J. Exp. Biol. (1969), 50, 375-386 275With 4 text-figures

Printed in Great Britain

THE NITROGEN BALANCE IN PLAICE, SOLE AND PERCH

BY L. BIRKETT

Fisheries Laboratory, Lowestoft

{Received 19 January 1968)

INTRODUCTION

In both ecology and fish-cultivation, a knowledge of the efficiency with which foodis converted into growth is of considerable importance. Conversion to growth, however,is but one aspect of the balance of nutrients in general, and it is therefore appropriateto consider the whole of this balance, and to do so with particular reference to selectedconstituents of living matter, the turnover and cycling of which may be traced fromprey to predator. Nitrogen—of which the major portion is contained in proteins—andenergy are the most commonly used constituents in such work.

The primary nitrogen ' balance' is between the input to the animal, i.e. that which isabsorbed into the bloodstream from the food, and the output, in which are includedthe nitrogen retained as growth or stored in the body and that which is metabolized.The balance is summarized by the following verbal equation, which forms a usefulstarting point to more detailed discussion:

total N consumed — faecal N = N absorbed = N retained + N excreted(I) (F) (A) (R) (E)

The ratio of growth to absorption RjA is the net efficiency of conversion, and thisratio is frequently used in ecology. It is easily determined in feeding experiments as amean value for the whole experimental period.

The real problem, however, is to determine not the net efficiency, but the grossefficiency, which is the ratio of growth to the excess of food eaten over the maintenancelevel. Determination of the maintenance level presents a practical problem, becausethe actively feeding and freely growing animal must use more food for maintenancethan its counterpart which is allowed only sufficient for maintaining a weight equili-brium. Dawes (1930, 1931) kept some of his plaice, Pleuronectes platessa, in a some-what uneasy weight equilibrium for many months, and determined the maintenancerequirement in relation to body size. Brown (1946) found that, in the trout, Salmotrutta, the maintenance requirement was a rather variable function, depending on therate of feeding in periods preceding the measurement of maintenance. The possibilitythat the nitrogen level in the body may be affected by the rate of feeding (as Gerking(1955) found for the bluegill sunfish, Lepomis macrochirus) indicates the impossibilityof making a direct determination of the nitrogen maintenance requirement.

PREVIOUS WORK

Ivlev (1939) reared young carp, Cyprinus carpio, in a pond at a fish-farm, and deter-mined both the energy and the nitrogen balance by periodically removing a sample

376 L. BIRKETT

of the fish, measuring their nitrogen excretion rate over a period of 3 or 24 hr., andestimating the nitrogen input by autopsy. Gerking (1952) studied the nitrogenmetabolism in long-eared sunfish, Lepomis megalotis megalotis, and green sunfish,L. cyanellus, and also (Gerking, 1955) in bluegill sunfish, L. macrochirus. Gerking'sexperimental methods were similar to those used in the work now reported. Menzel(i960) studied both nitrogen and energy conversion in the red hind, Epinephelusguttatus. The anchovy, EngrauUs encrassicholus maeoticus, was used by Shuleman (1962)in his study of nitrogen balance, in which the technique was essentially the same asIvlev's. Pandian (1967) studied nitrogen conversion in Megalops cyprinoides andOpMocephahis striatus.

MATERIAL

In this paper, the nitrogen balance of two marine species, the plaice Pleuronectesplatessa, and the sole, Solea vulgaris, and of the freshwater perch, Percafluviatilis, aredescribed from original experiments.

METHODS

For detailed balance measurements nitrogen must be accounted for not only in thegrowth and the food, but in the faecal matter and metabolic waste as well. It is there-fore preferable to keep the fish separately and, because growth alters the animq1( tokeep the duration of the experiment within reasonable limits. In the perch experimentthin was 6 weeks; in the remaining experiments, a shorter time, 10—20 days, was foundsufficiently long for measurable growth to occur. Initially, a comparison was madebetween the observed, or net, change in total body nitrogen during a given period, andthe total amount of nitrogen consumed or absorbed from the food. Each fish was pro-vided with an excess supply of live food, which closely resembled foods taken innature. Live food was used, partly because it would not decompose if not eaten withina short time, and also because the fishes involved in the experiments would not takenon-living food. The amounts consumed were only nominally maximal, however,because, as the results show, several of the fish lost weight during the experiment.

The live weight of a fish was obtained by first weighing it on a moistened pad, andthen re-weighing the pad without the fish; in this way, external moisture was kept to alow and standard level. Foods were weighed similarly. A damped torsion balance wasused for rapid weighing.

Faecal matter was collected by siphoning, using as little water as possible to carry itover into a suction funnel fitted with a previously dried and weighed Whatman no. 1filter paper. In the experiment with yearling plaice the faecal matter could not becollected and so the faecal nitrogen was not determined.

All materials were dried to constant weight in an oven at 40-500 C. prior to thedetermination of the nitrogen content by the standard Kjeldahl method. Nitrogen wasdetermined in samples of all the fish, including controls which were sacrificed at thestart of each feeding experiment and which represented the initial condition. Exceptfor the perch, there was no significant systematic change in the nitrogen content pergram live weight. Regular samples of the food and faeces were also analysed fornitrogen content.

Nitrogen balance in plaice, sole and perch 377

EXPERIMENTS(1) Perch, Perca fluviatilis

In this experiment, which was carried out in June—July 1964, mainly to assist withthe design of the later experiments, six perch, caught in Oulton Broad near Lowestoft,by kind permission of the Norfolk and Suffolk River Authority, were fed for a 6-weekperiod on earthworms, Lumbricus sp. The temperature of the experiment was notcontrolled, but varied little from 170 C.

(2) Plaice, Pleuronectes platessa

The plaice and sole experiments were carried out at the Ministry's FisheriesExperiment Station at Conway, North Wales, during June-July 1965, using fishcaught off Penmaenmawr. Two size-ranges of plaice were used, 'small' and 'yearling'fish. The former were fed on lugworms, Arerticola marina, which were dug freshly atLlandudno West Shore, and allowed to defaecate for 2-3 days in several changes of seawater before use, so as to eliminate the possibility of including their faecal matteralong with that of the plaice. The yearling plaice were fed on Artemia from a stockculture. The temperature was again approximately 170 C. One batch of small plaicewas fed ad libitum and the others were fed at lower rates.

To estimate the total weight of Artemia eaten, the numbers eaten daily were multi-plied by the mean dry weight, which was checked at frequent intervals because theArtemia were growing rapidly. Large feeds resulted in frequent attacks on Artemiawithout ingestion, so that these fish were fed 'little but often', and it was then foundtaht the numbers killed but not eaten were small.

Table 1. Plaice, Pleuronectes platessa, initial and final live weights,and amounts of food eaten throughout the experiment

Serialnumber

l Ai B2

3A3B45A5B6789

1 0

Live weight

Initial

2389267315-2627-3629711124274728-78178316-17169817-36I9-75

(g.)

Final

26-732470168529-7128891411287833-6416-22i6-99165917-5319-54

Fresh weightof food

eaten (g.)

39992I575O3718635-521256313I-33819-17063816

150911-311275688185196

Duration ofexperiment

(days)

171325171517172 1

91 0

1 0

1 0

1 0

Plaice that are taken direct from the sea and placed in isolation tend to refuse food,sometimes for quite long periods. However, if kept together they feed satisfactorilyalmost at once and can later be separated, and this method was used in order toaccustom the small plaice and soles to the experimental conditions; it was not neces-sary to do this with the yearlings, however, as they had no inhibitions about feedingin isolation.

L. BlRKETT

A few plaice were kept without food for 11 days in order to study the fasting loss rateof nitrogen.

(3) Sole, Solea vulgaris

Small soles were also fed on lugworms. Yearling soles were used in an experimentat Lowestoft during January-February 1966. These fish were reared at the White FishAuthority's hatchery at Port Erin, Isle of Man, and fed on whiteworm, Enchytraeus sp.The faecal matter was collected from only a few of these fish; the mean value ofefficiency of nitrogen absorption was used for the remainder in order to calculatenitrogen absorption.

Tables 1—5 summarize the observations on growth and feeding in these experiments.

METHOD OF CALCULATION

The mean daily rates of growth (R), intake (/), absorption (A) and excretion (E) areall expressed in this paper in units of mg. nitrogen per live weight per day, abbreviatedto mg. N/g.-d. The following efficiency ratios are used:

efficiency of nitrogen absorption, a = A11, (1)

net efficiency of nitrogen conversion, £ = Rjl. (2)

Ivlev (1939) used the symbol K to denote net efficiency, and this symbol is frequentlyused in ecology. These ratios are means for the whole period of an experiment. Thegrowth rate, R, is calculated on the assumption that growth was exponential; using nto denote the nitrogen content in mg. N/g. of the body, then

T W

R = n\ log,-j^mg. N/g.-d., (3)

Table 2. Yearling plaice, Pleuronectes platessa, initial and final live weights,and amount of food eaten throughout the experiment

SerialNumber

1 1 A

i a A1 3 A1 4 AI S A1 6 A1 7 A1 8 A1 9 A20 A

1 1 B

1 2 B

13B14B15B16B17B18B19B20 B

Live

Initial

0-2350-1710-2240-2250-1891-2091-4701-5771-5451-454

O-7330-468I-OO20-859O-7372-3103-0582871

3-2472-680

weight (g.)

Final

O-7330468I-OO208590-7372-310305828713-24726801-160I-OOI1816148215032624

3759335234293240

Fresh weightof food

eaten (g.)0-3620-2890-6060-474040809841-4251-274

1-4191-028

0-44405020-64005860626063308110806

08330-756

Duration ofexperiment

(days)

171 7

171717

171 7

17

1 7

1 7

181818181811

18181 2

18

Nitrogen balance in plaice, sole and perch

Table 3. Sole, Solea vulgaris, initial and final live weights,and amounts of food eaten throughout the experiment

379

Serialnumber

2 1 A22 A23 A2 4 A25 A21B22 B23 B2 4 B25 B

Live weight

Initial

56602871

9 6 816-25

5 0 0

53-493021

9 8 814976-21

(g.)

Final

53 493021

9 8 81497

6 2 1

530335-74116718-13

9 0 4

Fresh weightof food

eaten (g.)

34-1773I-538

62539712

16-106

35-2056158221-37235-25024-780

Duration ofexperiment

(days)

1717171717

2 0

2 0

2 0

2 0

19

Table 4. Yearling sole, Solea vulgaris, initial and final live weights,and total amount of nitrogen in the food consumed throughout the experiment

Serialnumber

26

2728

29303 i32333435363738394 0

4142434445

Live

Initial

0989063807081 0660465o-68i123902480-3430-3730-487068105680481038905520-67307030-5120686

weight (g.)

Final

1-3920-83009321-352O-7751-2531-843O-5550-674O-7970-86809841-15600070-625I-IOI

1-276076907951-143

Total nitrogenconsumed

(mg. N)

18-909u-5561826319556108152378526-14313655I4-I941651818883I7-747193731593614-1461903524150IO-54417-81317829

Duration ofexperiment

(days)

1515151515151515151515151515151515151515

Table 5. Perch, Perca fluviatilis, initial and final live weights,and amounts of food eaten throughout the experiment

Serialnumber

4647484 9505 i

Live

Initial

109-15I43-72ioo-oo125-00101-79111-52

weight (g.)

Final

1277515238110-281340912887in-77

Fresh weightof food

eaten (g.)

11724611670857812

1442369-11

Duration ofexperiment

(days)

424242424242

380 L. BlRKETT

where Wo, Wt are the initial and final body weights of the fish and t the duration ofthe experiment in days. Then

/ = U/g and A = al. (4)

The excretion rate is given byE = A-Rmg. N/g.-d. (5)

Some nitrogen is excreted from the body independently of the food intake; providedthat the animal is in a resting and post-absorptive condition, this nitrogen loss wouldbe equivalent to the basal metabolic rate. However, the fish used here were active andfreely growing, and it is likely that they excreted more nitrogen from the body thanrequired for basal metabolism. This nitrogen loss from the body is replaced by themaintenance food, which is sometimes described as the food consumed and used fornon-productive processes. The nitrogen absorbed in excess of maintenance gives riseto a net gain which is R. Assuming that the ratio of growth to absorption in excess ofmaintenance is constant, this gives a second measure of conversion efficiency:

gross efficiency of conversion, £• = R/A—A^ (6)

where A^, is the maintenance level.Rearranging equation (6) gives

R = f{A-AJ, (7)

which is a linear regression of growth on absorption. Its slope, £*, is a gross efficiency,and the negative intercept on the ordinate,

i*Am = E» (8)

is the constant nitrogen excretion rate, independent of food intake, already mentioned.Equation (7) may also be written as

R = g*A-E0. (9)

The maintenance rate, A^, is the intercept on the abscissa but should be calculatedfrom the regression of A on R, with A as the dependent variable:

A = (Rl^)+Am. (10)

i/£* is the nitrogen absorbed per unit of growth.From equations (5) and (9) the total excretion rate E is seen to have two components:

E = A-R = (:-£*) A + Eo, (11)

(1 — £*) being the excretion of nitrogen derived from the food.From equations (5) and (10), the excretion rate is also seen to be related to the

growth rate:£ = , 4 - ; ? = [(1-£•)/£•] * + 4n- ( « )

Here again, there are two components to the excretion rate—one, A^ which isindependent of the growth rate and is clearly equal to the maintenance rate of absorp-tion as it should be; the other is directly proportional to the growth rate in the ratio(1 — £*)/£*, for which the same metabolic cost of growth is proposed.

To summarize, equations (9), (11) and (12) form a basic system within which theover-all nitrogen balance of the fish may be described. The constants in these equa-tions are not implicitly dependent on body weight.

Nitrogen balance in plaice, sole and perch 381

RESULTS

(i) Nitrogen contents

Most of the fish had a mean nitrogen content of 24-28 mg. N/g. live weight (Table6). The exceptions were the hatchery-reared yearling sole, which contained signifi-cantly less nitrogen per g. The foods usually had significantly lower nitrogen contentsthan the fishes to which they were given.

Table 6. Mean nitrogen contents offish and foods(mg. Nig. live weight) and of faeces (mg. N/g. dry weight)

Nitrogen content (mg. N/g.)

Fish

I. Small plaice ( i - s ) #

Small plaice (6-io)f2. Yearling plaice3. Small sole4. Yearling sole5. Perch (experimental)

Perch (controls)• Fedt Fed1 Not

In fish

25-392414

23-20

25-6519-03

26-772849

In food

9 2 29 2 2

7-879-22

18261569

in excess of demand.at controlled rates.measured.

In faeces

20-122O-I2

t28719-02

6-55

(ii) Efficiency of nitrogen absorption

Figure 1 shows the relationship between the nitrogen absorbed and the nitrogenintake rates, both as mg. N/g.-d., in the small plaice. The slope of the regression is themean absorption efficiency. Similar regressions were calculated for the other two fishspecies and the following mean efficiencies were found: small plaice, 0-918; small

"3 i-o2i1&s<

0 10Intake (mg. N/g.)

Fig. 1. The relation between the absorption r»te, A, and the rate of intake, /, (in mg. N/g.-d.)in small plaice. The slope is the mean absorption efficiency.

382 L. BlRKETT

sole, 0-850; yearling sole, 0-960; perch, 0963. The difference between the small soleand plaice is noteworthy in view of the fact that these two species were using the samefood—lugworms. High efficiencies of nitrogen absorption seem to be characteristic offishes; Gerking (1952, 1953), Menzel (i960) and Ivlev (1939) all reported values of theorder of 0-90.

There was a possibility that the faecal nitrogen could have included some derivedfrom cells abraded from the alimentary canal lining, or from secretions and bacteria,but the proportion may be assumed to have been insignificant.

Assuming that only 90% of the produced faeces could be collected, because of itsdispersed state, the absorption efficiencies would have been underestimated by about1 %, because of the high values obtaining. Absorption was not estimated for the year-ling plaice.

0-4

02

sO -0 -2

- Small plaice

0-4 o 0 8 VI 1-6

Absorption (mg. N/g.)

U)

so

Yearling plaice

CO

z

so

0-4

0 2

- 0 - 2

2 4

Intake (mg. N/g.)

Small sole

•0-4 0 8 1-2 1-6

Absorption (mg. N/g.)

z"

:h (

mg

.

0

O

10

n

Yearling

v°'A .

sole/

/

o°

1 . 1

0 10 2 0 30

Absorption (mg. N/g.)

0-1

OO

oV

- 0 0 4

Perch

/ 0 2 0-4

Absorption (mg. N/g.)o

Fig. 2. The relationship between the growth rate, R, and the rate of nitrogen absorption, A, orintake, /. The rates are all in mg. N/g. body weight/day. The slope of the regression is £•, thegross efficiency of nitrogen conversion; the regression cuts the ^4-axis at the maintenance rateof absorption, A^, and the i?-axis at the basal excretion rate, E,.

Nitrogen balance in plaice, sole and perch 383

(iii) Relation between growth rate and absorption rate

Figure 2 shows the relationships between R and A for the plaice, sole and perch.The regressions (equations (9) and (10)) yielded estimates of the gross efficiency, £*,the maintenance rate, A^, and the constant rate of excretion, Eo, which are summarizedin Table 7.

Table 7. Constants calculated for the regression in Fig. 2

(For the yearling plaice the faecal nitrogen was not determined,and the calculations are based on the unabsorbed intake (/).)

Small plaiceYearling plaiceSmall soleYearling solePerch

Grossefficiency

(£•)0-2750417039104900-413

Maintenance,Am

(mg. N/g.-d.)

0-2030-4990-38102440173

Basalexcretion,

Eo(mg. N/g.-d.)

0-0560-2080-1490-120

O-O72

Table 8 gives the loss in weight of eight plaice during an 1 i-day complete fast. Themean loss rate of these fish averaged 7-54 ± 2*07 mg./g. live weight per day, which isequivalent to 0-056 mg. N/g.-d. This result agrees well with the negative growth-rateof some of the plaice in the feeding experiment, those which failed to consume enoughfood for maintenance; those plaice with R negative were excluded from the regressioncalculations.

Table 8. Loss of weight by plaice during an 11-day fast

Live weights (g.)

Initial

24844 1 1

38-222-47i'75

15-353-450-88

Final

22-293-69

36-482-251 5 9

14273-25o-8i

lviemi 1U3B ruic

(mg./g. per day)9-89-84"i8-58-7675-37-4

Over-all mean rate: 7-54±2

(iv) Relation between excretion and growth

Figure 3 shows the relationship between the excretion rate, E, and the absorptionrate, A, and growth rate, R, for small plaice. The slope in the E-A diagram is (1 — £*)(equation (11)), and that in the E-R diagram is the metabolic cost of growth (1 — £*)/£*of equation (12). The estimates of metabolic cost are: perch, 1-42; small plaice, 2-64;small sole, 1-56; yearling sole, 1-04.

(v) Estimates of gross conversion efficiency and maintenance rate from thework of other authors

The data published by Gerking (1952, 1955), Ivlev (1939) and Menzel (i960) gaveestimates of the gross efficiency and maintenance rate in a number of species. Figure 4

L. BlRKETT

shows the relationship between R and A in Gerking's sunfish, MenzePs red hind atthree temperatures, and Ivlev's carp.

10

Iao

10

710

Absorption (mg. N/g.)

- 0 3 0-3

Growth (mg. N/g.)

Fig. 3. The relationship between the rate of nitrogen excretion, E, and the rate of nitrogenabsorption, A, and the rate of growth, R, in small plaice. The rates are all in mg. N/g. bodyweight/day. The slopes are (1 — £•) and (1 — £•)/£•, respectively.

Table 9.summarizes the estimated values of the constants for some of these species.Ivlev's work was less critical than that of Menzel or Gerking, and it was carried outover a wide range of ambient temperature. He kept many fish in an outdoor pond andobtained their feeding rates by autopsy at roughly fortnightly intervals; the nitrogenexcretion rate was determined from the increase of nitrogen in 1 1. of water containinga fish taken direct from the pond. The curve through Ivlev's data is therefore fitted byeye.

Table 9. Constants calculated for the regressions in Fig. 3

MaintenanceGross efficiency Am

(£•) (mg. N/g.-d.)

Green sunfishLong-ear sunfishBhiegill sunfishfCarp

0-5*

0-500-74

0-2590-0130-3111-17

f Faecal nitrogen not determined.

DISCUSSION

Dawes (1930, 1931) determined the gross efficiency of conversion in the plaice bydeducting from the food consumed by growing individuals amounts equivalent tomaintenance; the maintenance ration was found by trial-and-error with other plaice.Using the values for nitrogen content given above, 25-39 mg- N/g. live weight in theplaice, for nitrogen absorption efficiency, 0-918, and a recent determination of thenitrogen content of Afyfr'/itf flesh, 18 mg. N/g. live weight, Dawes's value of £* = o-2g.plaice growth per g. of MytUus edulis flesh can be transformed to a nitrogen efficiencyas follows:

g* = O-2X 0-918 = 0-259,

which is quite close to the nitrogen efficiency of 0-275 found in the above experiment.

Nitrogen balance in plaice, sole and perch 385

Comparison of the results for small plaice and small sole shows that, although thesetwo species were using the same food with apparently the same nutritional value, theywere benefiting in different degrees from it. The plaice absorbed a larger proportionof the nitrogen than the sole; however, they had a lower gross efficiency. Each gram of

0-5 r

Z

- Green sunfish

y ^ 0-5

Absorption (m

t

1 0

g- N/g.)

0-5 - Long-eared sunfish

z

Io

0 5 1 0

Absorption (mg. N/g.)

0-5 - Bluegill sunfish00

z

eo

o 0 5 10

Intake (mg. N/g.)

O

2

0

- Red

- 19°

-

hind

and 23° C. >

2

Absorption

1

4

"/)

^28°C.

i i

6

do

z

8O

- Carp

0 1 2 3 4 5Absorption (mg. N/g.)

Fig. 4, The relationship between the growth rate, R, and the rate of absorption, A, or intake, / .Data recalculated from various authors—green, long-eared and bluegill sunfish from Gerking(1952, 19SS); red hind from Menzel (i960); carp from Ivlev (1939). The rates for the carp andsunfish are mg. N/g. body weight/day, red hind in mg. N% of body N/day. The slope is thegross efficiency of conversion £•; the regressions intercept the A- or /-axis at the maintenancerate.

Arenicola consumed contained 9-22 mg. N; calculating retention at 9-22.a.£*,and metabolism at 9-22.a.(1 — £•), the plaice derived 2-32 mg. N for growth, and6-14 mg. N for metabolism, per gram of Aremcola, while the sole derived 2*98 and4-78 mg. N respectively. The daily maintenance requirements of these two specieswould be derived from 88 and 128 mg. Aremcola, per gram of plaice and sole respec-tively. Such differences as these point to the need for more comprehensive comparativeinvestigation of the convertibility of foods. The yearling sole and plaice used above

386 L. BlRKETT

cannot be compared directly, since they were given different types of food; further,the fact that they were kept in the same size of tanks as the larger individuals may havepartly accounted for their higher maintenance requirement, since the smaller fishwould have had proportionally more scope for activity.

Although Ivlev's (1939) methods were less critical than those used in tank experi-ments, his ideas have been widely adopted in ecology, probably largely because of theiroriginality. He introduced the use of the balanced equation into fish energeticsfollowing Terroine & Wiirmser (1922) in their energy balance method for fungi. Thesystem of equations used here (equations (9), (11) and (12)) provides a more convenientmethod for estimating the fundamental ecological factors of gross efficiency andmaintenance requirements. The relation between growth and food revealed byequations (9) and (10), and in Fig. 1, shows that however much nitrogen is absorbed,i.e. above or below the maintenance level, a certain fraction E* is retained and (1 —£,*)metabolized; in other words, the maintenance food and that available for growth arenot subtly distinct portions of the total food and are not differently utilized as seemsto be implied by the common concept of maintenance.

SUMMARY

1. Plaice, sole and perch were fed on live food for 20-40 days, and their growthrates were compared with their rates of absorption, both measured as mg. N/g. liveweight/day.

2. The gross efficiency of nitrogen conversion—that is, the growth per unit ofnitrogen absorbed in excess of maintenance—was between 27-5% and 49-0%

3. The maintenance rate of absorption was between 0-173 an<^ O'499 mg. N/g./day.4. The basal rate of excretion amounted to o-o56-o-2o8 mg. N/g./day.5. At maintenance, total excretion is equal to the amount of nitrogen absorbed from

the food. The metabolic cost of growth was I-O4-2-64 mg. N excreted per mg. Nretained.

The author is grateful to Professor S. D. Gerking for reading the original text andmaking some valuable remarks.

REFERENCES

BROWN, M. E. (1946). The growth of brown trout (Salmo trutta Linn.). 2. The growth of two-year-oldtrout at a constant temperature of 11-5° C. J. exp. Biol. 22, 130-44.

DAWES, B. (1930). Growth and maintenance in the plaice (Plewronectesplatessa L.). Part I. J. mar. biol.Ass. U.K. 17, 103-74.

DAWES, B. (1931). Growth and maintenance in the plaice (Pleuronectes platessa L.). Part II. J. mar. biol.Ass. U.K. 17, 877-947-

GERKING, S. D. (1952). The protein metabolism of sunfish of different ages. Phytiol. Z06I. 35, 358-72.GERKING, S. D. (1955). Influence of rate of feeding on body composition and protein metabolism of

bluegill sunfish. Physiol. Zodl. 28, 267-82.IVLEV, V. S. (1939). Balance of energy in carps. Zool. Zh. 18, 303-18. (In Russian.)MENZEL, D. (i960). Utilization of food by a Bermuda reef fish, Ephinephelus guttatus. J. Cons. perm. int.

Explor. Mer as, 216-22.PANDIAN, T. J. (1967). Intake, digestion, absorption and conversion of food in the fishes Megalops

cyprinoides and Ophiocephalus striatus. Mar. Biol. 1, 16—32.SHULBMAN, G. E. (1962). Nitrogen balance and food consumption of the Azov Sea anchovy (Engraulis

encrassichoha maeoticus Pusanov). Dokl. Akad. Nauk SSSR ifj, 724-6. (English version—NationalScience Foundation, New York, 1963, pp. 1290-1.)

TERHOINE, E. F. & WTJRMSER, R. (1922). L'6nergie de croissance. 1. Le developpement de VAspergillusniger. Bull. Soc. Clam. biol. 4 (9), 519-67.