Fisheries Research, 5 {1987) 235-242 235 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

G r o w t h and Maturat ion of N o r w e g i a n Coastal Cod and Northeas t Arct ic Cod under Di f ferent Condi t ions

OLAV RUNE GODO

Institute of Marine Research, P.O.Box 1870, N-5011 Nordnes (Norway)

ERLEND MOKSNESS

Flodevigen Biological Station, N-4800 Arendal (Norway)

ABSTRACT

God~, O.R. and Moksness, E., 1987. Growth and maturation of Norwegian coastal cod and North- east Arctic cod under different conditions. Fish. Res., 5: 235-242.

The Norwegian coastal cod and the Northeast Arctic cod have been considered as two popula- tions which can be separated by otolith studies. They are, in addition, characterized by differences in growth, age at first spawning and migration pattern. The environmental conditions under which the two groups live before maturation differ considerably.

In this paper, growth and age at first spawning of cod from both groups, as well as of progeny from crossings between them, were studied under controlled laboratory conditions. The results show that growth increased and age at first spawning decreased considerably under favourable laboratory conditions compared to observations in the field. It is further indicated that the differ- ences in growth and age at first spawning for coastal and Northeast Arctic cod reflect differences in the environment rather than genetic variations.

INTRODUCTION

In the history of fish management, the separation of stocks or management units has been an important subject of investigation. Biological parameters like colour, growth, age and size at maturity, number of vertebrae as well as variation in migration and year-class strength have been used to characterize cod populations ( Schmidt, 1930; Rollefsen, 1933, 1934 ). In western and north- ern Norway, the Northeast Arctic cod (NAC) and the Norwegian coastal cod ( CC ) have been separated by differences in otolith patterns. This method was later confirmed by analyses of haemoglobin polymorphism (Moller, 1968, 1969). The two groups are further characterized by differences in growth and age at first spawning, i.e., CC grow faster before maturat ion and reach maturation at an earlier age (Rollefsen, 1934; Godo, 1984). The genetic basis of these differ-

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ences has recently been questioned by Mork et al. (1985). An environmental effect has been observed in otolith zonation (Trout, 1957 ), on biological char- acters (Iversen and Danielssen, 1979; Howell, 1984; Braaten, 1984) and on haemoglobin polymorphism ( Karpov and Novikov, 1980).

In this paper, the influence of environmental factors on growth and matur- ation was examined utilizing laboratory experiments with NAC and CC as well as the offspring obtained by crossing.

METHODS

Laboratory data

In the period 1976-1985, several groups of cod were raised in the laboratory at Flodevigen Biological Station, Arendal, Norway. Most of the groups were offspring of CC caught outside Flodevigen, while two groups were progeny of a crossing between NAC and CC caught off More (Fig. 1). The crossings were carried out in 1981 and 1984 with a number of individuals, which were sup- posed to be representative of actual spawning populations in the area. One laboratory group consisted of 1-year-old NAC caught in the Barents Sea in October 1984 (Fig. 1 ). One group of NAC, mainly 3 year olds, was transferred from the Barents Sea to Bergen in 1978 (later called the Bergen Group) and kept under environmental conditions similar to those described for the Flodevigen experiments. The number of fish in the different laboratory groups varied between 6 and 137.

The fish were kept in tanks from 601 to 5 m 3 and transferred to bigger tanks according to increasing fish size. The salinity was fairly constant ( 33.8-34.0%~ ) and the temperatures varied from 4°C in March/April to 12°C in September/October, throughout the years. The larvae were fed wild zooplank- ton, while the juveniles and adults were fed artificial food. All groups were given an excess of food. Length and weight of the individuals were measured every second month, except for the Bergen Group which were measured three times: September 1978 and June and December 1979.

Field data

Data on the natural cod populations off Arendal were obtained from the literature (Dannevig, 1953; Moksness and Oiestad, 1984). Biological data on cod off More were collected in the period 1979-1983 and information on NAC in the Barents Sea was taken from published sources and 1978-1984 survey results (Hylen and Nakken, 1982; Nakken and Raknes, 1984).

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RESULTS

Natural populations

Growth curves and maturity ogives for CC off More and NAC are presented in Fig. 2. The CC maturity ogive represents the percent mature in the various age classes in the population in January-February prior to spawning. Most CC are mature by the fifth year of life. The maturity ogive for the 1982 spawning stock of NAC is adopted from Hylen and Nakken (1982) and it shows that NAC mature roughly 3 years later than the More CC.

The growth of NAC is demonstrated by the age-length relationship of the 1975 year classes (Fig. 2 ). Growth observations (mm day- 1 ) of NAC in the field are also given in Tables I and II and a considerable difference in growth between year classes is indicated.

238

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Fig. 2. Growth curves {broken line) and maturity ogives (solid line) for CC caught off More in 1979-1983 ( O ) and NAC caught in the Barents Sea in 1979-1984 ( • ) .

Field observations on growth and maturation of cod from various areas are presented in Table I. The table shows that NAC generally have slower growth and mature at a later age than the CC populations investigated.

Laboratory groups

Some variation in growth between the different laboratory groups was observed. The growth rates were, however, higher than those observed in the field and all individuals hatched and raised under laboratory conditions matured as 2 year olds. The results are summarized in Table II.

Otolith studies showed that the Bergen Group consisted primarily of age -3 fish. However, some older individuals were present in the group and resulted in a slightly higher mean length than was observed for age-3 fish in the field (1975 year class ) at the time of transfer (Fig. 2, Table III ). Assuming that the presence of individuals older than age 3 in the Bergen Group did not result in bias, the results imply a nearly 100% increase of the growth rate compared to the field observations (Table II). Table II also shows that for the 1983 year class, its growth rate tripled from 0.40 mm day-1 in the field observations to

239

TABLE I

Calculated mean growth ( mm day- 1 ) from hatching to maturation and age (years) at first spawn- ing for cod from various areas

Area Mean growth Maturation (years) Reference

(ram day -1 ) 50% Range

North Sea d" 0.49 3 2-5 Daan (1974a) ;

4 3-6 Daan (1974b)

Southern Norway Skagerrak 0.42 -0.50 3 2-6 Dannevig

(1953) Flodevigen 0.42-0.49 Moksness and

Oiestad (1984)

Western Norway More Barents Sea

0.45 3-4 2-6 0.27 6-14 0.28 9 3-14

Rollefsen (1953) Ponomarenko et al. (1980)

1.16 m m d a y - 1 in t h e l a b o r a t o r y , w h i c h was t h e h i g h e s t g r o w t h r a t e o b s e r v e d

t h r o u g h o u t t h e l a b o r a t o r y s tud ie s .

TABLE II

Growth and maturation of the fish in the laboratory groups. Field observations of the growth rate of the 1975 and 1983 year classes of NAC are included. The mean number of individuals in each group (N) and the mean fish length at the beginning of the experiments (L) are given

Group Year Year N L Growth Age at class (cm) (mm day- 1 ) spawning

CC Fledevigen Vat. 43 0 0.52-0.74 2 CC × NAC More 1981 137 0 0.57 2

1984 30 0 0.75 2 NAC nat. pop. a 1975 0.30 6-8 NAC lab. b 1975 58 41 0.59 5 NAC nat. pop. c 1983 0.40 NAC lab. d 1983 6 18 1.16

a Mean growth in the period May 1975-September 1978. b Mean growth in the laboratory in the period September 1978-December 1979. c Mean growth in the period May 1983-October 1984. d Mean growth in the laboratory in the period October 1984-April 1985.

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TABLE III

Growth and maturation of the Bergen Group and the NAC 1975 year class. Figures in parentheses refer to the month of measurement

Year Mean length (cm) Percent mature

Bergen Field Bergen Field a Group Group

1978 41 b (9) 32(2) - 0 1979 58 (6) 42(2) + 5

65 (12) 1980 52(2) 100 10 1981 61(2) 34 1982 65

aAssuming that the 1975 year class followed the maturation ogive of the 1982 spawning stock. bSize at transfer.

DISCUSSION

Growth of cod is dependent on temperature and food availability. The tem- perature dependency is demonstrated by Iversen and Danielssen (1979) and Howell (1984) and an optimal temperature of 12-15 °C for the Flodevigen cod was indicated by the former authors (0-group cod). Braaten (1984) showed that excess feeding enhanced the growth rate of CC. Increased growth, as a result of increased food availability in the field, is discussed for the NAC by Ponomarenko et al. (1980). Blindheim et al. (1981) studied environmental differences and variations along the Norwegian coast. Considerable geographic and seasonal differences in temperature were found and variation in growth in cod populations, both within and between the areas studied here, must be expected. The variations in the field observations of growth indicated in Fig. 2 and Table I are thus according to expectations.

The observations on growth rate and maturation (Fig. 3, Tables I and II) indicate an inverse relationship between growth rate before maturation and the age at first spawning, i.e., CC grow faster and mature at an earlier age than the NAC. Periodic variations in growth and age at first spawning for cod have been observed and discussed by various authors (Borisov, 1978; Ponomarenko et al., 1980. Hylen and Nakken, 1982). In addition, variation has also been observed among CC from different locations (Dannevig, 1953; Otterbech, 1953 ). An important question in this connection is to what extent the observed variations are of genetic origin or a result of different environments.

Unfortunately, our laboratory groups did not include a group of NAC raised from eggs to mature individuals. If no environmental effect on the age at first spawning is assumed, the age at first spawning for the CC × NAC groups should be between theage of first spawning observed for NAC and CC. It is assumed that no selection diminished the part of the NAC × CC group with highest age

241

at first spawning specifically. This possibility cannot be fully excluded, but we consider the probability for such selection to be low, based on the observation that no specific mortality was observed in this raising experiment compared to the others. Furthermore the NAC groups transferred to the laboratory from the field showed vigorous growth under similar conditions.

The results show that under similar and favourable growing conditions, when various CC and NAC X CC groups are compared, no systematic differences in growth and age at first spawning are revealed. This implies that cod groups with considerable differences in growth and age at first spawning (about age 7 and age 4 for NAC and CC, respectively) in the field, will, under favourable conditions, have similar growth rates and spawn for the first time at an age of 2 years.

When transferred to the laboratory from the sea, a doubling and tripling of the length increment per day was observed for two groups of NAC. The 1983 year class showed the highest measured growth when all experiments are com- pared. The transfer of the 1975 year class (Bergen Group) as 3 year olds indi- cated that an improvement of environmental conditions at any stage in the immature life of the cod accelerates maturation considerably.

In the data of Ponomatenko et al. (1980) and Nakken and Raknes (1984), an inverse relationship between growth rate and age at first spawning in NAC was indicated. Our field data indicate the same relationship and the laboratory data demonstrate the dramatic influence of the environment (temperature) and the presence of unlimited food availability on growth and maturation.

The differences between NAC and CC observed in the field seem to be elim- inated under similar environmental conditions, indicating that the differences observed in the field are probably not of genetic origin.

ACKNOWLEDGEMENTS

We would like to thank the Fish Behaviour Group at the Institute of Marine Research Bergen, through their leader P. Solemdal, for kindly letting us use their data on the 'Bergen Group' of Northeast Arctic cod. We would also like to thank D.S. Danielssen and J. Gjoseter for valuable criticism of the manucript.

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