
species the methods to prevent their approaches can be adopted.
Bamboo hurdles may be erected along the edge of the pond; strands
of wire can be stretched across the pond; the muter depth can
be increased; or the height of the edge of the pond can be in-
creased. Alternately they can be caught by using traps, utiger-
trapsn; or they can be killed by poison. For birds such as
kingfishers and others, which live near the water and come at
ail times, a good method will be a use of bird-lime on the
stakes near the water on which they are likely to come to perch./185
87
Chapter X. On the Method_of_Examination_of . Technioues_in_Eel
Culture
1. Purpose of the study
The advance in the techniques of culture means an increase
in. the production of the creature under culture and an. improve-
ment in their quality. The author .interprets the term techni-
ques of culture to include not only techniques directly ,concerned
with fish culture such as the quantity released e selection, feed-
ing, handling of fish released s or the changing of water but
also other items such as equipment used to increase production,
feed, construction of ponds, and development and improvement
of varieties. The cost of production should properly be con-
sidered as an administrative item and is excluded from the tech-
nicues of culture. The present study excludes problems asso-
ciated with cases such as the rearing of ornamental fish in
which the object is not the increase in the overall produc-
tion but the production of individual specimens, problems
associated with extensive farming in lakes, rivers, or re-
servoirs, or problems associated with the increase in the
production of stocked fry. The present paper will deal with
the examination of techniques of eel culture in the context of
the management of extremely intensive fish culture.
In the past techniques of culture have been handled in
an abstract sense as of adjectival significance and there have
been no bases to be used as standards for examination.
88
2. Various factors controlling the techniques of eel culture
In order to establish standards for the examination
of the techniques in eel culture one must obtain all the elements
which are closely related to the increase in the production of
eels and the improvement in their quality, clarify the rela-
tions among them, and determine the coefficients.
A. Rate of weirrht gain
This is the element which determines the standard for
wl w0 the increase in production. The rate of weight gain W =
w0
is obtained on the basis of the quantity released (wo) and
yield (wi ) . It determines the overall increase in weight.
An examination of the effects and value of the rate of weight
gain on the techniques of eel culture is as follows.
Natural losses occur during the rearing on account of the
propriety of techniques or due to causes beyond control. Thus
there is a close relation between the rate of natural decrease
and the rate of weight gain.
If (WI) represents the rate of weight gain when the
wt t wl x n0' • hence WI can be n1
natural decrease is zero,
w" w0 obtained from the expression WI
WO
, where wII is
the total weight at the time of harvest with zero natural
decrease, no is the number of heads released, and ni is the
number of heads harvested.
89
The relationship between the rate of natural decrease
no - ni N = x 100, W, and W" is shown in a tabular form in
no
Table 166. /185
The relationshin between the percentap:e (P) of W with
respect to Wî and the rate of natural decrease is graphically
shown in Fig. 76. An examination of the figure shows that the
relation between P and N can be expressed by the expression of
the type P = a N . The calculation of constants a and b reduces
. the expression to P = 1.601 Ni016 . It appears that the ràte
of weight gain contains the factors which control the variations
in weight due to natural decrease. Thus it is not necessary
to define the rate of natural decrease as an element for the
examination of the techniques of eel culture. It can be rep-
resented by the rate of woight gain.
The population density influences the production of eels.
The relationship between the population density and the rate
of weight gain is shown in a tabular form in Table 167.
The relationship between these factors is shown graphi-
cally in Fig. 77. The relationship - between the rate of weight
gain, W e and the population density, X , can be represented
-0.147 by the following expression : W- = 1.150 X Thus one may
assume that the effects of the population density on production
can be represented by the rate of weight gain.
It is needless to mention that the size of the fry which
will become the breeding stock influences the rate of weight /186
gain. The effects are clearly observed if one examines the
90
relationship between the rate of weight gain and the number
of years of rearing or the number of feeding days. The relation-
ship between these factors is Shown.in a.tabularform in Table
168, and graphically in Fig. 78. /187
Thus the relationship between these factors are of the
form W axb and by calculating the constants a' and b one
-3.502 obtains the expressions W = 16.648 (
200
X _m)49 Wt = 15.741 ( Thus since the number of rearing
200 years or the number of feeding-days are related to the size of
the fish, the difference in the techniques of raising eels as
reflected in the size of the eels can be represented by the
rate of weight gain. Further the degrees of variations in the
size of the breeding stock affects the production. The rate
of weight gain of the group with a. larger degree of variations
is inferior'to that of the group with a lesser degree of vari-
ation (cf. Chapter V). The relationship between these factors
are shown in a tabular form in Table 169.
An examination of the table indicates that because the
rate of weight gain of the larger group is inferior to that
of the smaller group, the rate of weight gain of the larger
group, with a larger degree of variations on account of the
inclusion of smaller specimens, is initially better in compa-
rison to that of the group wdth a smaller degree of variations.
However in the former it decreases gradually due to the natural
decrease.Thus it is possible to represent thé effects of the
degrees of variations in the size of the breeding stock on
and
91
rp
the production by the rate of weight gain.
The relation between the amount of feed administered and
the production is shown in Table 170.
The amount of feed administered and the rate of weight
7ain are closely related and it is possible to represent the
effects of the amount of feed on the production by the rate
of weight gain (cf. Chapter VI).
In short the rate of weight gain represents all the ele-
ments of various kinds which control the growth of the body of
fish. Consequently it can be mentioned as the most important
factor among the factors for the examination of the techniques
of eel culture.
food_quotient
This is a coefficient which represents the amount of
food intake (or the amount of feed administered) required for
an increase of an unit measure of weight during the period of
rearing. The food quotient, F, can be calculated by the following
expression: F - wl wo
„ where f represents the total
food intake or the total feed administered during the period
of rearing. Thus the food quotient explains the relationship
between the amount of feed administered and the increase in
flesh; a small value of F means a small amount of feed required
for an increase in flesh of an unit measure and better ef-
ficiency in the use of feed. The cost of feed, whlch occupies
11› 39.5% of the cost of production of adult eelS, depends on the
efficiency of the feed, and constitutes an important element
92
not only with respect to the production but also with resPect to
the management of eel culture. It is a factor in the examina-
tion of the techniques of eel culture.
The relationship betwee n the food quotient and the number
of years of rearing and the number of feeding days is shown in
a tabular form in Table 171.
The relationship between the two factors is shown gra-
phically in Fig. 79.
If F represents the food quotient and D represents the
number of feeding days, the following relationship exists bet-
ween the two: F = an. The calculation of a results in the /188
following expression: F = 3.210 + 0.126D. It will be observed •
that the food quotient is proportional to the increase in the
number of feed days. In other words the food quotient increases
with the growth in the body of fish; consequently the efficiency
in feed decreases.
The relationship between the population density and the
food quotient is shown in a tabular form in Table 172.
The relationshdp between the two is linear. If X repre-
sents the population density, the relationship between the
two can be represented by the expression F = 1.042X ± 2.50 . It
will be observed that the food quotient increases with the in-
creaseAm the population density. /189
_ The relationship between the degree of variations in the
size of the fry and the food quotient is shown in a tabular
form in Table 173.
The food quotient of the group with a smaller degree of
variation in the size of the fry is smaller in comparison to
93
- that of the group with a larger degree of variation in size and as
in the case of the rate of'weight gain, the degree of variations
in the size of the fry is closely related to the food quotient.
As was explained in the previous sections it is clear that
the rate of weight gain and the food quotient are closely related
to the various elements which control the production of eels.
The relationship between the two is shown in Table 174 (cf. Fig.
81).
The relationship between the rate of weight gain (W) and
the food quotient (F) can be expressed by the expression
W = aFb Thus by calculating the constants, a and b $ one
obtains the following expression : W = . 10.469F-0.181
. The
larger the rate of weight gain and the smaller the food quotient,
the better the techniques of eel culture. These are important
functions in judging the quality of techniques of eel culture.
C. Variations in quantity of g,rowth
The quantity of growth is expressed in terms of length,
weight, or by using both, or in terms of the "Quantity of body"
(length of body1 3 ,or the degree of obesity ' weight However as it is
complex to calculate the latter, its adoption is difficult. /190
The degree of variation measures the quantity of growth of
the total stock harvested and is shown by the coefficient of
variation obtained by multiplying by 100 the quotient result-
ing from the division of the standard deviation (3 ) by the
mean (M) of the quantity of growth, i. e., it can be expressed
94
by the equation C - x 100. When eels are raised in-
tensively under identical conditions and by the same method,
the characteristic feature is a prominent variability. The
variability bec ornes more prominet as the method becomes more
intensified. However in intensive commercial farming the
commercial value of the product has a fixed limit with respect
to its size. Thus it becomes necessary to produce specimens
which exceed the minimum size within a fixed limit. It is
inappropriate to pay attention only to the increase in the
overall production and neglect to pay attention to the variations
in the quantity of growth. The quantity of growth in terms
of the length of the body (cm) of the specimens reared for 138
days under the same conditions and by the same method is shown
in Table T75.
It will be observed that the variations in growth as the
result of rearing under the same conditions and by the same
method are 15.93% in A, 15.04% in Bp and 9.81% in C. Thus
marked variations in growth are observed. Further the factors
which are mainly responsible for the variations in growth are :
a. Population density, b. amount of food intake, c. various
factors specific to the creature, d. diseases which cuase
damage to development, e. size of the fish released, or f.
the period of rearing.
The population density is an important item in the tech-
niques of fish culture. The propriety of the population has a
great deal of effect on the yields. The relationship between
the population density and the variation in the quantity of
growth is shown in Table 176..
The relation between the degree of variations and the
population density is graphically shown in Fig. .82.
An examination of the relationship indicates that the
variation is proportional to the population density; one
observes the tendency wherein the variation becomes smaller
with the increase in the population density.
A large density is a . characteristics of intensive
eel culture. However a large population density results in
the phenomenon in which the strong preys upon the weak and in
the lack of balance in the distribution of feed.' Further it
appears as though it causes variations in growth because 'the .
influence of individual variations manifests itself'strongly. /191
In order to prevent these phenomena one can take such measures
as to minimize the population density within thé" manageable
range, to increase the frequency of selection, or to increase
the frequency of feeding. Economic considerations must'be
made in the implementation of these measures. It will be one of
the factors, which indicates the advanced stage of techniques
of eel culture, tb produce eels with the least degree of vari-
ations in the quantity of growth.
The relationship between the amount of food intake and
the variation in growth is shown in Table 177. Experiments
were carried out by using "shirasun-eels. They were used at
the rate of 375.9 g for every "t -subon (= 3.305 m2 ). They were
divided into 3 groups by the amount of feed adminstered. One
group was fed with feed equivalent in quantity to 5% of the
heads of eels released; the other two groups were fed at the
rates of 10 and 15%. The table shows the results of rearing
them for 59 days e feeding them with ground sardines twice a day.
96
An examination of the results shows that the coefficient of
variability of growth varies directly with the amount of feed
administered; the coefficient increases with the increase in
the amount of feed administered.
As factors, which are'specific to a creature, and which
control the degree of variations in growth, one can mention
such items as the difference in. sex, hereditary factors, in-
dividual variations, or time of oviposition. These factors
are very closely related to growth. With reference to the dif-
ference in growth due to the difference in sex WALTER (1934)
experimentally establishedusing carps ) that the growth was much
better in the female than in the male, and BELLEN1 (1910)
observed that the females of European eels attained much larger
sizes than the males. Marukawa (1916 a) and the author obtained
similar results with Japanese eels in Part Two.
With respect to the relationship between hereditary factors
and growth., WILLER, QUEDNAU, and KELLER (1930) using trout and
the author using carps experimentally obtained superior varie-
ties. Similar results were observed with eels (cf. Part Two,
Chapter IV). /192
The length and weight of the body and the fatness of
"shirasun-eels which were collected at the same area in the River
Oi and at the same time are shown in Table 173.
An examination of the table shows that there are also
variations in the size of the fry. Similar results are obtained
by the season, area, and year. BELLENI (1910) observed a great
deal of variations among the fry of European eels at the time
of the upstream migration; i. e., 51-61 min group constituted
13,3%, 65-73 mm group constituted 82.1%, and 78-81 mm
group constituted 4.6%.
SCHMIDT (1925) in. European eels and jESPERSEN (1943) in
eels from the Indian Ocean and the Pacific Ocean observed
that there were considerable differences in the length of the
body of their leptocephali even among the specimens which had
been collected at the same time and in the saine area, concluded
that the variations were due to the differences in the time
of oviposition, and assumed that the variations would persist
over a long period of time. The author also drew a similar
inference from the fact that the groups of Japanese eels with
the same body length are observed over an extended period of
upstream migration (Part Two, Chapter IX). They differ with
respect to the period of migration and to the quantity of growth
(Table 179). Thus the fry themselves are equipped with the
factors responsible for the variations in the quantity of natural
growth.
Contraction of diseases, which hinder growth, leads not
only to death but also a marked delay in the growth of the
diseased fish. Thus, since the growth in the healthy group
is not intnrfered, there results an increase in the variations
in growth of the two groups. The eels which had been reared
for a year were divided into 3 groups on the basis of growth.
A group consisted of specimens with r,. extremely good growth,
B group consisted of specimens with fair growth, and C group
was made of specimens with inferior growth,whose growth had
had been impaired by parasites on the whole of their - _ bodies,
to obtain information on the degree of variations
e 98
in growth (Table 180).
In. the above table if one assumes that the growth in A
and B grouns was normal, the width of the variations will be
13.70-21.96 cm in length and 3.00-15.78 gr in weight. However
because of the presence of an abnormal group with diseased
fish the widths of the variations, 9.58-21.96 cm in length
and 0.95-15.78 gr in weight, became extremely wide.
Thus the variations in growth are extremely closely
related to techniques in fish farming. /193
3. The method of examination of techniques of eel culture
In previous sections examinations were made of various
functions which were closely related to techniques of eel
culture. Of these as functions, whose smaller values indicated
the superior techniques of fish farming, there were the food
quotient (F) and the degree of variations in growth (V), and
as a function, whose larger values indicated the superior tech-
nique, there was the rate of weight gain (W). Thus if one lets
C represent the technique of eel culture, it can be obtained
from the following expression.
F x V That is to say, from the equation C Thus
the magnitude of the numerical values of C can be a coefficient
which shows the standard of the quality of technique of eel
culture; the smaller the value of C, the superior the technique.
However these functions vary considerably depending on the
size of the fish and their age. Thus the comparison by means
of the values of C must be carried out with fish of the same age.
I
.. t . Ag
..." 99
An attempt at a comparison of C with nshirasulf-eels which
were reared for a year is as follows As an example of superior
results in eel culture one can cite a- value of C of 3.9 with
the rate of weight gain of 46.43, food quotient of 5.42
and variation in growth of 33,9; on the other hand In an -Inferior
example the value of C was 32.7 with the rate of weight gain
of 13.1, food quotient of 7.51 and degree of variation in Frowth
of 57.0. It appears as though it is appropriate to judge
the standard of technique of eel culture by classifying them
into the following categories on the basis of the valuesof
C (Table 181). /194
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