FISHERIES RESEARCH BOARD OF CANADA

Translation Series No. 2356

Causes of the mass mortality of the oyster (Ostrea edulis L.) in limski Canal, Istra, 1960

by Miroslav Nikolic

Original title: Uzroci Masovnog Ugibanja Kamenica

From: Acta Adriatica (Adriatic Journal), 11(31) : 227-238, 1964

Translated by the Translation Bureau(VR) Foreign Languages Division

• epartment of the Secretary of State of Canada

Department of the Environment Fisheries Research Board of Canada

Marine Ecology Laboratory Dartmouth, N. S.

1973

19 pages typescript

*DEPARTMENT OF THE SECRETARY OF STATE SECRÉTARIAT D'ÉTAT

BUREAU DES TRADUCTIONS TRANSLATION BUREAU

MULTILINGUAL SERVICES

DIVISION

DIVISION DES SERVICES

MULTILING.UES

CANADA

TRAN5LATED FROM — TRADUCTION DE

Croa -,:ian INTO — EN

English

TITLE IN ENGLIsH — TITRE ANGLAIS

•--' TT 2- :-11 -1 - ) -3 7;R C;trel-p. Lu , . •

NUMBER OF TYPED PAGES NOMBRE DE PAGES

DACTYLOGRAPHI ÉES

19

PLACE OF PUBLICATION LIEU DE PUBLICATION

Split (Yugoslavia)

TRANSLATION BUREAU NO. hiSTCRE-CLIENT NOTRE DOSSIER NO 143 •"! 11.-• REQUESTING DEPARTMENT Envi ronment

Fisheries Research 1;oard of Canada, BRANCH OR DIVISION Marine Ecology Laboratory, TRANSLATOR (INITIALS) DIRECTION OU DIVISION

PERSON REQUESTING Mr. R. E. Drinnan DEMANDé PAR

med-fo-rd Oceanography, Dartmouth, Nova Scotia

V.H. TRADUCTEUR (INITIALES)

pips a-7) Lp

AuTHOR — AUTEUR

M. - 12 ,:o11c

TI[LE IN FOREIGN LANGuAGE (TRANSLITERATE FOREIGN CHARAcTERS) TITRE EN LANGUE ÉTRANGÈRE (TRANSCRIRE EN CARACTÈRES ROMAINS)

UZROCI UGLDANJA LNICA

REFERENCE IN FOREIGN LANGUAGE (NAME OF BOOK OR PUBLICATION) IN FULL. TRANSLITERATE FOREIGN CHARACTERS. R.F..FRENcE EN LANGUE E'TRANGiRE (NOM OU LIVRE OU PUBLICATION), Au COMPLET S TRANSCRIRE Eel CARACTÈRES ROMAINS.

Acta Adriatica

REFERENCE IN ENGLISH — RÉFÉRENCE EN ANGLAIS

Adrietic Journal

PUBLISHER — ÉDITEUR DATE OF PUBLICATION DATE DE PUBLICATION

PAGE NUMBERS IN ORIGINAL NUMÉROS DES PAGES DANS •

LORI GINA L

227-238 YEAR

ANNÉE

ISSUE NO. NUMÉRO

VOLUME

1964 31

YOUR NUMBER VOTHE DOSSIER NO

DATE OF REQUEST SePteMber 22, 1972. DATE DE LA DEMANDE

For in ao-n TRADUCTION NON REVISE':

Information seulement

505.2 CC-1 (1-s In cv. 2./esi

SECRÉTARIAT D'ÉTAT

BUREAU DES TRADUCTIONS'

DIVISION DES SERVICES

MULTILINGUES

- OEPARTMENT OF THE SECRETARY OF STATE

TRANSLATION BUREAU

MULTILINGUAL SERVICES

DIVISION •

_ CLIENTS NO. DEPARTMENT. DIVISION/BRANCH CITY

N° DU CLIENT MINISli. RE DIVISION/DIRECTION VILLE

-Environment FRE . Dartmouth, N. BUREAU NO. LANGUAGE TRANSLATOR (INITIALS)

N° DU BUREAU LANGUE TRADUCTEUR (INITIALES)

i j 143514 Croatian V.H. D et.7A. ", 4 f

1Q72

A.driatica (Adriatic Journa1) Vol.XI, Mo. 31, 1964, op 227-233 (Lusoslavia)

• CAUSES OF 7a7L i'iASS MORTALITY OF THE OYSTER (Ostrea edulis L.) ILÏ Ln.SKI CANAL, ISTRA, 1960.

Ey Miroslav NikoliC, Institute of • Marine Biology in Rovinj, Yugoslavia.

Introduction 227*

-Durin?.7: the period molluscs have been reared in Limski

Canal . (Lim Inlet), each year the average mortality rate in

oysters has fluctuated between 10 and 20% (A 1 1 6 d i 1905).

The causes for such mortality, which is considered to De

within a normal range, are the following:

a) careless handlinP; durin the required operations

In oyster production;.

• ID) buffeting of waves which causes damage to individual

sections of the equipment in a rearing station and

consequently, to the molluscs also;

Numbers in the right-hand margin indicate the corresponding pages in the original.

7530-21-02g-533Z

2

c) Continuous attacks of varying intensity by fish

Particularly by the gilt-head bream (Sparus auratus L.):

d)- occaSional attaCks. by the -Hspoiler" - the turbellarian

Stylochus pilidium Lang. A more widespread attack

by this organism was recorded in the literature in

19314 (j.ytinski-Sal z, 1935). The mortality

rate of 87, which was registered at that time applied •

only to the oysters 8 to 35 mm in length, while

" - other - sizes- did not smffsr.

Near the end of 1960, the sudden death of oYsters of

all sizes occurred in the Limski Canal. Because of controversial

opinions about the causes of that mass mortality research was

initiated. Scientistst investigations in the field, obemical

analyses by the InStitute of Chemical and Technological Research

in Ljubljana (Yuc7oslavia) and our own analyses provided the

basis for conclusions outlined at the end of this study.

Mortality rate fluctuation

In ord ,, r to r.hef%ir the mo ,-tality rate, ten sites were

chosen in the valleys where the rearing ponds are situated.

The number of animals involved in this investigation 228

totalled 16,452, the research extending over a period of

seven months.

The highest mortality was recorded in 1960, in the

months of October and Novemper while in December it went down

considerably. At the beginning of 1961 it continued to drop.

PEP.I.17011.0

NAVI

K- Art i.GADEP.A I

3

Table 1 - Mortality rate (percentage) shown for eaCh site, in each month and for different age grouc.

Site (Station) (Age 1960 1961 r,-rouc) Nov. Dec. Jan. Feb. Mar. Apr. Total

Dno Lima

1( 'I. --- r.nj eria i _ • _

1031. °it) t.-)Ë morlanly XII 1 i.t 111 IV I..14:upno

---

V ES KO•12•0*

•P

33,7 3,3 (33,4 C-.1059 30,2 1,0 1,0 2,4 1,1 2,2 33,3

•

B6,5 61 - -4,9 • 1,5 3,1- 2,0 05,5 C-19:39 16,1 11,9 ' 7,3 0,6 30,5

K-1 iM9 47,5 4,7 5,4 • 1,3 2,8 2,9 75,1

0-1053 02,4 14,7 7,8 5,6 13,8 4,3 77,0

ADEUA II 1.:-.1.550 1),8 3,7 3,5 -1,9 10,3 C-1939 46,7 8,1 5,3 4.8 3,0 . 3,4 67,3

• - C-1950 68,4 • 6,9 70,0

.L-1LISLA',•_4.1ZO3'A •

C-1950 (33,1 611 1

, . C ER1ZER A I 0-1959 6_1,4 12,11 7,0 8,0 3,4 1,7 74,9

60,0 13,a 1,3 1,3 0 . 1,3 02,5 ..

C-1833 07,7 . 0,5 • 4.5 . 1,3 73,0 CERIZERA. II 0-1953 63;5 0,3 9,1 4,3 4,3 7,2 77,3

1:.-1.3.30 ,. , 5,1 1,1. 1.3 17.7

C-. um 03,3 13.3 11,5 4.4 72,9 7;11‘.41J A • C-1950 ' 03,0 12,2 10,4 3,9 3,1 4,7 - 74,1

- K-1060 1,1 .,,- 3 1,7 5,9 .

C-19:1 0 00,,Ï3 '1,9 7:9 1 , 3 .t 3,8 3,3 70,7 . 3 5.9 0-1930 54,3 3» 8 , 0,3 i .8 61,8

- s ST .1; G A . K-1039 42,0 5 5 5,7 q9 -,-, 2,9 3,3 53,5 . • C-1050 •

K-1000 00,0 •13,0 5,6 3_2 1,1 3,0 p0,1

Note: C = cemented in fours; K = attached to sticks.

until April', when a normal mortalitY rate was reached again.

It can be seen from these data that the oldest'animals

suffered most (two and a half years old - 76.66%), one year

old somewhat less (5346 - 61.81%) and the six months old

the least (36.66). The variation in mortality rate for

' .rom lifferent control sitee (oyster beds) was minimal.

In order to find the cause of this high mortality, 229

the oysters themselves had to be examined first of all.

Their shells were not damaged and had no unusual deformities.

A thin layer of brown silt was easily visible on them. On the

external surfaces of the shells no living organisms were

observed. The "spoilers" - turbellarians of the species

.Stylochus °indium Lang - were not found. Likewise, eggs of

this orP;anism were not found, although it leaves them usually .

on the inside surface - of the shell. The animals were well

nourished it is therefore assumed that death occurred.

quite rapidly. The .edible portion usually decayed very-soon

(within a day or two), so that in most cases only the adductor,

muscles and the shell were found. Gills and other -parts of

the body were covereCL wibh a layer of silt and .mucus. No other

changes were noticed.

For chemical analyses, eleven samples were collected

representing both live oysters and dead ones with dried,edible

portions from the Limski Canal, and live specimens from the

natural oyster beds elsewhere. These samples were sent to the

Institute of Chemical and Technological Research in Ljubljana

for chemical analyses.

Table 2. r_Qhemical analysis of the edible portion of oysters • (Ostrea edulis L.)

•Sample No. Inorganic ash SiO

2 e •

Tabc7. ,i 2. - KcJi:s1.•.; fflzulizi.z. ,jc,‘th - ih tlij:?lovo. k' ;:;c (asu ed.7.ais L.)

. .

1r..t•1.z.,1( Peil,2.n S102 l'e203 C.....0 11.•pf) Na Cu. C1 P205 Sa -

1 )3,40 0,04 0,15 3,07 0,111 3,34 0,0027 5,40 1,53 1,04

2 • 1 2 ,75 (J,20 0,1 13 1. 2 0 Mg 3,16 el 1,01 1 , 2: 0 3 10,60 0,18 0,17 1,69 0,97 4,04 0,0141 (3,53 1,81 1,10

4 12,82 0,11 0,11 0,87 0,74 3,07 0,0138 5,06 1,66 0,95

12,17 0,10 0,12 1,77 0,65 2,70 0,0197 4,16 1,53 0,08

• • C 14 • 8.3 0,133 0,14 2,54 0,7'i 3,23 0,0146 5,28 1,53 0,91

7 16,40 0,32 0,25 0,83 0,85 4,78 0,01 88 0,09 1,70 0,08

8 17,7,3 0,73 0,25 1,14 1,00 • 4,21 0.0171 7,00 - 1,25 1,13

9 14,70 0,16 0,13 0 ,72 1,01 3,30 0,0037 6,12 1,79 0,96

10 10,88 0,41 0,89 • 0,46 0,90 5,50 0,0027 6,25 2,14 0,99

11 16,70 0,52 0,33 1,50 0,02 3, 89 0.0090 6,77 1,51 1,14

Svi rezultati su u izraèunati na sulm tvar Uzorci 1, 2, 3 i 4su 21e sheU;;.e iz nanaln. tizorel 5 1 6 su 'iive Skoljke a prirodnill stbsniSta lizorei 7, 8, 9, 10 ill su mrtve Prûljke iz

. . _

All the results are shown in % of dry matter. Samples 1, 2, 3 and. L. are live oysters from Limski Canal. samples 5 and 6 are live oysters from the natural beds. Samples 7, 8, 9, 10 and 11 are dead oysters from Limski Canal.

From the results obtained; it can be concluded that

there is no great variation in the chemical composition of the

edible portion.. Oysters from the natural beds have a slirehtly

higher content of CaO - 1.77 to 2.54%, as compared to 0.46 -

1.69 in the oysters from the artificial breeding grounds.

That.increase for oysters from natural beds may be attributed .

to a greater age and to life in the sea where there is a higher

content of calcium. The increased content of asli in the dead

oysters from Limski Canal - 14.70 to 17.23%, as against 12.17

eto_16..60A_in _live:oysters from both-natural-grounds-and Limski -

Canal - could be attributed indeed to the increased content • -

of silicon, iron and aluminum which were also found in the

'sediment layer.

Oceene-re -ehio -eeserch 230

he follewing measurements' were taken in thè Limski

Canal: temperature, degree of oxygen content of sea water,

salinity and turbidity (transparency).

The sea temperature and oxygen content fluctuated

within a range correspondim3 to the normal conditions prevailing

in Limski Canal. The recorded temperature in 1960 ranged from.

9.6°C (winter) to 25.3 00 (summer), while the oxygen content

was somewhat higher than it had been recorded in other years -

84.2 to 106.8 (1960). As will be seen in further presentations,

salinity is a very important factor in the - life of oysters.

In the outer hall of the inlet (from the mouth towards the

interior) the sea water reached a maximum salinity of 37.50 °/00

and 38.e °Apo(D ,, c.-Feb.), /ihile the minimum was rather

irregular - 35.61700 to 36.17 Q/oo (pril) and somewhat more

stable in August (34.16 °/oo - 34.18 °/oo). Since the breeding

area is located in the second (or inner) hall of the inlet, -

this latter section is of greater interest to us. The salinity

of sea water in that section varies; it is subject to -

substantial and sudden changes, particularly at the surface,

and deuends on the amount of precipitation and on the under-

water sources of fresh water. It happens sometimes that the

231

:0,1> ,

",IRSAR

,

5 4 3 -

ROVIN)

i>)

water On the surface is nearly fresh (Vatova1950,

Mar1nkov1c1959).

The Limski Canal has a great number of underwater

sources of fresh water."Vatova-(1950) registered 15'of

them. The more important ones are at Punta Simija and Pija

Fera. ;.ccor.ir to V a t o v a'(1950), At Punta Simija the

fresh water .es?ows in f rom an underwater cavern. In the

immediate vicinity of that source the salinity was 5.260/00

(Nov., Dec.) .and 4.24°/oo . (February)_, whilel.n March the water

was completely fresh. The salinity of the sea water at Pija

Fera ranged from 0 to

(July to'September) and 29..18°Jc° (November). The zreater the

amount of precipitation the greater the flow of the fresh

water sorings and the more the salinity is reduced in the

vicinity of these sources. - !

5.4e/oo (Feb. to April), 3.17 to 6.49 °/oo

hûrtiroldim iy. /. 1. ,)! Ut • . . _ . .

Pfmtaja br. 1 — I>no /Arno, Stailuel „ 2 -- Veq:r.ovo

„ „ 3 —

„ — r .

•.„ Year4p.icicre, „ 7 — •13ijcla „ — CL.ra 1. ,

8 „ D • - (....'erlzern u. „ „ 10 --

Sodium (Nem) - • •

.

" re•

grams • . 10:520 • . . 0.3915 • . . 0.9642

milliwavesl 457. 0 19..6 79.3

555.9

501.0 -- - 51,7--

•0.01 3.21

The Institute of Chemical_and Technological :Research_

performed the analysis of sea water. The sea water used for

analysis was clear and colourless. One litre of that water

contained the followin7:

Cations

motal

Chloride ( 0 1 - ) • • • "7'111 e ( SO/ = ) ilydrophosphue (rieG4 ) . Hydrocarbonate (HCO 3- ) .

Anions • • • •

11.8757

• 17.780

• 0.0o14 • 0.196

Tota l

• • • • • 0

• • Aluminum oxide CA2 ,70 1 ) • • Sum of all solid mâtler . . Density at 19°C • 0

PH . • • • • residue after evaporation •

. 20.4574

• 0.0084 • 0.0023 • 0.007 • 32.345 • 1.023 • 7.7 0 • 33.357

Ferric oxide (Fe9 0.1 )

1This may mean either millivolts or °/oo, or perhalbs mhos. The term has been translated exactly as it stands, since the method of determination is not given. Revisor.

Precipitation

In addition to temperature, oxygen content, salinity,

turbidity and Chemical analysis of the sea water, the total .

amount of precipitation in this region has also been measured..

By studying the monthly precipitation values for the

year 1960 it became evident that the greatest amount of

precipitation oCcurred in the months of March, July, September,

October, November and December, and in April 1961. For

comparison, precipitation in other years is also given•

9

(1927-1928 and 1953 to 1961). It is evident from these data

that the total annual precipitation was within the range of

622.2 to 907.0 mm. 1n1960, however, it increased to 1207.4 mm,

that is, it was approximately one third more than in the

rainiest year previously recorded. Naturally, because of the

increased amount of precipitation in 1060, te underwater

springs of fresh water described earlier, had increased also

and, in addition, much of fresh water had poured down the steep

banks as run -off during the rainy days. As a consequence,

there was a significant freshening of the sea' water at different

levels below the surface and, particularly, at the surface.

Accordinn: - to V

on the decrease

for a period of

occurred.

a t o'v a (1950), the,rain had a great effect

of salinity of sea water in the Limski Canal

more than a month after the precipitation

Table 3. - Monthly fluctuation in salinity of the sea water in years 1927-1928 and 1957-1961.

;7 1.a .r./.1rt .i rr) gcc7i. -n.c.':1:.u. 1927-1928. :1951- 1 961.

232

3. —

19131. 1,110 inc,

)

- . .

JAN 1:1UAR

A1:11111, 3.42-36,8k

JU • 36.5.;5-37.1.5

AJrI UST 3-1,“3—:!6,39 SEP . FEMBAR 34,25-36 0::TO.F; 33.06-3 NOV:;',M1,3A .R. 29,34 --nr.c o

\23.

M,12-31,89 25,97-37,P0 3 0,32-3(,:.;5

32,21-37,43

33.33-37,01 32.25—:37.34 2:S.44-37,57 37,05

2

34.43-37,63 36.'3P-37,975 36,92-57.90 32,07— • ,56

,64—?7,0(5

21,6-37.55

I

20,E6-34.93

:r • 35-35.17

35,70-36,0a

31,17-35,05

31,20-372,23 i3-37‘,P1 35.58‘-35 1;7

33.7,5-37,21 3.C,29-36..58

35,99-37,r*

2.7

ej

,1A

2/1.5

92,0 043,5 1

11

1

U ;WA GODIN.,1 1H

0 8,■ Cfel 1.1.1 fi 0 Vil,

Table 4. - Precipitation in the region of Rovinj in the Period 1927-1928 and 1953 to 1961. -(for each month and yearly)

1 0

tube/a 4. — Prcr,-17.d obrp-ina. n vocirus.ju za 1 927—.1.92;;1'. 1953-1961. g.

Preep.il..icm.: 1927. 111 25 . 1253. 1053. 1055. 1953. 1957.

JA.1:,"AR

1953. 1 950; 1095 . 1931.

23„9 25.3 , 59,n 23.0 39,0 40.9 30.0 52,3 65,3 60.5

...- 20.3 57 .8 1:.., - - - 2(ki - - - 9.54 • 45,3 - - 10,0_ PT 1 ' 9 3 110.3 3•A2 64..3 85.9 17.3 55.7 5 -,1 11`.'.7

API'..1. 7_, ::. 3.1 17.0 ....,.• .,.„,, 7:),P r 1.9 T i;1.; () 5.) 114,0 7 0 3:-)„-3 1)..5 5.1.0 1:3,1

2 7,4 5".7 15,9 42.7 1 25,:1 . 22,0 67 ..3 62.1

JUI,I 1,7 14.0 92,0 68.5 14'.«:.7 31.3 4 7. ; 57,8 25. 3 122 . 4 A f_1(.7,UST 15.8 -1!4.:1 4:1.9 /1,3 1.32.0 23.2 :,',5,1 1 -.,.1 1,3'.,3 /:.."1:,PVT.- ',1. ".P.R., 74,7 1 65.7 50.0 31,1 67,7 19,1 34,4 32,7 9.1 , 129.4

oKToBAr. 6J

33,5 t; 112,1 132.1 .:i 1 .1 60, ; ,i _

80,8 111,1 54,2 20:),2

N 0 V E 'T.1-; A R 51 2 105.5

, P2.‘; 55.5 - 114.1 — 110.3- - 62.3 - - 126.2 —98,1- - 154.2 DEC1 7:7■113 AR :39,2 42,9 ■■ 3,13 9,5,1s 22.9 i 6,6 36.3 113;2 2.02.9 - 1'27,0

U I:: 15 2 N 0 5/1,1: 0 91.1.- 6 2 1,6 6.,'.2.1 920 2 510,2 622,2 793,0 957.0 1 207,4 T-otal

500 rrm, - 1000

1227

,COrnin - 70 100

1953 -; c;11

(.0

19•55 ...• 6:90

1956 .2! r)

• 1.957 • 72 ?

1 950 7.9.9

1.061 ..

1

' 1 17.4

X i ••

. 13 91

X1 1 _

-SR E Diy ..1/1.1 ( MJES EN L--:. M [

CSOHINA 1-: 5h )2 3.) 60/..).,.V.1;

1:11 1. Ulzupga 1;s ■ IfOina go ii!:11i , l ■ c,rnin n flosiniu uw. 1. 'h (lrP.):" 1 11 jo«t•cq pu c: t ions IU F:ocinj (rdnu total)

M 2. SrecInjnlz. ni.ic.1::C.21:t 1 - Fo. 2 . Thc ,n,:ounr Of pri . eipilaf/o; s at govinj (a niqc - poqr averag (')

Fig.l - The total annual precipitation in Rovinj. Fig.2 - The average monthly precipitation in Rovinj

(for a 9-year period).

-èteml

11 ■.■

Turbidity. of the sea.

The Limski Canal is a sea inlet approximately 11 kilo-

metres long, penetrating the land in a west-east direction.

With the influx of fresh water (surface run-off and underwater

springs), large amounts of red soil and clay enter this inlet.

mbe ..eicler.2 of soiD in the fres'nened sea ater create

of "emulsion" and settle to the sea bottom only when the

salinity of the water is restored. For that reason we find

a layer of grayish mud at the bottom of the first third of the

inlet, (i.e. the innermost section), and grayish mud with a

surface layer of brown-coloured mud in the second third of . the -

inlet. The bottom of the last third e the inlet is'grayish

mud with a reddish surface layer. Very soon after.e•tering

into the sea water,_ the suspended particles of red soil and

clay.also settle on the shells of various organisms and

Penetrate into them, particularly when the . animal is filtering

the -surrounding water in its feeding and breathinçe operations.

A large amount of sediment was found on the shell surfaces

facing the surface of the sea above. The wet sediment had a

dark brown and occasionally, a grayish-red colour. According

to the analysis by the Institute of Chemical and Technological

Research, this sediment, when dried at 105 °C, had the following

composition:

Silicon dioxide (Si02 ) • . 29.4% Ferric oxide (Fe203) . . . 9.0 Aluminum oxide (Al2 0 3 ) . . 7.8 Lime (CaO) 16 L.

Iviagnesium (Mg0) . . . 9.8% Phosphoric pentoxide

(P205). . 0.2 Lost dVirlg drying

(112 0- 4- CO, 4- .organic mÉitter). • .27.4

12

234 . A Sample of turbid sea water taken October_18th,_1960, -

--at the innermost end of the Limski - Canal -(the -Inn° Lima'

station) showed that, on that day, there was about 0.68 grams

of Silt in one litre of sea water.

of the s ,,e in the Lirrslei Canal

-.:.ecause an artificial enrichment of the sea with

nutrient salts was performed in the Limski Canal in the period

1959-1960i -it was- necebbary - tO study'some details of that -

operation.

The artificial fertilization was carried out by a

previously proven method (B u 1 j a n 19.57), in several

applications in approximately 2,600,000 cubic metres of water.

According to our calculations, the quantity of Phosphate in

each lot thrown into the water in 1959 amounted to 28 -101 mg

per cubic metre, and in 1960 - from 14 to 33 mg per cubic metre

of sea water. The fertilizers that were used, mainly . super-

phosphates, were diluted in the sea water, according to the

scientist who carried out this fertilization program, in the

ratio of 1:1,000,000. From the results of chemical analysis

by the Institute of Chemical and Technological Research we can

see that the superphosphate which was used can be dissolved

at the rate of 2.56 grams of P20 5 per litre of distilled water

(pH = 4.0), and at the rate of only 1.74 grams of P20 5 2- 5 per

litre of the sea water (pH = 5.0). The density of sulphuric

acid used was 1.835, which corresponds to a 94% sulphuric

acid solution. The_mixture of 100 grams_of _superphosphate and

3.-5 millilitre of sulphuric acid in 100 millilitres of sea

water resulted, after mixing, in a very thick paste.

ce ,zallnity of the sea on the

vitality of oysters

The outimal salinity of the sea for the s -oecies Ostrea

edulis L., according to Ra n s o n (1951), ranges from 31 to

3 5 KD. The reduction of salinity affects the life and health

of molluscs. As we have seen in the enclosed tables, the

salinity in the Limski Canal fluctuated between 4.72 °/oo and

38.06°/oo (1960).

From the experiments carried oit in separate ponds the

following could be learned: oysters cannot tolerate -

a substantial reduction in salinity without harmful effects

upon their health or 111 e. The critical point at which their

life becomes threatened occurs when the salinity of sea water

reduced to 20 0/00 . At 38°/oo the oysters survived as long

as two months, deDending on the degree of their fitness and.

the temperature of the sea water in the ponds in which they

had a sufficient amount of oxygen but no food. In the 22 °/oo to

20°/oo salinity range that period was much shorter and varied

depending upon the temperature of the water. At higher

temperatures (20-25°C) oysters survived a shorter time, while

at lower temperatures (10-15°C) they were able to survive

longer. At 19°/oo salinity and less, the time was noticeably

14

shortened. At higher temperatures oysters could survive onlY

3 to 7 days, while at lower temperatures that period extended

to 14 days.

Mussels as members of the animal community in

the Li ki Canal

Mussels (Mytilus galloprovincialis Lmk) were also in

the same breedinr area, in the'region which was most exposed

to Changes in salinity. Although one could expect that in

such an environnent the mussels would suffer in the saMe way,

this however was not the case.

On the basis of - the examinationef the inussel populations

and their habitat, the following can be concluded:

1) during the mass mortality of oysters, only a small 236

number of mussels over one year old lived in the Limski Canal

and a great number of young ones, less than a y.ear old,

• 2) the mussels are much more resistant to changes in

salinity and can live in sea water with much lower salinity,

up,to 4°/00 (according to Lambert1950 and Ranso , n

1 951 ),

3) the mussels can filter the sea water more effectively.

In comparison with oysters this ratio is 9•:1 (L a m b e - r t 1950),

4) the mussels from Limski Canal have, on their Shells, •

fewer commensal organisms than have the 'oysters Or, scimetimes,..

do not have any at all, particularly the young ones. "

X Secchi

15

235 Table 5. — Sea transparency in the Limski Canal (Secchi 0.5 m)

Tcbcia. 5. -- .R-nzirrost mom, If, ka.nolu (Secci 0,5 In)

128. 1057. 1855. 1959. 1930. 1951.

January__ _ .

' .....A U NAR — ------ - q-,o------- 3.-r,5 - -- - - 0.L,5 -7,5 8 — 9 - February .F12.auli.A.ra 10 — 1'7 . .,.,:,

' - 9 5 11 —12 4.5— 7,:; March Mi • ., 11T 0,4-2 10 —11 12 —14' 5,5— 7,5 April : ,..1=1-7W-.. 5.5-10 15.3 0 —15 May- — . ' • 2 — 7- :. 5 9 --11 3 --1:'. June . _ ....,. , , . \ _ 1') 7• —13 3.5--•,:', july .T -7_;Li :2,5-17 11 —12 8,5-11 :3,5— 7,5 7

August - - ; AT-7--;I:r2T- — -- - 1 4 —19 - 8 —4 9 7 — 7.5 --7 —13.5 • 4.5— 9 4„5 — 8.5

5eptember SE 7-7-ri'..%1BArt 10 — 1 7 10,5-12,5 5 — 7,5 8,3— 9 5 —10.5 Uct ouer OKTOBAR 15,3-24 10,5-11,5 9 8 — 6.5 0,5— 3 4 -- 7

' November NOVirsIBAR 6,3-10,5 3 — 4,5 4 ...._... 5

Deceniber DEC2M- B..•=1.:R 5,5— 9,5 7,5— 8 11 7 — 7,5 1,2— 5,5 5 — 7

Table 6. — Salinity and transDarency of the sea at the'control stations in October, November and December 1960.

Station: Octob'er November, December S °/oo Secchi S°/oo Secchi S'Yoo Secchi

_ • •

Tabela 6. — Sanos j prczirr.ost rnora r.a izontroinint postajconct

za X. i i X7I !;CO. g0clinc

XI Xii Seccht S 0 : 00 Secchi

• DNO LIMA 23.0 1 33:10 3 4,72-21,47 1,2 \---ES.-7;•;:OVO 13,0 0.5 34.33 3 19,87-36.37 2.5

• PERUZOLO 18,0-19,0 3 • 34.29-34,45 4 — — '2■ AVI 22.5• — 3->.,45 — 34,23-38,92 3,5 . K..\...PjGA.1-DERA 1 • 9`).0 — 21

_ .34 — •—

. IAI1GADIM A II .19,7. ...... 3 — —

; B -1,TEL.A. GUOTA 19,0 2-3 34.38 - 3 • __, _. _ . _

! CERIZERA I' 22,5 ' 3 32,93 4 — -- 4. CERIZERA II 9 2 .73 — 34.11 — —

! ,5> I5IIJA ' 23,0 — ,

-- — —

Conclusions

1) A mass mortality of oysters in the Limski Canal

suddenly occurred in 1960, in the following manner:

oysters cemented in fours,' year of birth: 1958, 76.66% died,

oysters cemented in fours, " 1959, 61.81,

oysters attached to sticks, " 1959, 53.46%,

ovstrs ,r.,,ttecrl to stick, " 1960, 36.66%,

16

2) From the - inspection of dead oysters it was established

that they were not damaged and that they did not bear any signs

indicating the presence of "spoilers" or other parasites.

A lengthy illness can also be excluded as the cause of death,

because the dyin2: animals were sufficièntly fat.

5) :1'esuis of c'nstcal analyses of tIls sea water s

of the edible portion of oysters and of the sediment on their

shells prove that it was not the poisoning by the chemicals

used as fertilizers nor by the substances which were brought

into the sea by the influx of fresh water and Mud.

4) The whole year 1960 was rainy (1207.4 mm),:and the

hiP:hent precipitation was in the months . bf Februarv, March,

July, September, October, ',November and December. Althôugh the

remaining months had less rainfall; we can conclude, according

to Vatova(1950), who claimed that heavy rainfalls

continue to have an effect on salinity for oVer a month after

the precipitation occurred, that during the year 1960 there

was a -cOntinuous reduction - in the salinity'of the sa water

in Limski Canal. •

5) Large amounts of silt enter the water of Limski

Canal with precipitation, which then settle on the sea bottom

and on the shells and other parts of oyster bodies. The mucus

found on the bodies of molluscs is orobably a protective

material produced by the gills, according to Loosanof f.

(1961).

On the basis of what has been said above we can recon-

struct the orccess of destruction:

The reduction of salinity caused, first of all, the . _

_ destruction of the.commensal.organisms. This adversely affected

the micro-climate which, in turn, produced a negative effect

on the oysters' life. At the.same time, the reduction of

salinity had a direct effect on the oyster's body. Experiments

• iladf nrov -ed t-hst - a salinity of less than 20 °/oo is detrimental

--to their life. This, and lower salinity concentrations were

registered in 1960. The presence of silt in the water with •

reduced salinity made the already disturbed living conditions

even worse.

These. great disturbances occurred at a time when the

older animals, weakened after spawning, had not yet had time

to recover. For that reason they had a higher mortality rate.

.The young ones, which had not spawned yet, having grater

vitality and thus greater adaptive powers, had suffered a

lower percentage of casualties.

The mussels did not suffer due to their ability to live

• - in water of low,salinity, and to the fact that they filter the

sea water more effectively. Also, they have fewer commensal

organisms on them.

SuPporting the above statements are the studies of the

following researchers: Loosanoff 1952 and 1961,

Lambert 1950, Ranson 1951, Vatova 1950,

Bytinski-Salz 1935, and Korringa 1956.

18

18

Bibliogr.aphy

B 1 .1:.1 1,10 C1 R. A. IT IJ it. •

Aliod 1, I. 1.tl(J5: 1)1e AtisternzAtelit an e'er Nerci-Osilichm Ktlsie des Adriatisclien

Int.. 1• ISCLL \Viet). 1; ; ,1, 19;,7: • e,taj 0 rt.:,:ullattina eksp ....rimenata gnosiettia Mljetsllog iezera•

itt,% tt , t "VY, G,

13 •,‘. linski-Sal II. 1935: Ui 1.iicliIo (Stylocint.,.; pilidium Itanc,) dannoso ai

);_ r 1 • ; . :• al .Pr,-;31:eilvity. llapp. et Pi . oc:. •Ve.• ■ •:).

I 1.10 !L. Uxpl. JI.

, IliA' o :; h it 1' 1. 1_. V. iur 0l.trs it Vater of LOW Salinities,

Coro.. t,1 n I.,. "V. 11,1)1: 1 .11"ic..u13 rro,:tbidity en SO17■1C I.:arval and Adult Bivalves,

(:at 1 • 1h, 10 15n. •tturlh. ADP.

1 n k 0 1 é, 1\1 . Oet--atur...;ratiçït podruêju Rovinja i u Lims1,.:orn kr,flpi ta, Split.

1‘1 ;•. 1 . 1 ;; u i c. i 111f11: Ouc;:v.,,:fityrt 1:::1;a pocIruillt •

1 u 1., kuttlu 1./59-1081 godine, 11 drogriiski Godinjak 19 130.,

rt. n ri, C. 1951: Cul:ure. Pdris.

i ■ - a, A. i959: ul1 uutiCtiziont it -Intgraticile del Canal di Lenie in Istria, Nova Thalassia, Vol. 1 Ne. 8, Vene:da.

(Croatian entries - e, 3 and 9):

#2. Buljan, M. 1957: Report on the results of fertilizing the Venetian Lake by the new method. Acta Adriatica (Adriatic Journal) Vol. VI, No.' 6, Split (YugoslaVia.)..

#8. Marinkovi(), M. 1959: Oceanographic research in the Rovinj reRion and in the rirski Canal, "The Hydrogra'ohic Car Look 1953, Split (Hidroezrafski Godi"ânjak).

Marinkovi6, M. and Nikol Oceanographic research in the Rovinj region and in the Limski Canal in the period 1959-1961. "The Hydrographic Year Look 1960", Split (lugoslavia), (Hidrografski Godinjak 1960).

• .4, • .19

ee -

- s umm a r - y :

_. During the....decadei, .th•.t_ have _e_ipp ,:•:.d since the rearing of shellfish has begun in Liniski Kauai, the annual mortality percentage observed in. oystcr

•eached frmu 10 to 20')/ü. This mortality perccntage can be considered normal u-ider (ircumstanccs:

-a) ci;reit--. lit;ndling by forcible trï2.atment el plants (especially by. - ---- • --- - b) -■vavcs i:utf.:ting some pfirts of the :;nstallations, c) Ilqic.ss•of S'i;CrUS enratzie L.,

-ci) periodical aît.avizs by the harnif ,..!1 i libertarian Si7, iÙchlts piliditon Lang._

atl.ac.: r;:.4i.,t.c.rc•J in mortality at that Lime vas al.-..out. 87/0 in oysters from 3 to 35 nun in tength.

In October 1960 a iruss mortalit;\,7 (.>cetirrocl unexpectedly in oysters of all sizes.

Different opinions as to the causes were given. For this reason a number of investigations were carried out:

1. First of all, 10 positions were chesen at which data ‘vere taken on oyster mortality during a r...:1- iod of several months.

2. On thesc., posilions hydrographical data were m.easured. 3. Meteordgical information fol.' the saine area were registered.

.4. The oysters (shells F.ncl edible portions) were Lxarained. 5. The sett water, muddy sediments, employed fertiti,zers (superphosphate

and sulfuric acid), live oysters from the rearing.places and natural banks, and deaci oysters fi con the ut.:.;_tri tg piaces wet e analysed.

G. Different laboratory investigations of the influence exerted by low water salinity on oysters wet e-carrie.d out.

7. The mortality of mussels was e:c.ri-ipr.rcil with that of oysters. Hi re are the conclusions drawin en the basis of the above results:

1. The following irortality percent Mes were reSi...tcred .in 1960: inertality

oysters • year • 'percentage

cemented in fours 1953 76,66 cementcd in fours 1959 111,81 attached to sticks 1959 • 53,46 attached to sticks 19 5 0 30,66

2. Deacl oysters were not damaged. Parasites wt.‘re not found. Dying fat. ,

3. All chcmicat analyses (water, oystrs, fertiliser, etc.) excluded poisoning • by chemical subf"tances.

4. 1060 was very rainy,. pat ticulariy through most of February, Ma t ch, July, September, Octcher, November, and December. According to •Vntova (1950). the influence of salinity changes is r.otieod a mon th or more art.;m.. the .?nct of the main rainfalls. Therefore .we consider the whole of 1960 as a very rainy year.

5. The. surface ‘,va ter s.alinity in 1960 ‘..vas therefore orten between 4 11 '0 and .•

6. The. laboratory investigations have shown that salinity values under 20'u'd are harrnful to oysters. ' 7. Together with freshwater, lare_;e harmful quantities Of mud enter Lituski Kanal settling on shells and edible pertizuis of the animals.

!--3imilar results covcri ug other arpas other :peeic«,, were obiairml by Loosanoff 1952, 1961;.Lanthert 1950; «Ranson 1951; Vatova 1950; Bytinski-Sala 1935 ..indl(orringa1950. - . .