ICES mar. Sei. Symp., 198: 510-519. 1994

Vertebrae numbers as an indicator for the recruitment mechanism of coastal cod of northern

Solveig Løken, Torstein Pedersen, and Erik Berg

Løken, S., Pedersen, T ., and Berg, E. 1994. Vertebrae numbers as an indicator for the recruitment mechanism of coastal cod of . - ICES mar. Sei. Symp., 198: 510-519.

Cod (Gadus morhua L.) in northern Norway are divided into two main groups (coastal cod and Northeast cod), but both groups are managed as a single stock. The coastal cod inhabit coastal and fjord regions and differ from Northeast Arctic cod with respect to growth pattern, age and size at maturity, and migration and spawning areas. Cod from the coastal and Northeast Arctic groups have been distinguished on the basis of meristic characters (average number of vertebrae), otolith type, and habitat selection during different life stages. We investigated whether the coastal cod are self­ recruiting, or whether this group also contains vagrants from the larger group of Northeast Arctic cod. Cod juveniles in coastal areas segregate vertically during the settling process, and the juveniles that settle in shallow-water sites have morphometric characteristics similar to those of adult coastal cod. The effects of topography and depth that influence separation are discussed. It is concluded that coastal and Northeast Arctic cod recruits have different early life histories.

Solveig Løken, Torstein Pedersen, and Erik Berg: Norwegian College o f Fishery Science, University o f Tromsø, N-9037 Tromsø, Norway.

Introduction CC also differ from NA with respect to growth pattern and age and size at sexual maturity (Rollefsen, 1934, Cod (Gadus morhua L.) in northern Norway are divided 1954; Hylen, 1964a). Jakobsen (1987) points out that the into two main groups: Northeast Arctic cod (NA) and crucial question of whether CC and NA should be coastal cod (CC) (Rollefsen, 1934, 1954). CC inhabit regarded as separate stocks is dependent upon “whether coastal and fjord areas and undertake only short-range fry from some of the main spawning grounds of North­ migrations. The larger NA stock, however, migrates east Arctic cod drift to nursery areas on the coast or in from the feeding areas in the Barents Sea and near the fjords and acquire the characteristics of coastal cod Svalbard to spawning areas along the Norwegian coast from the environment”. In order to identify whether (Hylen, 1964a, b; Godø, 1986; Jakobsen, 1987; Berg­ vagrant juveniles originating from NA spawning areas stad et al., 1987). CC spawn in several fjords along the inhabit the CC habitat in fjords and along the coast, an coast (Rollefsen, 1954; Jakobsen, 1987), and large num­ identifying marker for NA juveniles is needed. bers of them spawn in the most important NA spawning Meristic characters have frequently been used in in­ area in Lofoten-Vestfjorden (Hylen, 1964a). The larval vestigations of the structure of cod stocks in the western and juvenile drift from the NA spawning grounds on the Atlantic (Templeman, 1981, 1983; Lear et al., 1981; Norwegian coast towards the NA juvenile habitat in the Lear and Wells, 1984). Vertebrae number in fish is fixed Barents Sea and near Svalbard have been well described during the early part of the embryonic period (Fahy, (Sundby et al., 1989). In contrast, there is very little 1976), and differences in the average number of ver­ information on larval and juvenile drift from the CC tebrae (AV) between the groups of cod have been spawning in fjords and it is uncertain whether the CC attributed to differences in temperature during the em­ group is mainly self-recruited or recruited by vagrants bryonic period (Dannevig, 1947; Brander, 1979; Lear et from the NA spawning. The separation of cod into CC al., 1981). In support of this. Brander (1979) found a and NA is based on differences in the sizes and shapes of negative correlation between field data on temperature otolith zones (Rollefsen, 1934), which are influenced by during the spawning period and AV in cod stocks. environmental factors (Godø and Moksness, 1987). The Investigations carried out in northern Norway during ICES mar. Sei. Symp.. 198(1994) Vertebrae numbers as an indicator 511 the 1920s and 1930s indicate that NA cod have higher Bottom-settled cod juveniles were caught in Malan­ AV than CC (Schmidt, 1930; Rollefsen, 1934, 1954). gen, Sørfjorden, and other localities around Tromsø in The biological significance of the differences in AV September and October 1992 (Fig. 1). Bottom trawling found in adult NA and CC are uncertain. The aim of this was used to obtain samples from deep water (>55 m), study is to investigate whether AV could be used as a and beach seining was employed in shallow water (0-6 natural mark to trace cod juveniles settling to the bottom m). Cod juveniles collected by beach seine in in the CC habitat. After settlement, the group identity during 1992 were obtained from Finnmarksforskning, (CC or NA) could be determined by the otolith struc­ , and six samples taken by Danish seine on ture. Vertebrae numbers were measured to assess the the coast of Finnmark in 1990 were provided by the variability in vertebral numbers of CC and NA in their Institute of Marine Research in . feeding and spawning areas. Young cod (age 1 + ) were sampled from various locations ranging from the inner parts of fjords to the outer coast of Nordland, , and Finnmark in 1991— Materials and methods 1993 (Fig. 1). Samples were taken with bottom trawling, and with Danish seine, at various depths. Spawning cod were collected on the spawning grounds in In the laboratory, fish length, weight, and sex were Lofoten and in March and April during the recorded and then flesh was cleaned off the bones to years 1991-1993 (Fig. 1). Cod were fished with a pelagic facilitate counting the number of vertebrae. 0-group trawl at a depth of 70 m in Lofoten, and using a bottom cod were examined under a binocular microscope. All trawl in Malangen. vertebral counts include the urostylar half-vertebrae. Pelagic juveniles collected from the Norwegian coast Otoliths were used for age determination and for charac­ and Barents Sea during July and August/September in terizing the fish as either coastal cod or Northeast Arctic 1991-1992 were obtained from the Institute of Marine cod based on the growth pattern criteria described by Research, Bergen. Juveniles were also collected at these Rollefsen (1933, 1934). If available, 30 fish were ana­ times at locations along the coast and in the fjords of lysed in each sample of 0-group fish. Troms and Finnmark. All pelagic cod were taken in Analysis of variance (ANOVA) was used to test for pelagic trawls at 0 to 60 m depth, in hauls lasting one half effects of group identity, year class, and year of sampling to one hour. on vertebrae number for cod sampled on the spawning

BARENTS SEA

500 Tromsøflaket -200 -

Ullsfiôrd Tromsø / •70‘ Altafjori Varanger Vågsfjord Sørfjori

69' lalanqen

2 00 '

40 80 km 68'

20 ' 24' Figure 1. Overview of the area sampled 512 Figure 2. Comparison of number of vertebrae (average vertebrae number AV ± 2 X SE(AV)) in different year in year different classesX2 SE(AV)) of ± coastal AV number vertebrae (average of of vertebrae number Comparison 2. Figure cod (circles) (CC) Arctic (squares) sampled cod and Northeast on (NA) the spawning in1991-1993grounds Lofoten and Malangen 1992-1993.sizesSample given.are

. øe, . eesn adE Berg E. and Pedersen, T. Løken, S.

VERTEBRAE NUMBER 55 54 53 52 54 55 53 55 52 54 52 5* 120 112 ooe 19 Mlne 1993 Malangen 1993 Lofoten 1991 Lofoten 117- 4- ooe 19 Mlne 1992 Malangen 1992 Lofoten G (er) G (years) AGE (years) AGE 1 H116 0 3 93 30 0 7 03 113 l|4 ,114 3« 30 2 54 53 54 53 52 52 70 020 20 50 - B20 2 B 2 - ' - - 300,00 250 3» ------50 0 2 CSmr e.Sm. 198 (1994) Symp., Sei. mar. ICES 20 ICES mar. Sei. Symp., 198 (1994) Vertebrae numbers as an indicator 513 grounds. Because of the unbalanced data set it was Table 2. Analysis of variance testing the effects of year class necessary to restrict the ANOVA to year classes that (1986-1987), sampling year (1992 and 1993) and sampling were present in all sampling years. A probability level of locality on vertebrae number in coastal cod (CC) sampled at the spawning grounds in Lofoten and Malangen. SS = sum of 5% was taken as indicating statistical significance. All squares, DF = degrees of freedom, MS = mean square, F = statistical tests were performed using SYSTAT (Wilkin­ Fisher ratio, p = significance level, n = 103. son, 1990). Source SS DF MS F p

Year class 0.002 1 0.002 0.003 0.959 Results Sampling year 5.938 I 5.938 7.419 0.008 Sampling locality 20.512 1 20.512 25.629 <0.001 Cod sampled on the spawning grounds Error 79.234 99 0.701 In cod sampled on the spawning grounds in Lofoten and Malangen, there was no statistical significant difference in vertebrae number between female and male fish for either NA or CC (ANOVA). ANOVA on the material from Lofoten was restricted The cod juveniles sampled along the coast of Finn­ to the year classes 1983, 1984, and 1985. The ANOVA mark in 1990 at depths of 15-30 m had AV values revealed that there was a highly significant effect of group ranging from 52.6 to 53.0. Samples of settled cod juven­ identity (p < 0.001), a just significant effect of year class iles that were taken at intermediate water depths (26- (p = 0.044), but no significant effect of sampling year in 110 m) in autumn 1991 had AV values ranging from 52.9 the material from Lofoten (Table 1). To investigate to 53.3. These values increased with increasing depth. whether vertebrae number differed between spawning Cod taken in northern Norwegian fjords showed con­ locality, years of sampling, and year classes, an ANOVA sistent variations between the AV seen in different was performed on the CC year classes 1987 and 1988 sampling gears (Fig. 4). Cod taken in beach seine sampled in Malangen and Lofoten. This analysis was samples in shallow water in 1992 had consistently lower restricted to CC because in the samples from Malangen AV than those caught by bottom trawling in deeper in 1992 and 1993, 83 and 95% of the cod were character­ water (Fig. 4). In Sørfjorden, the AV of pelagic juveniles ized as CC. The ANOVA revealed a significant effect of in July was intermediate between that of bottom-settled sampling year (p = 0.008) (cod sampled in 1992 had juveniles sampled in the sublittoral by beach seine, and lower value than those sampled in 1993), and sampling the AV of juveniles sampled by bottom trawling in deep locality (p < 0.001) (CC sampled in Malangen had lower water during August-September at depths greater than vertebrae counts than those sampled in Lofoten) (Table 60 m (Fig. 4). 2, Fig. 2). Cod (age 1+) on the feeding grounds Pelagic and bottom settled 0-group juveniles In most samples taken in various areas and depths, AV The juveniles that drifted towards the Barents Sea and was below 53.0 (Fig. 5). In Malangen, Altafjord, and in Svalbard area had relatively homogeneous AV values samples taken along a depth gradient from the Finnmark both in 1991 and 1992, compared with the cod juveniles coast to the Barents Sea, there was a positive trend sampled in the fjords and coastal areas (Fig. 3A and 3B). between AV and depth (Fig. 5).

Discussion Our results for AV of CC and NA are similar to those Table 1. Analysis of variance testing the effects of year class reported earlier for cod in northern Norway (Schmidt, (1983-1985), group identity (CC or NA), and sampling year (1991,1992, and 1993) on vertebrae number in cod sampled on 1930; Rollefsen, 1934, 1954), indicating a relative stab­ the spawning grounds in Lofoten. SS = sum of squares, DF = ility in the observed AV values in CC and NA over a long degrees of freedom, MS = mean square, F = Fisher ratio, p = time period. The AV values found in CC in northern significance level. n = 429. Norway are similar to or slightly higher than those of cod in western and mid-Norway (Schmidt, 1930). Source SS DF MS F P The differences in vertebrae number between spawn­ Year class 3.513 2 1.757 3.138 0.044 ing CC and NA caught in Lofoten and between spawn­ Group identity 10.768 1 10.768 19.235 <0.001 ing CC caught in Lofoten and Malangen (Fig. 2, Tables 1 Sampling year 0.732 2 0.366 0.654 0.521 and 2) suggest that the fish may have had a different Error 236.813 423 0.560 prehistory. NA in Lofoten had higher vertebrae num- 514 S. Løken, T. Pedersen, and E. Berg ICES mar. Sei. Symp., 198 (1994)

75

Sørfjord n = 3 1 74' « Sørfjord/ i_ Q n = 90 □ cc < l_ Sørfjord/ 73' q Ullsfjord n=66 _i ^ Malangen/ O Balsfjord n - 150 <5 LL) a; ^ Kvalfjord cr n -30 < 2 Sørøya/ z Revsbotn. n = 72 lZ 70' 52 53 54 Vertebrae number 69' Figure 4. Vertebrae numbers (average number of vertebrae (AV) ± 2 x SE(AV)) of pelagic juveniles caught in July 1992 (circles), bottom-settled juveniles caught by beach seine in 0 0 3 a # shallow water (triangles), and juveniles caught by bottom trawl 52.0 52.5 53.0 53.5 54.0 in deep water (squares). 67 20 22 24 26 28'

bers than CC caught in Lofoten, and CC in Malangen had lower values than CC in Lofoten. It is known that exposure to low temperatures during embryonic devel­ opment induces high vertebrae numbers (Brander, 1979; Løken and Pedersen, unpubl.). However, water temperatures at the CC spawning grounds in fjords tend ^Svalbard to be lower in April than those recorded on the NA spawning grounds (Sundby and Bratland, 1987). The data that exist on the timing of spawning by CC in fjords Barents Sea indicate that the CC spawn during the same time period (March-April) as cod in Lofoten (Skreslet and Danes, 1978; Falk-Pedersen, 1982; Kjørsvik et al., 1984). Since low temperatures during egg development induce high vertebrae numbers, it could be expected that CC would have higher AV than NA, if the juveniles that recruit to the CC group are mainly a result of CC spawning in the fjords in March-April. Contrary to this expectation, the lowest AV values were found in samples of pelagic juveniles taken near the coast and in the fjords (Fig. 6), and it is unlikely that differences in water temperature during embryonic development have induced the differ­ ences in AV between CC and NA. Some samples of pelagic juveniles in the fjords and at 0 0 3 ®# the coast were characteristic of NA (high AV), some 52.0 52.5 53.0 53.5 54.0 were intermediate, indicating mixing, and some were Figure 3. Horizontal distribution of average number of ver­ characteristic of CC (low AV) (Figs. 3 and 6). N A spawn tebrae (AV) in samples of pelagic juveniles in 1991 and 1992: in several localities from the Møre area (ca. 63°N) in the (A) juveniles sampled in June-July, (B) juveniles sampled in August-September. The main concentration of pelagic juven­ south to Sørøya in Finnmark (ca. 70°30'N) in the north iles in August-September 1991 is indicated by a stippled line (Godø, 1986; Sundby and Bratland, 1987), and the (ICES, 1991). Samples taken in 1992 are hatched. northward drift of eggs, larvae, and small juveniles ICES mar. Sei. Symp., 198 (1994) Vertebrae numbers as an indicator 515

Lofoten/Vesterålen Vågsfjord Malangen

100

200 i • h * ~ l

300

400

500 Balsfjord Sørfjord Ullsfjord

100

E 200

300

400

500

Altafjord Porsanger Finnm.coast/Barents Sea

100

200

300

400

500 52 53 54 55 52 54 55 53 54 55 Vertebrae number Figure 5. Distribution of average vertebrae number (AV) in age 1+ cod with increasing depth in different geographical regions (average number of vertebrae AV ± 2 x SE(AV) are shown). covers a large area that often continues into the fjords in AV values in samples of pelagic juveniles in July 1992 northern Norway (Sundby et al., 1989). Eggs spawned were intermediate to the low AV values for cod sampled on CC spawning grounds in the fjords may partly be in beach seine and the high AV recorded in samples advected out of the fjords during the egg stage (Skreslet from bottom trawling (Fig. 4). Thus, juveniles with low and Danes, 1978), and are likely to mix with offspring and high vertebrae numbers appear to segregate during from the NA spawning grounds. The difference in ver­ the bottom settling process, probably reflecting differ­ tebrae numbers between mature CC caught in Lofoten ent settling strategies in juveniles with low and high and CC caught in Malangen, as well as the difference vertebrae numbers. A vertical segregation of low and between sampling years, suggest that the CC group is a high AV 0-group cod was maintained in larger cod (age heterogeneous group with regard to vertebrae number. 1+) in some deep fjords (Fig. 5). High AV cod were 516 S. Løken, T. Pedersen, and E. Berg ICES mar. Sei. Symp., 198 (1994)

SPAWNING COD

O Lofoten NA ft* 11 E3 CC w m i=— ■ Malangen CC

w

PELAGIC JUVENILES July-Sept. 1991

□ Tromsøfl.-Bar.Sea ■ Coast & Fjords

[23 Bar.Sea 1992

BOTTOM SETTLED JUVENILES ISl Danish seine 1990 0 B.Trawl 2 6 < Z < 1 1 0 m 1991 ■ Beach seine 1992

H_ EH B. Trawl Z>60m 1992 54 VERTEBRAE NUMBER

Figure 6. Overview of average number of vertebrae (AV) from samples of cod from the spawning grounds in Lofoten and Malangen, and samples of pelagic and bottom-settled 0-group juveniles. For spawning cod, each value represents a year class sampled in a particular year. For juveniles, cach AV-value represents one sample. The bottom-settled juveniles were caught off the coast of Troms and Finnmark. Samples with less than 10 individuals are not included.

found deeper than ca. 300 m in some fjord areas. In these 1987a, b, 1990; Keats et al., 1987), and the predation risk fjords, there is a gradual decrease in AV towards shallow may be maximal at intermediate depths, where the bulk depths, which indicates a mixture of juveniles with low of the large cannibalistic CC live (Bax and Eliassen, and high vertebrae numbers at intermediate depths 1990). Settlement at shallow depths similar to CC in (150-300 m). The results and interpretation of AV northern Norway is observed in the North Sea (Riley distribution with depth and age of cod are summarized in and Parnell, 1984), off the Skagerrak coast (Tveite, Figures 6 and 7. In the Barents Sea, most of the area 1971, 1984), and in (Godø et al., 1989). inhabited by the NA have depths between 150 and 400 m This shallow water settling strategy is very different from (Nakken and Raknes, 1987), while at the coast and in that found in NA juveniles in the Barents Sea, but it is the fjords, cod inhabit depths between 0 and 500 m uncertain whether this inter-stock difference in settling (Fig. 5). strategy is due to genetic differences between stocks. We suggest that in the fjords and at the coast it may be Haemoglobin allele frequencies (H bl1) found in cod advantageous (i.e. maximum fitness obtained) for an juveniles settled in deep and shallow water in northern individual to settle in the sublittoral while in the Barents Norway resembled H bl1 allele frequencies in larger Sea it may be advantageous to settle far to the east and juveniles and adult NA and CC, respectively (Møller, north in the Barents Sea, where the density of large 1966, 1968, 1969). Other polymorph proteins and mito­ potential cannibalistic cod is low (Nakken and Raknes, chondrial DNA reveal small and, rarely, statistical 1987; Mehl, 1989). In late August-early September, differences between samples from CC and NA (Jørstad, juveniles (ca. 4-8 cm length) in the fjords had settled at 1984, Mork et al., 1985; Jørstad and Nævdal, 1989; shallow depths and were caught with beach seine Johansen et al., 1990; Dahle, 1991), and the scale of samples (Fig. 4), while the juveniles in the Barents Sea genetic differentiation in cod in northern Norway is were still pelagic (Olsen and Soldai, 1989; Sundby et al., disputed (Mork et al., 1985). The degree to which 1989). The macroalgae belt in the sublittoral may pro­ vertebrae number, depth preference, and other life vide hiding places for small cod juveniles (Gjøsæther, history characteristics in NA and CC are genetically ICES mar. Sei. Symp., 198 (1994) Vertebrae numbers as an indicator 517

AGE (year)

DEPTH (m) Spawning

Figure 7. Conceptual model of CC recruitment mechanism. The interpreted relation between age, vertebrae number, otolith type, stock group (CC and NA), and depth distribution. O Coastal cod (low AV), immature; • coastal cod (sexually mature); □ NA cod (high AV), immature; ■ NA cod (sexually mature). Unfilled symbols denote pelagic individuals. Grey-filled symbols denote bottom-settled individuals. AV = average vertebrae number.

determined is unknown. The influence of parental group cod group. The juvenile segregation into CC and NA on vertebrae number in offspring can be tested experi­ appears to be due to different bottom settling strategies mentally by comparing AV in offspring from CC and in juveniles recruiting to these two groups. NA reared under identical environmental conditions during the embryonic period, and is the target of further studies by the authors. Age distributions at the spawning areas in Lofoten Acknowledgments (Hylen, 1964a) and fjords in Finnmark (Jakobsen, 1987) indicate a different mechanism in the recruitment of N A We thank A. Hylen, H. Bjørke, S. Sundby, T. Jakob­ and CC. In NA, year-class strength (measured as re­ sen, A. V. Soldai, and S. Olsen at the Institute of Marine cruits at age 3 years) is largely determined during the Research, Bergen, Y. Svendsen at Finnmarksforskning, early pelagic phase (Sundby et al., 1989). Recruitment in Hammerfest, and J. E. Eliassen at the Norwegian Insti­ NA is variable and strongly influenced by climatic vari­ tute of Fisheries and Aquaculture for providing us with ability, with high temperature during spring being a cod juveniles. We thank the crew on board RV “Johan prerequisite for the production of strong year classes Ruud” and RV “Hyas” for their help during the sam­ (Sætersdal and Loeng, 1987; Ellertsen et al., 1989; pling of fish, and T. Jakobsen, A. Hermannsen, and N. Nilssen et al., 1994). Current data are too scarce to assess K. Leth for technical assistance. J. Marks is thanked for the long-term variability of CC recruitment, but the linguistic help, and Professor J. G. Pope for valuable influence of climatic variability on recruitment could be comments and help with the statistics. We thank Pro­ expected to be different in NA and CC because of fessor K. Olsen for the initiative, advice, and inspiration different habitats and life histories from the egg stage for this study. The project (NFFR-no. 1201-500.001) onwards. was supported by the Norwegian Fisheries Research It is concluded that cod recruiting to the coastal cod Council (NFFR, I), and is a contribution to the Research group in northern Norway have a different early life Programme on North Norwegian Coastal Ecology history than those cod recruiting to the Northeast Arctic (Mare Nor). 518 5 .Løken , T. Pedersen, and E. Berg ICES mar. Sei. Symp., 198 (1994)

References Jørstad, K. E., and Nævdal, G. 1989. Genetic variation and population structure of cod (Gadus morhua L.) in some Bax, N., and Eliassen, J. E. 1990. Multispccies analysis in fjords in northern Norway. J. Fish Biol., 35 (Suppl. A): 245- Balsfjord, northern Norway: solution and sensitivity analysis 252. of a simple ecosystcm model. J. Cons. int. Explor. Mer, 47: Keats, D. W., Steele, D. H., and South, G. R. 1987. The role of 175-204. fleshy macroalgae in the ecology of juvenile cod (Gadus Bergstad, O. A., Jørgensen, T., and Dragesund, O. 1987. Life morhua L.) in inshore waters off eastern Newfoundland. history and ecology of the gadoid resources of the Barents Can. J. Zool., 65: 49-53. Sea. Fish. Res., 5: 119-161. Kjørsvik, E., Stene, A., and Lønning, S. 1984. Morphological, Brander, K. 1979. The relationship between vertebral number physiological and genetical studies of egg quality in cod and water temperature in cod. J. Cons. int. Explor. Mer, 38: (Gadus morhua L.). In The propagation of cod Gadus 286-292. morhua. Ed. by E. Dahl, D. S. Danielssen, E. Moksness, Dahle, G. 1991. Cod, Gadus morhua L., populations identified and P. Solemdal. Flødevigen Rapportser., 1984(1): 67-86. by mitochondrial DNA. J. Fish Biol., 38: 295-303. Lear, W. H., and Wells, R. 1984. Vertebral averages of Dannevig, A. 1947. The number of vertebrae and rays of the juvenile cod Gadus morhua, from coastal waters of eastern second dorsal fin of fishes from the Norwegian Skagerrak Newfoundland and Labrador as indicators of stock origin. coast. Annls biol., Copenh., 2: 131-146. J. Northw. Atl. Fish. Sei., 5: 23-31. Ellertsen, B., Fossum, P., Solemdal, P., and Sundby, S. 1989. Lear, W. H., Wells, R., andTempleman, W. 1981. Variation in Relation between temperature and survival of eggs and first- vertebral averages for year-classes of Atlantic cod, Gadus feeding larvae of northeast Arctic cod (Gadus morhua L.). morhua, on Flemish Cap. J. Northw. Atl. Fish. Sei., 2: 57- Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 209-219. 60. Fahy, W. G. 1976. The morphological time of fixation of the Mehl, S. 1989. The Northeast Arctic cod stock’s consumption total number of vertebrae in Fundulus majalis (Walbaum). J. of commercially exploited prey species in 1984-1986. Rapp. Cons. int. Explor. Mer, 36: 243-250. P.-v. Réun. Cons. int. Explor. Mer, 188: 185-205. Falk-Petersen, I. B. 1982. Ecological investigations on the Mork, J., Ryman, N., Ståhi, G., Utter, F., and Sundnes, G. zooplankton community of , northern Norway: 1985. Genetic variation in Atlantic cod (Gadus morhua) planktonic eggs and larvae from March 1978 to February throughout its range. Can. J. Fish, aquat. Sei., 42: 1580- 1980, Sarsia. 67: 69-78. 1587." Gjøsæter, J. 1987a. Habitat selection and inter year-class inter­ Møller, D. 1966. Genetic differences between cod groups in the action of young cod (Gadus morhua) in aquaria. Flødevigen Lofoten area. Nature, London, 212: 824. Rapportser., 1987(2): 27-36. Møller, D. 1968. Genetic diversity in spawning cod along the Gjøsæter, J. 1987b. Habitat selection of juvenile cod (Gadus Norwegian coast. Hereditas, 60: 1-32. morhua), whiting (Merlangius merlangus) and some littoral Møller, D. 1969. The relationship between Arctic and coastal fish in an aquarium. Flødevigen Rapportser., 1987(1): 17-26. cod in their immature stages illustrated by frequencies of Gjøsæter, J. 1990. Selection of foraging sites by cod, Gadus genetic characters. FiskDir. Skr. Ser. HavUnders., 15: 220- morhua whiting Merlangius merlangus and goldsinny- 233. wrasse, Ctenolabrus rupestris, in aquaria. J. appl. Ichthyol., Nakken. O., and Raknes, A. 1987. The distribution and growth 6: 204-210. of Northeast Arctic cod in relation to bottom temperatures in Godø, O. R. 1986. Dispersion and mingling of cod from various the Barents Sea, 1978-1984. Fish. Res., 5: 243-252. nursery and feeding areas along the Norwegian coast and in Nilssen, E. M., Pedersen, T., Hopkins, C. C. 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