Rapp. P.-v. Réun. Cons. int. Explor. Mer, 166: 13-19. Mai 1974.

A C O N T R IB U T IO N T O T H E F E E D IN G E C O L O G Y O F J U V E N I L E C O D (G AD U S

M O R H U A L.) IN T H E W E S T E R N B A L T IC

W. E. A r n t z

Institut für Meereskunde, 23 Kiel 14, Düsternbrooker Weg 20, Federal Republic of Germ any

IN T R O D U C T IO N

The cod (Gadus morhua L.) is by far the most frequent­ly found demersal fish in Kiel Bay and, beside the herring, it is also economically the most im portant fish in the western Baltic. For this reason its feeding ecology is of special importance : as the principal end of the food chain on the bottom of Kiel Bay, the cod best reflects the condition and variations of the food available in this area and, because of its large numbers, it also influences constantly the extent of the annual consumption in the benthos and thus regulates, to a certain degree, the food available to the other species of demersal fish.

In order to determine the quantities involved in these processes in the limited area of the deeper parts of Kiel Bay, in 1968 a start was m ade simultaneously on the collection of benthos samples, the stomachs of dab and the stomachs of cod. Following the publication of the macrobenthos investigations (Arntz, 1971a) and the study on the food intake of the dab (Arntz, 1971b), the present paper gives an initial insight into the cod’s feeding conditions. Similar studies concerning whiting, plaice and flounder will follow. To obtain as compre­hensive a picture as possible of the food intake of the cod, the stomachs were processed from three main aspects. An examination was m ade of

(a) the composition of the food in the course of the year,

(b) the periodicity of the daily food intake and(c) the diel quantity of food consumed (as a

percentage of the body w eight).

T he literature will be dealt w ith a t the beginning of each section.

M A TE R IA L

The stomach m aterial came from 1007 cod caught in standard trawlings carried out by the research cutters “ H erm ann W attenberg” and “ Alkor” and the fish cutter “ Kiel 5” , in the period Jan u a ry 1968 to February 1969, in six trawling areas in Kiel Bay (Fig. 5, Table 5). To determine the periodicity of the food

intake, the stomachs of another 548 cod caught in two 24-hour fishings in November 1968, and M arch 1971, were examined. The fish studied were between 11 and 35 centimetres long (age groups 0—I I ) . A large am ount of m aterial from cod over 35 cm in length, collected and processed already on the vessels in the same period, will be published a t a later date. T he stomachs were prepared on board and were preserved individually in 10°/0 formalin-sea water. In the laboratory, the stom­ach contents were counted over according to syste­m atic groups and species, and were weighed in 0-01 grammes (wet weight). T he difference between the total stomach contents and the identifiable share was registered as “ m acerated share” .

C O M P O S IT IO N O F T H E FO O D

T he literature records the outstanding im portance of small crustaceans as food for juvenile cod. According to Todd (1907), in N orth Sea cod 15 to 30 cm in length they constitute 95 °/0 of the total weight of the food consumed, and according to R eim ann (1956) there is an equal consumption in Baltic cod up to 25 cm. R ae (1967a) found crustaceans in 91 °/0 of the stomachs of N orth Sea cod 11 to 20 cm in length. Blegvad (1916)

15 m - l in e

20'10*30 ’

Figure 5. Situation of the trawling areas and bottom grab sta­tions in Kiel Bay. Shaded: Investigated area (Abra alba-coenosis).

14 W . E. Arntz

Table 5. N um ber of stomachs processed according to trawling areas in Kiel Bay. Cod up to 35 cm, n = 1 555

T raw lingareaM onth

Süderfahrt Boknis Eck Dorschm ulde Vejsnäs R inne Millionenviertel H ohw achterBucht

J a n 1968 ___ 33 24 22 _

Feb - . . . . 12 - - 35 _* _*

M ar - . . . . _* 50 30 _* 28 -A pr — 24 - - - - -M ay - . . . . 26 10 8 22 24 11fan - . . . . 16 - - - -

J u l - . . . . 31 5 11 17 27Aug - ----- 25 - — - - -Sep - . . . . 40 18 31 32 6Oct - . . . . 50 - - - 5 —

Nov - . . . . - 30 41 - 225** 13Dec - . . . . 40 - - 30 - -

J a n - ----- 50 16 4 50 49 8Feb - 33 - - - - -M a r - . . . . - - - 323** - -

T otal ................ 350 130 128 179 + 323 155 + 225 15

- No fishing. - * No cod up to 35 cm in the haul. ** 24-hour fishing.

and H erd ing (1928) give further, similar indications. T he im portance of the small crustaceans (mostly cumaceans, mysids, amphipods and euphausiids) de­clines w ith the increasing size of the cod. They can be replaced by decapods of m edium size (Crangonidae, Pandalidae, Palaemonidae) and these in tu rn by still larger decapods (Brachiura, A nom ura, Nephrops). Thus, in R ae’s investigations (1967a, b ; 1968) crusta­ceans also constituted the m ain food of cod up to 50 cm with frequencies of 62 -68°/0, while Hessle (1923) even alleges tha t 75°/0 of all the cod in the central Baltic contained crustaceans (in this case mostly the large isopod Mesidotea entomon). However, even in areas in which large crustaceans are frequent, fish are

more in evidence with the increasing size of the cod, and in the oldest fish they are usually m ore im portan t than the crustaceans.

T he picture changes in sea areas in which large crustaceans are lacking or rare. H ere, their p a r t is taken on by fish a t a very early age (Hertling, 1928, 1929).

According to the literature, therefore, crustaceans and fish constitute the m ain elements in the food of the cod. T he other systematic groups play a smaller role: the share taken by polychaetes is almost invari­ably under 10°/0, while th a t taken by molluscs, echi- noderms and other benthos is even smaller.

Figure 6 reproduces the annual m ean values (al

mm.vivKviviy]

MOLLUSCA CRUSTACEA POLYCHAETA PISCES MISCELLANEOUS

Figure 6. Shares of the various systematic groups in the food of Kiel Bay cod according to size groups. Annual average 1968.

A contribution to the feeding ecology of juvenile cod (Gadus morhua L.) in the western Baltic 15

fishing areas) for juvenile cod in Kiel Bay. The follow­ing are the values for the whole range of fish from 11 to 35 centimetres in length : crustaceans (in the sequence Diastylis rathkei, Gastrosaccus spinijer and Crangon cran­gon) account for 35-6 °/0 of the food weight, poly- chaetes (mainly Harmothoe sarsi, Nephthys spp. and Pherusa plumosa) for 26-8°/0, molluscs (almost exclusive­ly soft bodies of Cyprina islandica) for 21-9 °/0 and fish (mostly Pomatoschistus minutus) for 15-3°/0. T he rest account for 0-4°/0.

W ithin the various size groups, shiftings are obvious: crustaceans, which constitute 62-2 °/0 of the weight of food in cod 11 to 15 cm in length, diminish to 20-6 °/0 in the 31 to 35 cm group. Cyprina islandica is not con­sumed by the youngest cod, whereas in the oldest it constitutes as m uch as 39-6°/0 of the total food intake. T he pa rt taken by polychaetes varies, as a rule, the least, although it diminishes from 30-8°/0 to 21*3 °/0- O n the other hand, the share of fish increases from 7-0°/o to 18-1 °/0. In all size groups the im portance of the other food is minimal.

In the case of cod over 35 cm in length, not dealt w ith in detail in this paper, a further rapid increase in the share taken by fish is to be traced with increasing size, and it ultimately exceeds the share of Cyprina islandica, while the share taken by crustaceans and polychaetes shows a considerable further decline.

Thus, in contrast to other areas, Kiel Bay displays, above all, the following peculiarities:

a) T he very rap id and early decline of the share of crustaceans in the food. T he principal reason is tha t in the deeper part of Kiel Bay decapods, with the exception of Crangon crangon, are almost com­pletely lacking. Eupagurus bernhardus and Hyas araneus are restricted to the northern pa rt of the Bay and even there they are very rare. Carcinus maenas and Leander adspersus are shallow-water de­capods.

b) T he astonishing im portance of Cyprina islandica as food for the cod. This phenom enon was first described by Bagge (1965) and an a ttem pt was m ade to explain it by Arntz and W eber (1970). It looks as if various favourable factors are associated here: a very large stock of ra ther thin-shelled bi­valves, which have been m ade available to the fish through the otter boards used in the intensive trawling fishery.

c) T he relatively large share taken in the food by polychaetes. T he stock of polychaetes in Kiel Bay is still very prolific in contrast to the Baltic itself (Kühlmorgen-Hille, 1963, 1965; Arntz, 1971a). In the biomass it is exceeded only by molluscs. Presumably in Kiel Bay polychaetes, as does Cy­

prina, fill in the gap which, in comparison with other areas, exists through the lack of larger crust­aceans.

SEASONAL CHANGES IN T H E FO O D C O M P O S IT IO N

Various authors record seasonal changes in the type of food, although the changes seem to concern mostly fish, whereas benthonic food remains fairly constant. For instance, according to Rae (1967a), in the N orth Sea certain kinds of fish are to be found in the cod’s food especially in the summer and other kinds in the winter, while the proportion of benthos (mainly con­sisting of decapods reaching an age of several years) shows hardly any change. Hansen (1949) says tha t the same applies to the G reenland cod.

The conditions in the Central Baltic appear to be ra ther different. According to Demel and Piatek (1957), adult cod in deep w ater show a preference for herring throughout the sum m er and the autum n, bu t from the onset of w inter up to spawning-time they sustain themselves on mixed food in the coastal area. In juvenile cod, there is scarcely any evidence, according to R eim ann (1956), of any seasonal changes because of their m arked preference for crustaceans, which a l­ways constitute 9 0 % of their food; for the rest, they prefer fish in the w inter and polychaetes in the summer.

According to the study of the Kiel Bay benthos, which, because of the large share taken by organisms which become extinct within one year, is subject to considerable seasonal changes (Arntz, 1971a), and the food of the dab (Limanda limanda L.), which similar­ly shows a very varying composition of the food groups in the course of the year (Arntz, 1971b), seasonal differences in the food of the cod were also to be ex­pected.

T he m onthly share taken in 1968 by the four m ain systematic groups can be seen from Figure 7 for the various size groups of cod. T he graph a t the bottom summarises the conditions relating to all sizes covered.

In general, the following picture emerges: from spring to au tum n crustaceans are frequently consumed and in w inter on a smaller scale. Cyprina clearly reaches a peak in M ay. Polychaetes constitute the m ain share of food in the sum m er; fish in the winter.

From a detailed study of the different size groups some interesting results emerge :

a) In the case of crustaceans, th e - in it ia lly -q u ite heterogenous picture is to be explained by the varying preference for the two most im portant species, Diastylis rathkei and Gastrosaccus spinijer. T he cod in the two smallest size groups take ad ­vantage of the good supply of Gastrosaccus in May, whereas in the case of the larger fish the mysids play only a small role. O n the other hand, in all

16 W . E. Arntz

% OF IDENTIFIABLE

100 CRUSTACEA

XI 1.69VII IXI. 68 III V

100 POLYCHAETA

4 0

XI 1.69IXV VIIIII1.68

100

ALL SIZES OF COD *3 5 cm

40

XI 1.69VII IX1.68 III V

Figure 7. Annual cycle of the food composition according to size

cod, with the exception of the very smallest, the frequency m axim um of Diastylis, which has attained its full size a t this time, is reflected in November. In this m onth the small cod still prefer Gastrosaccus.

b) T he m axim um of Cyprina in M ay is clearly due to the poor supply of other benthos food in this month. This is also seen very well in adult cod (Arntz and W eber, 1970) ; here, the increased intake is ap ­parently additionally favoured by the starved state of the fish following spawning time.

c) T he very synchronous course taken by the poly-

STOMACH CONTENTS (g)

MOLLUSCA

XI 1.69IXVIIIII V1.68

PISCES

XI 1.69IVVIIV1.68 III

11 -1 5 cm

-o 16 - 20 " size

21 - 25 “ groups

■+ 31 - 35 "

•---------• Mollusca

•---------♦ Crustacea food •---------• Polychaeta items

•.......... * Pisces ------

groups of cod. Below left: Composition for all cod up to 35 cm.

chaete and fish curve for all size groups is quite extraordinary. I t can be satisfactorily explained, in the case of the polychaetes, as far as the most im­portan t species-Harmothoe sarsi-goes, by the fre­quency fluctuations in the course of the year; the other two outstanding polychaetes -Nephthys and Pherusa- c a n live for several years w ith no annual rhythm detectable. For the seasonal changes in the fish intake, Pomatoschistus minutus, which seeks out the deeper area of Kiel Bay only during the cold season, is m ainly responsible.

A contribution to the feeding ecology o f juvenile cod (Gadus morhua L.) in the western Baltic 17

sunrise

g ident.

s u n s e t

g ident.

- I 1- - - - - 1_ _ _ I_ _ _ I_ _ _ I_ _ _ L _12 K 16 18 20 22 24 02 04 06 08 10 12 h

- • 11-15 cm

16-20 cm

- a 21 -2 5 cm

-* 26-30 cm

-+ 31-35 crri

s u n s e t sunrise

2,0

Figure 8. Diel rhythm of the identifiable stomach contents according to size groups. 24-h fishings in November 1968(left) and M arch 1971.

T he extent to which endogenous factors (e.g. caloric requirements) influence the choice of food consumed by the cod cannot be clarified in the present study.

P E R IO D IC IT Y O F T H E D IE L FO O D IN T A K E

I have dealt in detail elsewhere with the question of the periodicity of the diel food intake by the cod (W. Arntz, In press). Here, therefore, I summarize only the m ain results (Fig. 8) :

Ju s t as the N orth Sea cod does (Graham , 1924; Rae, 1967a), the Kiel Bay cod feeds a t the time of the clear change in the intensity of light, a t dawn and dusk. D uring the first 24-hour fishing (November 1968), considerably more was eaten in the morning, whereas during the second fishing the am ount of food consumed in the m orning and the evening was about equal. In each case the various sizes behaved in very m uch the same m anner.

Larger cod (over 20 cm in length) have very clearly- defined feeding periods ; on the other hand, the smaller (below 20 cm) feed continually. This is probably due to the kind of food : the laborious picking-up of small crustaceans leads less quickly to a full stomach than does the gulping-down of larger particles or a com bina­tion of both methods of behaviour.

In both feeding periods, various types of food an i­

mals can be taken in. For instance, in November 1968, Diastylis rathkei and Gastrosaccus spinijer were found in the stomachs only in the m o rn in g -a t the end of their nocturnal vertical m ig ra tio n - ; the other types in the evening as well.

D IE L Q U A N T IT Y O F FO O D

As in the case of the diel rhythm of feeding, the literature on this question is also scanty. Blegvad (1916) finds tha t the stomach contents of the cod are between 2 °/0 and 4°/0 of the body weight; Strzyzewska (1962) says tha t they are between 1 °/0 and 5 °/0 and R eim ann (1956) gives between 1-1 °/0 and 2-3 °/0 (maximum values) for cod up to 25 cm in length.

T he food curves of the two 24-hour fishings show a clear decline in the quantities of food, in line with the increasing body weight of the cod (Table 6, Fig. 9). In November 1968, the values were between over 5°/0 (cod of 11 to 15 cm) and l-4°/0 (cod of 31 to 35 cm) and in M arch 1971, between 5-0 % and l '0 ° /0-

Chrzan (1962) and Strzyzewska (1962) say tha t the stomachs of older cod contained relatively more food than those of younger fish. This seemingly contradicts the opinion frequently pu t forward (for flatfish, e. g. by Bückmann (1952), Hempel (1956, 1970), and Pandian (1970)) tha t young fish take in more fo o d -in terms of

2

18 W . E. Arntz

% of body weight

5

Nov. 683

2 _ March 71

1

11-15 16-20 21-25 26-30 31-35

size groups (cm)

Figure 9. Daily food rates of cod as a percentage of the body weight according to size groups.

T able 6. Relation between length and weight of Kiel Bay cod (« = 365) *

L ength(cm)

W eight

(g)

L ength(cm)

W eight

(g)

Length(cm)

W eight

(g)

11 12 21 94 31 30512 18 22 112 32 34013 22 23 124 33 36014 27 24 136 34 39815 32 25 157 35 42716 39 26 171 - 17117 50 27 199 - -

18 55 28 215 - -

19 68 29 244 - -

20 72 30 277 - -

* M ateria l from Ju l., O ct. and Nov. 1969 an d M ar. 1970; all traw ling areas. F rom B. A rntz (In press).

a percentage of body weight—than do older fish. In ­deed, it frequently happens, under in situ conditions, th a t larger fish take considerably heavier pieces of food (cf A rntz, 1971b, for d ab ); however, in the older fish the quantities found need not have been taken in during the last feeding period b u t can have come from an earlier one (cf, e.g. W agner, 1959). Finally, with all these comparisons it is to be observed tha t in the litera­tu re cited here it is not a question of daily rates re ­

sulting from 24-hour com parative fishings b u t of stomach contents registered fortuitously at various times of the day.

V A R IA T IO N S IN T H E Q U A N T IT Y O F FO O D

IN T H E C O U R SE O F T H E Y EA R

M any authors allege tha t most of the food is taken in in the second half of the year (Strzyzewska, 1962; Chrzan, 1962; Rae, 1967a, b ; 1968) or in w inter (Reim ann, 1956). T he general opinion is th a t the smallest food intake coincides with spawning time which, in the different areas, lies between February and Ju n e (Demel and Piatek, 1957; Wise, 1961; Strzyzewska, 1962; Chrzan, 1962; Rae, 1967a, b; 1968).

For the juvenile cod dealt w ith here, a limiting factor cannot be the spawning time b u t possibly the m uch lower tem perature of the bottom w ater a t the end of the winter. In agreem ent with the literature, the daily rates calculated from the findings of the two com parative fishings (Fig. 9) show considerably higher values for November than for M arch. As long as no further 24-hour fishing are available, no reliable de­tails can be given for the rest of the year.

T he percentage of the em pty stomachs (Table 7), which is, however, used only conditionally for state­ments about the quantities of food, shows an annual

T able 7. Percentage of em pty stomachs in the annual cycle 1968. All trawling areas

M onth

Cod 11- 35 cm Cod > 35 cm

N um berinvestigated

% em pty N um ber % em pty investigated

J a n ----- 79 1-3 279 34-4F e b . . . . 47 0-0 303 31-4M a r . . . 108 0-0 366 33-3A pr . . . 24 4-2 62 17-7M a y . . . 101 4-0 197 9-1J u n ----- 16 6-3 4 0-0J u l . . . . 90 3-3 146 4-8Aug . . . 25 0-0 282 12-1S e p . . . . 117 6-8 192 10-9O ct . . . 55 1-8 47 12-8Nov . . . 309 0-0 174 5-7Dec . . . 70 8-6 75 17-3

course only for cod over 35 cm in length. In the juve ­nile cod dealt with here, the em pty stomachs constitute under 10°/0 the whole year round.

F U T U R E W O R K O N K IE L BAY CO D

T he present study brings us a step nearer the aim, outlined a t the beginning, of an analysis of the benthos

A contribution to the feeding ecology of juvenile cod (Gadus morhua L.) in the western Baltic 19

exploitation in Kiel Bay. I t makes clear, however, tha t there are a num ber of im portant questions to be clarified.

a) First of all there must be an evaluation of the com­position of the food taken in by adult cod. The investigations m ade by Bagge (1965, 1966) provide some starting-points for the northern pa rt of Kiel Bay. I t is to be assumed that, w ith the exception of Cyprina, more benthos is consumed by juveniles than by adult cod because the share taken by fish in the older cod greatly increases.

b) M ore 24-hour fishings will have to be carried out, above all in the summer, in order to provide a reliable foundation for estimating the diel food intake a t various times of the year.

c) W ith the assistance of the population dynamists there must be a clarification of the dimensions of the 1968 stock of cod in Kiel Bay and to w hat extent there is an exchange with other areas and between shallow and deep water. A num ber of investigations into this complex of questions have been carried out {inter alia, by K ändler and Thurow, 1959; Thurow, 1963, 1970; Berner, 1967, 1968; Arntz and Hempel, 1972); for the moment, however, they allow only a very rough estimate to be m ade of the im portance the stock of cod has for the deeper part of Kiel Bay.

R E F E R E N C E S

Arntz, B. 1972. Fischotolithen als Hilfsmittel in der Fischerei­forschung. Diplomarbeit nat.-m ath. Fakultät Univ. Kiel, 56pp.

Arntz, W . E. 1971 a. Biomasse und Produktion des M akrobenthos in den tieferen Teilen der Kieler Bucht im J a h r 1968. Kieler Meeresforsch. 27: 36-72.

Arntz, W . E. 1971b. Die N ahrung der Kliesche (Limanda limanda (L.)) in der Kieler Bucht. Ber. dt. Wiss. K om m n Meeresforsch. 22: 129-183.

Arntz, W . E. (In press.). Periodicity of the Daily Food Intake by the Cod (Gadus morhua L.) in Kiel Bay. Oikos suppl.ser.

Arntz, W . E. & Hempel, G. 1972. Biomasse und Produktion des Makrobenthos in der Kieler Bucht und seine Verwertung durch Nutzfische. Verh. dtsch. zool. Ges. 1971: 31-37.

Arntz, W . E. & W eber, W . 1970. Cyprina islandica L. (Mollusca, Bivalvia) als N ahrung von Dorsch und Kliesche in der Kieler Bucht. Ber. dt. Wiss. K om n Meeresforsch. 2 1 :193-209 .

Bagge, O . 1965. O m molboøstersens betydning som fødedyr. Skr. Danm . Fisk.- og Havunders. 26: 12-16.

Bagge, O . 1966. Cyprina islandica as Food for Cod in the Western Baltic. ICES CM 1966/D:15, 3 p p . (mimeo).

Berner, M . 1967. Dorschmarkierungen in der Arkonasee im Ja h re 1962. Fischerei-Forsch. 5: 17-21.

Berner, M . 1968. Dorschmarkierungen in der M ecklenburger Bucht im Ja h re 1965. Fischerei-Forsch. 6: 73-75.

Blegvad, H. 1916. O n the Food of Fishes in the Danish W aters within the Skaw. Rep. Danish biol. Sta. 24: 19-72.

Biickmann, A. 1952. Vorläufige M itteilung über Fütterungs­und W achstumsversuche mit Schollen im Aquarium. Kurze M itt. FischBiol. Abt. M .P .I. Meeresbiologie 1: 8-21.

Chrzan, F. 1962. The Food and Feeding of Cod in the G ulf of Gdansk. Prace morsk.-Inst. ryback. Gdyni 11 /A: 161-201.

Demel, K . & Piatek, W . 1957. Cod Catches in the Bay of Gdansk in 1949-1953 in the Aspect of Hydrographic and Cli­matic Factors. Prace morsk.-Inst. ryback. Gdyni 9: 79-102.

G raham , M . 1924. The Annual Cycle in the Life of the M ature Cod in the N orth Sea. Fish. Invest., Lond. (2), 6: 77 pp.

Hansen, P. M . 1949. Studies on the Biology of the Cod in Green­land Waters. R app. P.-v. R éun. Cons. perm. int. Explor. M er 123: 83 pp.

Hempel, G. 1956. Studien zur Tagesperiodik der Aktivität von Fischen. II . Die Nahrungsaufnahm e der Scholle. K urze M itt. Inst. FischBiol. Univ. H am b. 6: 22-37.

Hempel, G. 1970. Introduction. In M arine Food Chains. Ed. by J . H . Steele, Oliver & Boyd, Edinburgh, 255-260.

Herding, H . 1928. Quantitative Nahrungsuntersuchungen an Pleuronektiden und einigen anderen Fischen der Ostsee. Ber. dt. Wiss. Kom m n Meeresforsch N.F. 4: 25-121.

Hertling, H. 1929. Die N ahrung der Ostseefische. M itt. dtsch. SeefischVer. 45: 101-122.

Hessle, C. 1923. Undersökningar rörande torsken (Gadus callarias L.) i mellersta Östersjön och Bottenhavet. M edd. Kungl. Lant- bruksstyrelsen 243: 19-74.

Kändler, R . & Thurow, F. 1959. O n the Stock of Flatfish and Cod and the Yields of the G erm an Fishery in the Baltic. R app. P.-v. R éun. Cons. perm. int. Explor. M er, 147: 24-38.

Kühlmorgen-Hille, G. 1963. Quantitative U ntersuchungen der Bodenfauna in der K ieler Bucht und ihre jahreszeitlichen V er­änderungen. Kieler Meeresforsch. 19: 42-66.

Kühlmorgen-Hille, G. 1965. Qualitative und quantitative V er­änderungen der Bodenfauna der Kieler Bucht in den Jah ren 1953-1965. Kieler Meeresforsch., 21: 167-191.

Pandian, T. J . 1970. Intake and Conversion of Food in the Fish Limanda limanda Exposed to Different Tem peratures. M ar. Biol 5: 1-17.

Rae, B. A. 1967 a. The Food of Cod in the N orth Sea and on West of Scotland Grounds. M ar. Res. 1967 (1): 68 pp.

Rae, B. A. 1967b. The Food of Cod on Faroese Grounds. M ar. Res. 1967 (6): 23 pp.

Rae, B. A. 1968. The Food of Cod in Icelandic Waters. M ar. Res. 1968 (6): 19 pp.

R eim ann, Z. 1956. N utrition and Food of Young Cod (Gadus cal­larias L.) o f the Southern Baltic. Prace morsk.-Inst. ryback. Gdyni 8: 171-186.

Strzyzewska, K . 1962. Nourishment and Food of Southern Baltic Cod. Prace morsk.-Inst. ryback. Gdyni 11 A : 387-421.

Thurow, F . 1963. Dorschmarkierungen in der westlichen Ostsee. Inform. Fischw., 10. Jg : 12-17.

Thurow, F. 1970. Ü ber die Fortpflanzung des Dorsches (Gadus morhua L.) in der Kieler Bucht. Ber. dt. Wiss. K om m n Meeres­forsch. 21: 170-192.

Todd, R . A. 1907. Second R eport on the Food of Fishes (North Sea 1904-1905). In tern .Invest.M ar.b io l.A ss.U .K . 2: 20-163.

W agner, G. 1959. Untersuchungen über die Tagesperiodizität der Nahrungsaufnahm e bei Pollachius virens (L.) K urze M itt. FischBiol. Univ. H am b., 9: 61-103.

Wise, J . P. 1961. Synopsis of Biological D ata on Cod, Gadus morhua Linnaeus 1758. FA O Fisheries Biology Synopsis 21, Rome.

2*