Life history and ecology of the gadoid resources of the Barents Sea

Fisheries Research, 5 (1987) 119-161

119

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Life History and E c o l o g y of the Gadoid Resources of the Barents Sea O.A. BERGSTAD, T. JORGENSEN and O. DRAGESUND

Department of Fisheries Biology, University of Bergen, P.O. Box I839, 5024 Bergen (Norway)

ABSTRACT Bergstad, O.A., Jorgensen, T. and Dragesund, 0., 1987. Life history and ecology of the gadoid resources of the Barents Sea. Fish. Res., 5: 119-161. Knowledge of the distributional patterns, reproduction, early life history, recruitment, growth, stock-size fluctuations and ecology of the main gadoids of the Barents Sea and neighbouring coastal areas is reviewed. Today, approximately one-third of the annual fish catch of ~ 3 million tonnes in these waters is gadoid fish, primarily Atlantic cod, haddock and saithe. A comparative analysis of the still limited information on the biology of the large gadoids indicates that differences in life histories and ecological strategies have evolved which tend to minimize inter-gadoid interactions. Intra-specific interactions may be more significant, although the available data do not allow assessment of their regulatory power. The understanding of the gadoids as elements of the Barents Sea food web is also highly limited. Accordingly, it is suggested that future research on gadoids should primarily focus on their relationships with other co-occurring species and on intra-specific regulatory mechanisms. Since selective exploitation by man affects important characteristics of the ecosystem (e.g., diversity and age/size structure of populations), such research may prove essential for a sound management of the resources.

INTRODUCTION

The Barents Sea is bordered to the north by the archipelagoes of Spitsbergen and Franz Josef Land, to the east by Novaya Zemlya and to the south by the coasts of northern Norway and the U.S.S.R. The western boundary is usually drawn along the shelf edge of the Norwegian Sea. Figure 1 illustrates the current pattern of the Barents Sea, which is characterized by inflow of the rather warm and saline Atlantic water mass and the Norwegian Coastal water mass from the southwest, and the cool Arctic water mass from the north (Loeng and Sundby, 1986). Water flows out of the Barents Sea as an undercurrent through the comparatively deep Bear Island Channel and as the South Cape Current to the south of Spitsbergen. The water mass distribution and characteristics have a major influence on the production processes and the current pattern largely determines the zoogeographical 0165-7836/87/$03.50

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Fig. 2. Landingsof fish (A) and of the different gadids (B) from the Barents Sea and adjacent shelf areas (ICES regions I, IIa and IIb). (Source:Bull. Statistique, ICES). GADOIDS OF THE BARENTS SEA Andriyashev (1954) lists 144 fish species as occurring in the Barents Sea. Of these, 19 are gadoids (Table I ), all members of the family Gadidae. Among these are included the peculiar subspecies Gadus morhua kildinensis Derjugin, 1920, inhabiting Lake Mogilnoe at Kildin Island and the undoubtedly rare Theragra finmarchica Kofoed, 1956, of which only four specimens have been reported (Kofoed, 1956; Svetovidov, 1959). Most of the abundant gadids are boreal endemics (Ekman, 1967), the only exception is the Arctic endemic, Polar cod, Boreogadus saida (Lepechin, 1774). [ Other Arctic species occasionally encountered are Onogadus argentatus (Reinhardt, 1837) and Eleginus navaga ( Pallas, 1811 ). ] Some species are only abundant along the upper continental slope of the Norwegian Sea and in the deeper parts of the Barents Sea. These are demersal

122 TABLEI Gadoid fishes of the Barents Sea (after Andriyashev, 1954). Species names have been updated according to J.C. Hureau and Th. Monod (Editors), 1979. Check list of the Fishes of the northeastern Atlantic and of the Mediterranean, 1-2, Unesco, Paris Species name Subfamily Gadinae Gadus morhua morhua L., 1758 G. morhua kildinensis Derjugin, 1920 Boreogadus saida (Lepechin, 1774) Eleginus navaga (Pallas, 1811) Gadiculus argenteus thori J. Schmidt, 1914 Melanogrammus aegle[inus ( L., 1758) Merlangius merlangius (L., 1758) M icromesistius poutassou ( Risso, 1826) PoUachiuspoUachius (L., 1758) P. virens (L., 1758) Theragra finmarchica Kofoed, 1956 Trisopterus esmarki {Nilsson, 1855) Subfamily Lotinae Brosme brosme (Ascanius, 1772) Ciliata mustela (L., 1758) CiIiata septentrionalis (Collett, 1875) Rhinonemus cimbrius {L., 1766) Onogadus argentatus (Reinhardt, 1837) Molva molva {L., 1758) Mo!va dypterygia ( Pennant, 1784) Phycis blennoides ( Briinnich, 1768)

English common name Atlantic cod Polar cod Navaga Silvery pout Haddock Whiting Blue whiting Pollach Saithe, Coalfish Norway pout Tusk Five-bearded rockling Northern rockling Four-bearded rockling Arctic three-bearded rockling Ling Blue ling Greater forkbeard

species like Molva dypterygia (Pevuant, 1784), Molva molva (L., 1758), Brosme brosme (Ascanius, 1772 ), Gadiculus argenteus thori J. Schmidt, 1914 and Phycis blennoides (Brfinnich, 1768) (Andriyashev, 1954; Rahardjo Joenoes, 1961; Bakken et al., 1975 ). The abundance and extent of distribution of blue whiting, Micromesistius poutassou (Risso, 1826), is variable [see Bailey (1982) for review ], although it occurs regularly in the warmer parts of the southwestern Barents Sea and off the shelf of Spitsbergen ( Godo et al., 1984). Of the boreal neritic species, some are u n c o m m o n or only moderately abundant. The Norway pout, Trisopterus esmarki, spawns in Norwegian coastal waters and 0group and adults occur regularly in the southwestern Barents Sea, although not in commercial quantities (Baranenkova and Khokhlina, 1968; LahnJohannessen, 1968 ). Merlangius merlangius ( L., 1758) and PoUachius poUachius (L., 1758) are uncommon. Of the boreal rocklings, Ciliata septentrionalis (Collett, 1875), C. mustela (L., 1758) and Rhinonemus cimbrius (L., 1766),

123

the latter is the more abundant, but its ecological significance in the Barents Sea can hardly be assessed due to lack of information. Of all the gadids, Atlantic cod, Gadus morhua L., 1758, haddock, Melanogrammus aeglefinus (L., 1758) and saithe, PoUachius virens (L., 1758) are clearly the most abundant. The populations in the Barents Sea are normally referred to as the Northeast Arctic stocks. Our discussion will primarily focus on these three species (and stocks), because of their obvious ecological importance and because half a century or more of research has been directed at studying their biology and population dynamics. The Polar cod will be considered when enough information exists to permit relevant comparison with the boreal species. DISTRIBUTION AND MIGRATION

Spawning areas, seasons and behaviour

Hjort (1902, 1905 ) published the first detailed account of the spawning areas of the Atlantic cod, primarily based on the occurrence of spawners in the fish landings. For haddock and saithe, some data resulted from extensive egg and larval surveys in the years 1900-1906 ( Hjort, 1909 and for more details Damas, 1909). Landing statistics, as well as series of egg and larval surveys (Wiborg, 1957a; Bjorke, 1984 and references therein, Serebryakov and Aldonov, 1984) and mark-recapture experiments (Dannevig, 1953; Ssetersdal and Hylen, 1959; Hylen et al., 1961, Olsen, 1961; Reinsch, 1976; Jakobsen, 1978c, 1981; Godo, 1984) have largely confirmed these early observations. Cod, saithe and haddock share the characteristic migration to southerly spawning grounds in the winter/spring. Still, competition among species on the spawning grounds is probably insignificant due to lack of spatial and temporal overlap. Cod Spawning areas of the long-range migrating cod are found on nearshore banks and in open fjords along the Norwegian coast from ~ 62 ° N to Soroy, Finnmark (71°N) (Fig. 3) (Hjort, 1909, Damas, 1909; Sund, 1939; Bjorke, 1984). Some spawning occurs east of Soroy, Finnmark and at the Murman coast (Rass, 1936). Outside Lofoten, the banks off More ( and neighbouring fjords ) and the coastal spawning areas between Lofoten and Soroy are the more important. Detailed studies on the spawning grounds, mostly at Lofoten, began with the pioneer studies by G.O. Sars (1879). At Lofoten, the cod arrives at the spawning grounds from late January and onwards (Rollefsen, 1938; Ssetersdal and Hylen, 1959; Anon., 1976). The spawning period lasts ~ 2 months, although most intense spawning occurs from mid-March to mid-April. The duration of' the period and the timing of peak spawning seem stable, although Pedersen (1984) found, based on statistics of the landings of roe, indications of a slight

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125

(1935) initial application of the echo sounder for mapping fish distribution, routine acoustic surveys have illustrated the dynamics of migrations and distributions on a local scale (Ssetersdal and Hylen, 1959; Hylen et al., 1961; Sundnes, 1964; Monstad et al., 1969; Blindheim and Nakken, 1971; Jakobsen, 1974; God~ and Toresen, 1980; God~ et al., 1982, 1983; God~ and Sunnan~, 1984; Nakken, 1984). Spawning may occur in midwater at 50-100-m depth or at the bottom. Dense schools form during the day and disperse at night ( S~etersdal and Hylen, 1959; Sundnes, 1964 ). Haddock Damas (1909) reported haddock eggs from off Mare and the northern boundary of haddock spawning was set at Haltenbanken. Andriyashev (1954) states that spawning areas are found along the entire range of the species over 80-250 m in waters of temperatures 4-8 °C. Early Russian workers (Schmit, 1936, 1937; cited by Sonina 1969) located spawning grounds off VesterAlenSor~ya and later studies, primarily egg and larval surveys by Aleev (1944; cited by Ssetersdal, 1952), Wiborg (1950, 1952, 1954, 1956, 1957b, 1960a, 1961 ) and Bjorke (1984) suggest spawning along the outer continental shelf from ~ 62 to 70 ° N ( Fig. 3 ). Large mature haddock were caught regularly in March-April at coastal banks at 68-70°N (Seetersdal, 1952) and Wiborg (1957b) reported running fish at ocean weather station Mike (66°N 02 °E) in the Norwegian Sea. Distinguishing early stages of haddock eggs from cod eggs visually is impossible and remains a source of uncertainty. Mork et al. (1983) and Sundby (1984) utilized a recently developed technique to identify eggs and found, in accordance with earlier studies, haddock eggs to be most numerous in samples from outer shelf areas, in this case off Lofoten. Spawning appears to take place over a greater depth range than indicated by Andriyashev, 50-600 m (Anon., 1979). The spawning period at the Norwegian coast is rather long, probably from mid-March to early June, with maxima in April and early May ( Wiborg, 1957b; Sonina, 1969). Saithe The early egg surveys located saithe eggs over depths of 100-200 m in the northern North Sea (Tampen, Viking bank ) and on the coastal banks off More (Fig. 3), while the northern boundary remained uncertain (Hjort, 1909). Mark-recapture experiments (Olsen, 1959, 1961; Reinsch, 1969; Jakobsen, 1978c, 1981) and acoustic surveys (Jakobsen, 1972) add Haltenbanken and Lofoten as rather important spawning grounds. Results from egg surveys are in agreement with this distribution of spawning areas (Wiborg, 1950-1961, 1962; Dragesund and Hognestad, 1966, Bj~rke, 1984). The spawning period lasts from January until March/April, but ripe fish have been caught in northern areas in May-June. Spawning probably peaks in February-March

126

(Reinsch, 1976). The spawners are normally found in water masses with temperatures in the range 5.5-10 °C and salinity > 35%o (Andriyashev, 1954).

Polar cod According to Rass (1968), the Polar cod spawns under the ice cover and the spawning areas include the southeastern Barents Sea and the waters off Novaya Zemlya ( Ponomarenko, 1968). Evidently, some spawning occurs in most years at the shelf off Spitsbergen (Hognestad, 1968). The spawning period lasts from late December to March-April (Rass, 1968).

Eggs, larvae and pelagic juveniles (O-group) The distribution of eggs, larvae and fry after spawning is primarily determined by the current pattern in the area (Fig. 1). For the boreal species, a northward and, for some species, a coastward drift is seen. The drift phase may last for up to 6 months and the larvae can be carried 500-800 nautical miles. During the summer and early autumn, fry are regularly found pelagically over large areas, usually in the warmer water masses in the southern Barents Sea and the Spitsbergen area.

Cod The highest densitites of eggs and early stage larvae are found in the upper 50 m in nearshore areas or over the shallow coastal banks (Damas, 1909; Wiborg, 1948a; Dragesund and Hognestad, 1966; Hognestad, 1969; Smedstad and Oiestad, 1974; Bjorke, 1984) in the Coastal water masses of temperatures 3-4°C (Ellertsen et al., 1981a, 1984; Sundby and Solemdal, 1984; Sundby, 1984). Ellertsen et al. (1981a), Sundby (1983) and Sundby and Solemdal (1984) offer good illustrations of the dynamics of the egg distribution in the Lofoten area, mainly in relation to wind-induced current patterns. A relevant study from coastal banks north of Lofoten was reported by Sundby (1984). The early stage larvae appear to be even more confined to coastal waters and fjordic environments than the eggs (Bj~rke, 1984). The best illustration of this resulted from recent efforts in the Lofoten area, summarized by Tilseth (1984) and Ellertsen et al. (1986). By June-July, the post-larvae and juveniles, now 10-50 mm SL (standard length), are found in the upper 75 m of the water column in the southwestern Barents Sea-Bear Island area, in the Norwegian fjords and over coastal banks (Damas, 1909; Corlett, 1958; Wiborg, 1957b, 1960b; Baranenkova and Khokhlina, 1964; Baranenkova et al., 1973; Mukhina, 1984; Bj~rke and Sundby, 1984). Bjorke and Sundby (1984) related the open sea distribution in July to the water mass characteristics. The bulk of the cod larvae were caught in the upper 40 m of the water column, still mainly within the Coastal water mass. By early August, the cod fry in northern areas (i.e., north of Lofoten) have

127 grown to a length of 30-85 m m (Corlett, 1958; Wiborg, 1960b). At More, a high fraction of the fry start to inhabit littoral and sub-littoral habitats at this stage (Godo and SunnanA, 1984). In the northernmost fjords and in the Barents Sea, most cod fry remain pelagic until late September-early October. Their open sea distribution in August-September has been very well mapped through 0-group surveys conducted annually since 1965 (Dragesund and Olsen, 1965; Benko et al., 1970; Anon., 1969-1981, 1982-1984; Randa, 1984). Most of the cod.fry are found in the upper 50 m of the water column in water masses with temperatures in the range 3-6 ° C. The geographical distribution varies mainly in relation to fluctuations in the water mass distribution. Figure 4 illustrates two extremes, 1970 and 1980, 2 years characterized by strong and weak inflow of Atlantic water masses, respectively ( Blindheim and Loeng, 1981 ). Haddock The records of haddock eggs from the large-scale egg surveys are few and due to the identification problems, often uncertain. The few reliable observations include Wiborg's (1957b) records from surface hauls at ocean weather station Mike and from vertical hauls at the shelf off Lofoten-Vesterfilen ( Mork et al., 1983) and further north ( Sundby, 1984). All these records are from the upper layers of the water column, but no studies give data on the vertical distribution of the eggs. Wiborg (1957b) caught both newly spawned eggs and eggs of advanced stages in surface hauls. Damas (1909), Wiborg (1950, 1954, 1956, 1957b, 1960a, 1961), Dragesund and Hognestad (1966), Hognestad (1969) and Bjorke (1984) report catches of early stage haddock larvae scattered over a wide area, including coastal and offshore waters. The impression is that the highest numbers were caught at or near the shelf edge off the large coastal banks, mainly in May-June. The same pattern is observed in June-July, although numbers increase northwards (Wiborg, 1960b; Mukhina, 1984; Bjorke and Sundby, 1984). By August-September, the haddock fry have grown to a length of 30-130 mm and are widely distributed in the southwestern and western Barents Sea and the shelf area to the west of Spitsbergen (Benko et al., 1970; Anon., 1969-1981, 1982-1984). As for cod, year-to-year and long-term variation related to the water mass distribution is seen. Although there is always extensive spatial overlap between cod and haddock fry, Randa (1984) demonstrates that haddock usually has a more westerly distribution than cod. Neither cod nor haddock form true schools at this stage, but appear as layers which are rather dense during the day and scattered at night (Dragesund et al., 1970). Saithe Early stages of saithe eggs can hardly be distinguished from eggs of Norway pout (and blue whiting), hence in many reports the data for these species are pooled. The only positively identified saithe eggs stem from the main spawning

128 A

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129

In June-July, saithe fry are abundant in littoral habitats, normally as small schools (Damas, 1909; Mironova, 1961 ). The change of habitat occurs at different times in different areas and years (Olsen, 1961). Lie (1961) reported litoral saithe in Late April near Bergen, while Mironova (1961) states that schools usually appears~in June-July at the Murman Coast. According to Damas (1909), the length of the littoral fry at this stage ranges from 20 to 60 mm, while the pelagic fry may be shorter, 5-35 mm. This is in agreement with other findings (Lie, 1961; Mironova, 1961 ) and points to a "normal" length of 30-50 mm at the change of habitat (Reinsch, 1976). Although saithe fry are not regularly abundant in the open sea in the autumn, they may in some years occur as large schools in surface waters, notably in shelf areas off Bear Island and Spitsbergen (Benko et al., 1970; Anon., 1969-1981, 1982-1984). Polar cod Eggs of Polar cod occur from January through June over wide areas of the eastern Barents Sea from the Kanin and Kolguev Islands in the south to northern Novaya Zemlya, in waters of < 0 to 1.8°C (Rass, 1968 and references therein). Larvae appear in May-July ( Rass, 1968) and are distributed in much the same areas as the eggs. Examples are given by Borkin (1983, 1984), who recorded most larvae in waters with temperatures - 1 to 4 ° C at 0-20-m depth. In August-September, 0-group Polar cod are found in two main areas, the southeastern Barents Sea and waters to the west and south of Spitsbergen. Most fry are found in the upper 50 m of the water column at temperatures from 0 to 4 ° C (Hognestad, 1968; Anon., 1969-1981 ). Immatures and adults

All the gadids, except Polar cod, can be characterized as benthic or rather bentho-pelagic by the end of their first autumn. The change of habitat occurs in summer for saithe, but gradually throughout the autumn for cod and haddock. Cod The distribution of the I-group cod in the spring is most probably determined by the extent of the 0-group drift during the preceeding autumn and consistent differences are seen between years of high and low strength of the Atlantic inflow (Nakken and Raknes, 1987). The distribution of older immatures is also highly influenced by the water mass distribution, as evidenced by fishery statistics ( Konstantinov, 1968, 1970; Mukhin, 1979) and experimental surveys (trawl surveys, acoustic surveys) (Eggvin, 1938; Lee, 1951; Hylen et al., 1961; Beverton and Lee, 1965; Shevelev, 1979, 1980; Godo et al., 1984; Nakken and Raknes, 1987). The youngest fish are found in the coldest water masses, which also show the greatest variation

130 in temperature. A gradual westerly movement is seen with increasing age {Nakken and Raknes, 1987). In the Bear Island-Spitsbergen area, the youngest fish are found in shallow water and the most dense autumn-winter concentrations of immatures to the south, west and southeast of Bear Island and on the banks to the west of Spitsbergen (Lee, 1951; Beverton and Lee, 1965; Dalen et al., 1977; Dalen and Smedstad, 1978; Randa and Smedstad, 1982; Godo et al., 1984). The cod is bound for Atlantic water masses with temperatures > 1.5-2°C (Woodhead and Woodhead, 1965b) and its general and local distribution varies spatially and temporally in relation to the position of the temperature gradients. Immature cod do, however, occasionally enter cold water masses, apparently in pursuit of dense concentrations of prey (Lee, 1951; Beverton and Lee, 1965 ). This is most often seen during the summer months, when the limiting temperature may be lower (Woodhead and Woodhead, 1965b). Regular seasonal north-south and east-west migrations between shallow summer feeding grounds and deep wintering areas are performed ( Trout, 1957; Maslov, 1968; Harden Jones, 1970; Nakken and Raknes, 1987). At Bear Island-Spitsbergen, the feeding migration goes northwards towards Hope Island and along the western shelf off Spitsbergen, while in the southern Barents Sea, it is northerly and easterly towards the frontal zone between Polar and Atlantic water masses. In the spring, a regular coastward migration, associated with the capelin spawning at the Norway and Murman coasts, is seen (Hjort, 1914; Eggvin, 1938; Hylen et al., 1961; Ponomarenko and Yaragina, 1978). The mature cod [ and a few old immatures (Woodhead, 1959; Woodhead and Woodhead, 1965a)] start their annual spawning migration in December-January and follow the boundary layer between the Coastal and Atlantic water masses along the shelf break towards the spawning grounds. Harden Jones (1970) provided an excellent review of the relevant information gathered from numerous mark-recapture experiments and from survey data. He emphasized the Barents Sea-Lofoten migration, since very little was known about migrations to other spawning grounds. Godo (1986) established, by several tagging experiments at More and Helgeland, that a high proportion of cod spawning at the southern spawning areas come from feeding area at Bear Island-Spitsbergen. This is in agreement with earlier findings from otolith studies (Trout, 1953, 1957; S~etersdal and Hylen, 1959). At the spawning grounds, there is extensive mixing with short-range migrators, commonly called Coastal cod (Hylen, 1961; Godo, 1984, 1986). Although the bulk of the spawning population leaves the spawning ground immediatelY after spawning (Harden Jones, 1970), a small fraction appears to remain in southern areas (Godo, 1984). Spent fish have been recorded on the Barents Sea feeding grounds from May onwards (Harden Jones, 1970).

131 Haddock Sonina {1969) describes in great detail the migrations and movements of immature and mature haddock in the central and eastern Barents Sea and offers a good review of Soviet research in the area. Up to an age of 3 years, no regular seasonal migration is seen. The demersal I-group is widely distributed in open sea areas of the western Barents Sea and the Bear Island-Spitsbergen shelves, but also in coastal areas. A gradual eastward movement is noted and the II-group is also found in the central Barents Sea. Three-year-old fish and older show regular seasonal migrations between eastern and coastal feeding grounds in summer and southern and western wintering areas in deeper waters. The distributions of cod and haddock overlap. Sonina states that the timing of the migrations is the same, although haddock seem to start migrating earlier. Good illustrations of the distributional patterns have resulted from numerous acoustic surveys (Hylen et al., 1961; Hylen and Smedstad, 1972; Midttun et al., 1981; Dalen et al., 1984). The haddock seems to be restricted to water masses warmer than 2 °C and on the feeding grounds, the highest densities are found in areas at 5-6 ° C ( Sonina, 1969). As with cod, pronounced year-to-year and periodic changes in the timing and extent of migrations related to the hydrographical regime have been noted (Sonina, 1969, 1980; Mukhin, 1979; Midttun et al., 1981 ). Tagging programs with haddock have proven unsuccessful (Sonina, 1969) and never produced a complete picture of the migration of the adults ( S~etersdal, 1956; Sonina, 1969). The spawning migration starts in January and February and the schools appear to follow the main branch of the inflow of the Atlantic water mass towards the coastal banks off Norway. They arrive at the spawning grounds in March (Sonina, 1969), but the southern limit of their migration is unclear. Very little is known about the return migration, but mature haddock are back at the eastern Barents Sea and Bear Island-Spitsbergen feeding grounds in late summer. The haddock is often characterized as a benthic species, but may frequently occur in midwater. Svetovidov (1948) states that the I-group is found pelagically in summer and Hylen and Smedstad (1972) reported the same for 1--3year-old haddock. Sonina (1980) decribes large midwater concentrations from the western Barents Sea. A floating long-line fishery takes place annually in July-August along the Finnmark coast. Saithe Immature saithe of age-groups 0, I and II remain in nearshore areas, although a gradual movement towards deeper waters with increasing age is observed (Olsen, 1961; Mironova, 1961; Lie, 1961 ). Mark-recapture experiments have shown that III- and IV-group (and some II-group) fish perform large-scale and long-range migrations towards feeding areas in the North Sea and the Barents Sea (Olsen, 1961; Reinsch, 1969;

132

Jakobsen, 1978c, 1981 ). A marked change of habitat takes place at this stage, as the saithe from as far south as 62 ° N, start to utilize open sea feeding areas, primarily in the southwestern Barents Sea, but also the Bear Island-Hope Island-Spitsbergen area (Mironova, 1961). The migratory patterns of juveniles are somewhat variable, since rather marked differences exist between results from the tagging programs in 1954-1964 and in the seventies. In the fifties and early sixties most fish tagged in the More region and northwards were recaptured in the Barents Sea ( Olsen, 1961 ) while in the seventies, most fish from More turned south to the North Sea feeding areas (Jakobsen, 1981 ). The immatures appear to become rather stationary throughout the year at their feeding grounds in the Barents Sea, although some summer movement towards eastern and northern areas is seen (Olsen, 1961; Mironova, 1961). Still, the saithe has a more coastal distribution than cod and haddock ( Mironova, 1961; Jakobsen, 1978c ). By October-December, the mature fish start their annual spawning migration (Maslov, 1956; Mironova, 1961) and reach the spawning grounds off More and Helgeland in January (Jakobsen, 1972; Reinsch, 1976). The schools seem to follow the shelf break and probably stay in Atlantic water masses. At the spawning grounds, the fish are found close to, but not at the bottom and form daytime schools which disperse at night (Jakobsen, 1972). Following spawning, most fish probably return to northern feeding areas immediately (Olsen, 1961). Some straying occurs, however, as evidenced by returns of tags from the North Sea-Skagerrak (Olsen, 1961; Reinsch, 1976; Jakobsen, 1978c, 1981 ). On their return migrations, the mature saithe regularly pass the banks off Lofoten-VesterAlen in May-June and most are back on their Barents Sea feeding grounds by August (Olsen, 1961 ). At this time, they utilize feeding grounds as far north as southwestern Spitsbergen and Hope Island ( Reinsch, 1976; Mironova, 1961 ). In late autumn, a general southward and westward migration is seen and large-sized saithe again appear near the coast of Norway (Mironova, 1961). As with cod, landing statistics indicate differences in the extent of the northward and eastward summer migration in 'cold' and 'warm' years ( Reinsch, 1976). Evidently, saithe schools may keep their identity for long periods of time and over long distances. Olsen (1961) frequently registered recaptures of two or more fish from the same tagging experiments on the same date and locality, sometimes a year or more hence and hundreds of miles from the location of their release. This strengthens the impression that saithe has a much more pronounced schooling behaviour than cod or haddock. Polar cod Polar cod is found in Arctic water masses and is widely distributed in the northern and eastern Barents Sea in late summer ( Ponomarenko, 1968; Hognestad, 1968; Monstad and Rottingen, 1977; Hamre and Rottingen, 1977;

133

Dommasnes and Rottingen, 1977). the distribution is extended southwards and westwards in years of weak inflow of warm water masses from the southwest ( Borkin and Shleinik, 1981; Anon., 1982b, 1983). There is a general westward and northward feeding migration in the spring and summer, followed by a southward and eastward pre-spawning migration in late autumn (Ponomarenko, 1968; Gjos~eter and Bjerke, 1973). Polar cod is often associated with ice and form nearshore concentrations on the wintering grounds ( Ponomarenko, 1968; Borkin and Shleinik, 1981 ). Although normally regarded as a predominantly pelagic species, Polar cod is often found in near-bottom concentrations ( Olsen, 1962; Hognestad, 1968; Gjosseter and Bjerke, 1973; Dommasnes and Rottingen, 1977; Hamre and Rottingen, 1977 ). STOCK UNITS

In the late 1870s G.O. Sars had compiled enough information to present the first picture of the migrations of the Northeast Arctic cod, i.e., long-range migrating cod with feeding areas in the Barents Sea-Spitsbergen area. What had earlier been recognized as independent populations was now thought to be different age groups of the same stock. The efforts made by Hjort, Damas and others to locate the spawning areas in the Northeast Atlantic and development of ageing techniques enabled them to present quite an accurate description of the stocks of the area as early as 1909 ( Hjort, 1909; Damas, 1909 ), with the possible exception of the Northeast Arctic haddock. As a result of increasing exploitation, the need for management emerged and as a consequence, the need for more detailed knowledge on stock separation. During recent decades, new knowledge has been gained by large-scale tagging experiments and studies of genetics. Cod From a management point of view, the Northeast Arctic cod is treated as one unit. Based on the structure of the otolith, it is possible to separate individuals from the eastern and the western parts of the area (Trout, 1953). As noted earlier, examinations of the otolith structure and extensive tagging experiments indicate that fish which spawn on the southern spawning grounds (i.e., More, Helgeland) mainly come from the western feeding areas (Trout, 1957; Ssetersdal and Hylen, 1959; Godo, 1984). Tagging experiments also indicate that repeated spawning takes place at the location of first spawning (Godo, 1984). Genetic studies have, however, not revealed any differences between fish from different parts of the feeding area which might indicate self-sustaining substocks (Reisegg and Jorstad, 1984; Jorstad, 1984a). Rollefsen (1933) could separate Coastal cod from the Northeast Arctic type

134 by the structure of the otoliths. Coastal cod spawns on the same grounds as the long-range migrators and at numerous locations where the Northeast Arctic cod does not spawn. The question of whether Coastal cod is a genetically distinct stock or progeny of Northeast Arctic cod growing up in coastal waters has not been answered. Moiler (1968) compared otolith structure and several genetic characteristics, notably haemoglobin frequencies. He concluded that Coastal cod and Northeast Arctic cod are genetically different. The results suggested great heterogeneity among samples of Coastal cod. Alternative explanations for this heterogeneity have been considered by several authors. Frydenberg et al. (1969) suggested that the Hb-polymorphism is maintained by stabilizing selection caused by heterozygote superiority, or by selection forces working differently on the two sexes. The latter view was supported by observations of great differences in Hb-gene frequencies between males and females, also observed by Mork et al. (1982). Karpov and Novikov (1980) found that the oxygen binding capacity of one of the Hb-molecule types strongly depends on temperature and thus the gene frequency should be expected to be influenced by selective forces varying in accordance with environmental temperatures. Mork et al. (1984) suggested that this selection could be strong enough to cause significant changes in gene frequencies, even within the same generation. Differences in gene frequencies thus may be a consequence of environmental forces as well as of heredity. Genetic studies of polymorphic proteins using new techniques have indicated distinct populations of Coastal cod, mainly in sheltered fjords (Jorstad, 1984b). Along the coast, the differences are not statistically significant, however, except for haemoglobin frequencies. Haddock Knowledge about the stock structure of haddock is mainly based on egg larval surveys, growth studies and distribution of meristic characters. As noted, haddock is not well suited for tagging experiments. In a Russian tagging experiment between 1953 and 1964, 100 fish were recaptured out of 17 000 released ( Sonina, 1969 ). Ssetersdal (1952, 1953 ) claimed that there is a single stock of the Northeast Arctic haddock based on vertebrae counts, brood strength variation and growth studies. This is also the present view, although it is possible that local stocks of haddock are found along the coasts. Saithe The saithe in the Northeast Atlantic is divided into five management units by area: one east of Scotland, one around the Faroe Islands, one at Iceland, one in the North Sea south of 62 ° N and another along the Norwegian coast north of 62 ° N. The latter is called the Northeast Arctic saithe. Large-scale tagging of saithe in the fifties indicated a substantial migration of large saithe from the Northeast Arctic stock to Iceland and the Faroes ( Olsen, 1959; Jakobsen and Olsen, 1987). Olsen linked this emigration to the migra-

135

tions of the Norwegian spring-spawning herring at that time. However, tagging experiments in the seventies have not shown similar migrations (Jakobsen, 1978c), but indicate a higher migration of both mature saithe and immature saithe from the Northeast Arctic stock to the North Sea. Thus, stock separation is even more difficult for the saithe than for cod and haddock. Polar cod Polar cod is a circumpolar species. The population in the Barents Sea belongs to the stock inhabiting the Kara Sea, the White Sea and the Barents Sea ( Ponomarenko, 1968). Information on the stock composition for this species is limited. REPRODUCTIVE BIOLOGY

Data on age and size at first maturity of the four principal species are summarized in Table II. Cod matures at a later age than the other species, although recent studies (Hylen and Nakken, 1982; Anon., 1984a) indicate that the difference between saithe and haddock and cod may at present be no more than ~ 2 years. The basis for comparison between species in earlier years seems weak. For cod, the data from 1942-1981 indicate a trend towards lower age at first maturity (Anon., 1984a). However, since sampling bias may prove highly significant, the ICES Arctic Fisheries Working Group (Anon., 1984a) recommended more detailed studies in order to assess the quality of the data involved. Polar cod matures relatively late in life and spawns no more than one to three times. It also matures at a comparatively large size, having reached 70-80% of its maximum length. Recent data indicate that very few fish grow older than 5 years (Borkin, 1984). Reliable information on the virgin stocks of cod, saithe and haddock are of course not available, but some data were collected in periods of comparatively low rates of exploitation. In the thirties and forties, 14-15-year-old cod still contributed significantly (1-5%) to the spawning stock, meaning that many females would spawn seven to eight times (using 8 years of age as mean age at first maturity) ( Rollefsen, 1953 ). Apparently, the cod which survive the larval and juvenile phase can start to spawn halfway in life and continue annually for 1-8 years, while Polar cod can somehow afford to postpone spawning until relatively late in life. Saithe and haddock have a strategy similar to cod, or they start to spawn even earlier in life and may thus have a comparatively longer reproductive phase. Under heavy exploitation, the number of spawnings has decreased. For cod, recent data show that of the fish reaching maturity, very few spawn more than once (Anon., 1984a). No satisfactory estimate of fecundity, nor its relation to age or length, exists for the Barents Sea populations. The figures given in Table II are probably

136 TABLE II Data on reproductive biology and life history of the Barents Sea gadids Cod Age at first maturity (years)

10.5 (6-14)' 6.5 (3-9) ~

Length at first maturity (cm)

70-856 75-90 s

Approximate absolute fecundity (millions of eggs per female ) Spawning season

1-1911

March-April

Approximate no. of spawnings Recent (1970s) 0-2 Virgin stock 0-8 Maximum recorded age ( years )

301:t

Haddock

Saithe

8-10 (6-10) 2 4-76'7 37-73 ~

5-6 (4-7) 3

65-75 (55-84)3

0.03-37

March-June

44

209 ,o

0.009-0.0214

5-84 1-16 ~2

January-March

January-March

0_2 TM

0-2 0-10

0-14 22 '4'7

Polar cod

273

Egg diameter (mm)

1.20-1.67 ~5

1.2-1.7 TM

1.03-1.22 TM

1.53-1.90 TM

Length at hatching (SL, mm)

4.0 TM 4.2-4.3 TM

3.5-4.015

3.4-3.8 ~5

5.5 '7

Length at EYS (SL, mm)

4.5-5.1 is 4.8-5.0 (3 C)18

5.5 TM

4 TM

6.517

Age at EYS (days)

9 (3 C) ,s

10 (3 C),9

?

?

Length at metamorphosis (SL, mm)

10-12 (7-10 C) 2°

12-15 '~

15-25 TM

32-15017

Length by AugustSeptember (TL, ram)

25-1402'

20-140 ~

20-17021

10-752'

~Anon. (1984a); 2Schmit (1936, 1937), cited by Sonina (1969); 3Reinsch (1976); 4Andriyashev (1954); 5Hylen and Nakken (1982); ~Sonina (1969, 1973, 1981 ) ; VBlacker (1971) ; Sponomarenko (1982) ; 9Hognestad {1968); '°Borkin (1984) ; 'lAldonov et al. (1982); '2Storozhuk and Golovanov (1974) ; '3Rollefsen (1953); ~4S~etersdal (1956) ; ~ZSolemdal and Sundby (1981) (naturally spawned eggs, Lofoten) ; 'SRussel (1976); 17Rass (1968) ; ~SEllertsen et al. (1980); ~gBigelow and Schroeder (1953); 2°Kvenseth (1983); 2'Benko et al. (1970); Anon. {1969-1981, 1982-1984).

imprecise and/or inaccurate and are included to indicate the order of magnitude only. Further studies are clearly needed to assess the significance of probable inter- and intra-specific differences in fecundity. Woodhead and Woodhead (1965a) described the seasonal cycle of the gonads and associated hormonal activity of the cod and pointed to a potential regulation of fecundity dependent on condition in the previous feeding season.

137 Data on early life history characteristics (Table II ) are scarce, especially for saithe, haddock and Polar cod. Solemdal and Sundby (1981) and Sundby (1983) studied eggs of cod in the field and found egg size to decrease and neutral buoyancy to increase through the spawning season. Within the temperature ranges on the spawning grounds, hatching should occur after 20-25 days for cod (Lofoten) and >f 13-15 days for haddock (Norway coast, Atlantic watermass) ( Russel, 1976; Dannevig, 1895 ). No data have been found for saithe eggs, but environmental data and egg size point to an incubation period equal to or shorter than that for haddock eggs. Polar cod eggs have a much longer incubation period, but no actual range can be given based on available data. AGE AND GROWTH The development of larvae and post-larvae of cod and haddock has been described based on several experimental studies (Ellertsen et al., 1980; Laurence, 1974 and others). Ellertsen et al. (1980) hatched eggs from Lofoten and presented data on morphological and anatomical development. Due to lack of sufficiently accurate and precise ageing methods, no reliable field estimates of developmental rates or growth rates are available. Some interesting comparisons can be made based on the length frequency distributions of pelagic juveniles (metamorphosed 0-group) in August-September (Benko et al., 1970; Anon., 1969-1981, 1982-1984). Polar cod is distinctly shorter (4-5 cm) than cod, haddock and saithe and mean lengths of cod are normally shorter than for haddock. The latter is also true in July-August (Wiborg, 1960b). This suggests that haddock has a significantly higher growth rate than cod during its first summer. The range of lengths is much broader for haddock than cod, possibly reflecting a protracted spawning season. The saithe caught in the 0group surveys are normally somewhat larger than both cod and haddock. Lie (1961) and Mironova (1961) give growth data for juvenile saithe from the Norway coast and the Murmansk peninsula, respectively. Some of the data for saithe, cod and haddock are given in Table II and Fig. 5. The data from Lie (1961) are excluded since he worked with fish from southern Norway (near Bergen ). The seasonal variation in growth rate illustrated for saithe is rather pronounced. In Fig. 6, mean lengths and ranges of 0-group cod in August-September are plotted for the period 1967-1984, together with temperature anomalies in the Kola Section (0-200 m ). The plot indicates considerable growth differences between 'cold' and 'warm' periods. Figure 7 gives generalized population growth curves for the four species based on several sources (see legend). Both saithe and haddock are longer than cod initially, haddock up to an age of 2-3 years and saithe until it reaches 6-7 years of age. Variation in growth between areas, year classes and time periods is frequently reported and has been related to changes in the temperature regime,

138 320 -

, ~ . m ,'It"

280

",,"" J

•

st

26,0,

. I"

| 2oo16o_1

120-

80~0" J J ASO ND'J FMAMJ J A SOND'J FMAMJ J ASO ND' 0" GROUP I" 5ROUP Q-GROUP HONTH - AGE GROUP

Fig. 5. Growth ofjuvenile saithe at the Murman coast (after Mironova, 1961 ) (broken line: interpolation based on a small number of data points). Vertical bar represents range of mean lengths from open-sea catches during the international 0-group surveys. (Source: Anon., 1969-1981, 1982-1984).

fish density or food supply. The problem of how to separate the effects of true growth changes from artifacts due to size/age-selective sampling often remains unsolved. Recent data for juvenile cod were presented by Ponomarenko et al. (1980) and Nakken and Raknes (1987), while Dementyeva and Mankevich (1965) give similar information from the thirties and late forties to the sixties. Ponomarenko et al. (1980) claimed to have observed density-dependent growth in the cod stock, but the results are not entirely convincing. The effect of temperature on growth rate was stressed by Dementyeva and Mankevich (1965), who considered temperature to be one of the main factors influencing growth rate. Nakken and Raknes (1987) related an increasing trend in the mean length at age since the mid-seventies to the general warming of the water masses. They also found indications of a slight depression of growth in the 1975 year class, the most abundant year class of the period. The mean length of 4 year olds differed by ~ 7 cm between the slowest and fastest growing year classes. Recent growth changes occurred during a period of low stock abundance, low to moderate recruitment, increasing temperature and expanding area of distribution. It would seem very difficult to determine the relative significance of the different (probably correlated) factors. A comparison of length at age for cod from the eastern and the western part of the feeding area indicates a higher growth rate for fish from the west ( Ponomarenko et al., 1980 ). No explanation for this difference is suggested, but the higher water temperature in the western area may be the prime factor.

139 1.0 t.J

O a:

5

-1.0

'

100

'

'

'

'

I

. . . .

I

.

1

. . . .

I

.

I

. . . .

I

.

-

i

50

.

.

.

1970

.

.

.

.

.

.

1980 YEAR

Fig. 6. Temperature anomalies in 0-200-m depth in the Kola section relative to the long-term mean for the years 1921-1984, (uppergraph) and mean length of 0-group cod in August-September (lowergraph). [ Sources:Temperaturedata fromthe KnipowichPolarResearchInstitute of Marine Fisheries and Oceanography, Murmansk; length data from the international 0-group surveys (all data made available by the Institute of Marine Research, Bergen) ]. For haddock, Sonina (1965, 1969) found an inverse relationship between growth rate and population density. She also found changes in growth rate related to altered feeding habits. The depressed growth reported for juvenile 120

~0

- -

cod

.....

sQJ~e

.......

haddock

.....

polar

cod

/,

AGE

(years}

Fig. 7. Growth curves for the Barents Sea gadids (fitted by eye based on several sources). (Sources: Aleev, 1944, Trout, 1953; Schmidt, 1957; Lie, 1961; Mironova, 1961; Hognestad, 1969; cited by Sonina, 1969; Blacker, 1971; Reinsch, 1976; Sonina, 1979; Ponomarenko et al., 1980; Godo, 1983;

Borkin, 1984).

140

cod in the seventies has been registered for haddock as well (Sonina, 1978, 1979). Sonina sees this as a consequence of the cooling of the water masses leading to a westerly distribution of the stock for greater parts of the year. Western areas are said to offer an inferior food supply for haddock. Olsen (1966) found indications of density-dependent growth of juvenile saithe from Finnmark, More and Lofoten and states that the underlying mechanisms probably act primarily during the pre-exploitation phase ( first 2 years of life). The deviation from the overall mean length at a given age in the years covered (1954-1963) amounted to ~ 10%. Two- and three-year-old fish of a fast growing year class could have higher average lengths than 3 and 4 year olds of a slow growingyear class. Reinsch (1976) recognized a decrease in mean length in age groups 3-10 from the mid-sixties to the early seventies, but offered no explanation for the trend. Although the data on which many of the reported growth trends are based may be of variable quality, clear indications of considerable changes are found for all species. Thus far, our knowledge of the underlying mechanisms of such changes is limited and provides a great challenge for future work. RECRUITMENT

Variations in recruitment of cod, haddock and saithe during recent decades are shown in Fig. 8, while Ssetersdal and Loeng (1987) presented similar data for cod for the period 1902-1983. Large fluctuations are evident for cod and haddock, with a strong year class exceeding a weak one by a factor of ~ 60 for cod and haddock. For saithe, this factor does not exceed three during 1960-1980. Efforts to establish stock-recruitment curves have so far not met with much success, regarding their predictive values. Deficiencies in data on spawning stock abundance and population fecundity may partly account for this; the effects on spawning stock estimates by virtual population analysis (VPA) of applying different maturity ogives have been illustrated by Anon. (1984a). Garrod and Jones (1974) fitted a Ricker stock and recruitment equation for Northeast Arctic cod using data for 1942-1968 (estimated egg production vs. number of recruits and estimated egg production vs. recruitment as potential egg production). The optimum size of the mature stock was calculated to be at the level observed in the early fifties (1.2-1.5 million tonnes). They also observed that recruitment becomes increasingly variable at low stock levels. Much effort has been directed at determining the time at which year-class strength of cod is established and at the study of recruitment mechanisms. Hjort (1914) hypothesized that resultant year classes are small when food for the larvae is scarce and large when food is abundant. His notion of a 'critical period' following yolk absorbtion has been central to recruitment studies of cod ever since. Nauplii of Calanus finmarchicus are the most important food for cod larvae at the time of first feeding (Wiborg, 1948a; Sysoeva and

141 2000-

1S00

1000

SO0:

,,o 1000o =¢ B00-

B

600¢3¢ z=

~.00~ 200.

S00~00300-

200100-

19S0

1960

1970

1900

YEAR CLASS

Fig. 8. Recruitment of the Barents Sea gadids estimated by virtual population analysis (VPA). (A) cod, no. of 3 year olds; (B) haddock, no. of 3 year olds; (C) saithe, no. of 1 year olds. (Source: Anon., 1985).

Degtereva, 1965). Recent investigations in the Lofoten area have indicated the importance of a match between time of maximum nauplii abundance and time of peak abundance of early larvae, although there is no strong indication of a short 'critical period' of abrubtly increasing mortality (Tilseth, 1984; Ellertsen et al., 1986). The importance of predation as a regulator of year-class size has only recently been given appreciable attention. Data from the Lofoten area indicate that herring and several invertebrates are predators on cod eggs and larvae (Melle, 1986). Cannibalism, notably at the time of settling, has been suggested by several authors as a regulatory mechanism. However, no quantitative studies have been published. Maslov (1956) found a positive correlation between the number of 0-group cod and the corresponding importance of the year class in the commercial catches. Randa (1984) found a high correlation between indices of year-class strength for pelagic 0-group cod and haddock just prior to settling and VPA

142 estimates for the corresponding year classes. Ponomarenko (1984) does not consider the 0-group estimates to be a reliable indicator of the strength of a year class at the age of 3. Considering recent recruitment patterns, the predictive value of Randa's relation seems to have been somewhat over-rated. Several authors have proposed abiotic factors as regulatory mechanisms of year-class strength. The importance of the discharge of freshwater has been mentioned by Helland-Hansen and Nansen (1909), Gran (1923) and Sund (1924). Skreslet (1976, 1981 ) claimed to have observed a co-variation between year-class strength of Northeast Arctic cod (survival indices) and the discharge from the rivers in the Southern Norway the previous year. Sundby (1979), however, did not find any significant correlation for a longer time span. Ellertsen et al. (1986) point to the significance of temperature due to its strong influence on copepod spawning time and incubation period and hence of the timing of peak abundance of nauplii. Since the time of maximum larval abundance varies little from one year to another, the temporal overlap of larvae and prey may primarily depend on variation in prey abundance. Ssetersdal and Loeng (1987) made an attempt to correlate estimates of year-class strength for cod with the climatic regime in the Barents Sea for the period 1902-1983. They found that stronger than average year classes have arisen when temperature anomaly cycles change from cold to warm and suggest that the cod may regulate year-class size in a manner dependent on climate-related variation in juvenile feeding area. POPULATIONSTRUCTURE Large, apparently stochastic, variations in year-class strength can totally alter the age and length distributions from one year to another and may also affect growth rate and age at first maturity. Figure 9 illustrates that effect on age composition and the ratio of mature to immature fish caused by the recruitment of exceptional year classes. Cod Prior to the development of distant water trawl fleets, the cod stock was moderately exploited. The fishery was mostly confined to coastal waters and based on spawners and large immatures. Since then, the stock has been heavily fished and the effort has been concentrated on recruits, especially when strong year classes appeared in the fishery. This selective exploitation has affected the stock in two ways: (i) mean age in the spawning stock has decreased significantly since the thirties (Fig. 10) and (ii) the number of age groups in the spawning stock has been reduced. Fish older than 12 years made up a considerable fraction of the spawners in

143

3000-

~

~

COD

3. 2000-

t000s* 6* "7.

' ' ' 6

. . . .

'6

. . . .

'

. . . .

"1°°°77

'

. . . .

'. . . .

/\

1960

1965

19"/0

1975

19B0

1400

SAITHE

1200-

1000-

I.

800-

2-

600].

¢00200-

19160. . . .

r

.

1965

.

.

.

j

.

1970 YEAR

.

.

.

~

,

1975

,

,

,

~

,

,

1980

Fig. 9. Age composition (by numbers ) of the boreal gadid stocks estimated by VPA (Anon., 1982a, 1984a,b).

the early thirties (Rollefsen, 1953 ), while in the seventies fish of this age were very rare (Jorgensen, 1982). According to Ponomarenko and Yaragina ( 1981 ) and Ponomarenko (1982), there are more males than females among the youngest spawners while the opposite is the case among spawners 10 years and older. The lower number of

144

o

I

~2 o 3

• 11

,,.,.,. / ,

~

5

~

....

i ....

1930

I''''

l ' ' ' ' l ' ' ' ' i ' ' ' ' l ' ' '

19/,0

5

COD

1950

' I ' ' ' ' I ' '

1960

•

' ' I ' ' ' ' I ' ' ' '

1970

I 'i

1980

YEAR Fig. 10. Mean agein the spawningstock. Cod: after Pedersen,1984. (1, Ssetersda|and Hy|en, 1964; 2, Ponomarenko, 1967;3, basedon age distributions given by Hylen, 1962and Hylen and Dragesund, 1973;4, basedon data from Jakobsen, 1978a,1978b;5, Ponomarenko and Yaragina, 1981). Haddock, basedon data from Meyer (1968-1975) and Wagner (1976-1979).

old spawners have accordingly reduced the fraction of females in the spawning stock. As a consequence, the reproductive capacity of the stock may be below the level indicated by the biomass or number of spawners alone. Length at age seems to have increased during the period 1929-1978 (Ponomarenko, 1967a; Rorvik, 1979). This higher rate of growth may, to some extent, mask the reduction of mean length to be expected as a consequence of the reduced mean age of the spawning stock. Haddock

Sonina (1969) reviewed the age distribution in the catches of haddock from the southern Barents Sea from the years 1930-1965. She clearly demonstrated the effect of varying year-class abundance on stock structure. No clear trend is evident, but a higher fraction of large fish was caught during the first postwar years, 1946-1949. This was probably caused by the low rate of exploitation during the early forties, an effect also underlying an observation by Soetersdal (1956) of old haddock (14-17 years) from the Goose Bank. There are normally more males than females in the youngest age groups (age < 6 years) both among immature and mature fish (Sonina, 1970-1980). Mean

145 age in the spawning stock, based on data from German trawlers fishing on the spawning grounds off Vester~len, for the period 1955-1979 is given in Fig. 10. No significant overall trend is discernible, only short-term fluctuations most probably caused by varying recruitment. According to Soviet investigators, firsttime spawners make up the bulk of the spawning stock in some years (e.g., 1980: 85%; Sonina, 1970-1980), while the average for the period 1968-1978 showed a slight dominance of repeat spawners (55.6%) (Sonina, 1981 ). Saithe Little information on stock structure of saithe has been published. Data given in the ICES Annales Biologiques usually consist of age and length frequency distributions based on pooled data from catches in all seasons and do not justify any conclusions concerning long-term trends. VPA estimates of stock biomass indicate a decreasing stock size since the early seventies ( Fig. 7 ), with spawners contributing a gradually smaller fraction to the total biomass of the stock (Anon., 1984b). This change is probably caused by high fishing mortality. STOCK SIZE FLUCTUATIONS Stock size estimates of cod, haddock and saithe based on VPA are given in Fig. 11. For Polar cod, no data on stock abundance are available, but catch statistics indicate a rapid decline of this stock under the heavy exploitation in the mid-seventies. A comparison of Figs. 8 and 11 clearly demonstrates the strong influence of variable recruitment on stock size, most evident for cod and haddock {notably the year classes 1950 and 1969 for haddock). For all boreal gadids, stock sizes have decreased gradually during the seventies (Fig. 11). Apart from this trend, no inter-specific co-variation is obvious. Large fluctuations of the yield from the cod fisheries are documented by historical records, especially for the fishery on immature fish (Hjort, 1914; S~etersdal and Hylen, 1964). Pronounced stock size variations are thus most likely also seen under low rates of exploitation. The recruitment failure of cod and haddock in the last decade, combined with heavy exploitation, has reduced the stocks to very low levels. Nevertheless, the 1983 and 1984 year classes are abundant for cod and average for haddock and increasing stock sizes are expected provided that the fishery is properly managed (Ulltang, 1987). The stock of Polar cod appears to have recovered ( Anon., 1984c ). The saithe stock has not shown the dramatic fluctuations seen for cod and haddock, although at present the spawning stock is at a very low level {Anon., 1984b).

146

[00

7..

{:APELIN

121

HERRIN5

0,8

1.2" 1.2] 08 O~

0.8-.

1950

1960

1970

1980

YEAR

19'50

19'60

1970

19~

Fig. 11. Total stock size estimates of the most important commercialfish resourcesof the Barents Sea. {Source:Anon., 1985). FOOD AND FEEDING The gut contents of first-feeding as well as the more advanced gadid larvae are, as in other waters, dominated by small pelagic crustaceans. Wiborg (1948) recognized from early studies of cod in the Lofoten area, that the selection of food particles was mainly a function of the ratio between size (width) of the prey and the mouth size of the larvae. With increasing size a gradual shift in dominant prey from copepod nauplii, through copepodites, copepods and amphipods to euphausids seems typical, mainly based on cod data (Wiborg, 1948a,b, 1949, 1960b; Sysoeva and Degtereva, 1965; Ellertsen et al., 1977, 1981b). The diet of larval and post-larval saithe has never been studied in the relevant waters and for Polar cod and haddock, very little information is available. Copepod eggs and copepods are significant in the diet of the comparatively large Polar cod larva (Ponomarenko, 1967b). The significance of noncrustacean prey is difficult to assess, but seems variable. This category includes phytoplankton ('green remains'), protozoa, rotatoria, appendicularia and polychaete larvae (Wiborg, 1948a; Ellertsen et al., 1977 ) and possibly bacteria (Olafsen, 1984) for larvae, and appendicularia (Frittillaria sp., Oikopleura sp. ),

147 medusae and other fish larvae (Wiborg, 1960b) for post-larvae and metamorphosed juveniles. There are no field estimates of the incidence of cannibalism during the early stages. Of special interest would be a comparison of the diets of co-occurring fry of different species. The only report available is Wiborg's (1960b), in which interesting indications of differences between haddock and cod fry of similar size (40-110 m m ) appeared. Further studies along this line would seem to be of great interest in order to determine how the gadid and non-gadid fry partition the resources in the pelagic phase when spatial overlap appears to be extensive. The composition of the diet of the settled 0-group and I-group fish obviously depends on the composition of potential prey in the habitat, be it littoral, sublittoral or an offshore bank habitat. For all the boreal gadids, a gradual increase with size in the importance of fish in the diet is seen, while it is characteristic that plankton, primarily euphausids, remains significant. Principal prey taxa based on literature data are listed in Table III in order of 'importance'. Mainly due to deficiencies in the data collection and statistics, it is impossible to determine quantitatively the nutritional significance of different prey and hence the degree of inter- and intra-specific diet overlap. In general, saithe and cod show stronger piscivorous habits than haddock although fish are probably more important to haddock than is usually assumed. Cod and haddock show clear seasonality in their prey selection. The more pronounced example is cod feeding on spawning concentrations of capelin in the spring and on feeding capelin near the polar front in summer. The primary summer prey may be the highly abundant euphausids, however, while Pandalus borealis is consumed throughout the year. Haddock shows similar seasonality of capelin and euphausid predation ( Sonina, 1969 ) and may, in addition, be the most important consumer of capelin eggs ( S~etre and Gjosmter, 1975 ). The large saithe has strong piscivorous habits (Mironova, 1969 ), but seasonal dietary changes, although probably present, have not been described. PREDATORS Predators known to consume gadids incidentally or regularly range from invertebrates and fish eating eggs or damaging/consuming larvae (Melle, 1986) to birds eating fry and mammals preying on adults and juveniles. There are no estimates of the consumption by any of these predators, except for man, who is the principal predator on juveniles and adults. Harp seal (Phoca groenlandica) and cetaceans [ Minke (Balaenoptera acutorostrata), killer whale ( Orcinus orca) ] have cod, haddock, saithe and Polar cod as part of their diet, but other fish, primarily capelin and herring, may be their usual prey (Bjorge et al., 1981; Jonsg~rd, 1982). The magnitude and effect of the pinniped and cetacean predation is unclear.

148 TABLE III Prey taxa of the immature and adult gadids of the Barents Sea (based on various sources, see below) Predator

Prey of different age (size) groups Settled 0- and I-group

II-group and older

Cod

Euphausids ( Thysanoessa inermis ) , decapods (Pandalus borealis), amphipods, isopods, mysids, copepods, polychaetes, fish 1.2.3.

Fish (capelin,polar cod, redfish, herring), euphausids, decapods (Pandalus borealis), hyperiid amphipods, polychaetes, pteropods, ctenophores 2.3.9.10.11.12.13.14.15.18

Haddock

Polychaetes, euphausids, amphipods, fish fry (redfish), capelin 4

Echinoderms, molluscs, polychaetes, euphausids, hyperiid amphipods, decapods, fish (capelin, capelin eggs) 11.16.17

Calanus finmarchicus,

Fish (capelin, herring, blue whiting, norway pout, juv. cod, haddock), euphausids, decapods, copepods, appendicularia 5.19

Saithe

harpacticoid copepods, littoral amphipods, isopods, mysids, euphausids, polychaetes, molluscss,6 Polar cod

Calanoid copepods 7

Calanoid copepods, euphasids 7,s

1Wiborg (1948b, 1949); 2ponomaranko (1965, 1973, 1974, 1984); 3Mehl et al. (1986, workshop proceedings) ; 4Antipova and Baranova (1982); SMironova (1961); 6Lie (1961); 7Ponomarenko (1967b); SHognestad (1968); 9Hjort (1914); l°Zatsepin and Petrova (1939), cited by Ponomarenko and Yaragina (1978); nBrown and Cheng (1946); 12Ponomarenko and Yaragina (1978,1979, 1984 ); 13ponomarenko et al. (1978); 14Grahamet al. (1954); lSBeverton and Lee (1965) ; 16Sonina (1969); ~VSeetreand Gj~s~eter {1975); lSKlemetsen {1982); 19Nedreaas (1984). C a n n i b a l i s m is g e n e r a l l y t h o u g h t to be extensive, a l t h o u g h no reliable estim a t e s o f its significance as a r e g u l a t o r o f p o p u l a t i o n s i z e / r e c r u i t m e n t are available. GENERAL DISCUSSION Several decades of r e s e a r c h o n t h e B a r e n t s Sea gadoids h a v e p r o v i d e d considerable i n f o r m a t i o n o n t h e i r life history, d i s t r i b u t i o n a l p a t t e r n s a n d ecology. In e a c h section we have, however, p o i n t e d to t h e m o r e obvious gaps in our knowledge or u n d e r s t a n d i n g . In general, m o s t a t t e n t i o n has b e e n focussed on cod, less on h a d d o c k , saithe a n d P o l a r cod a n d v e r y little on a n y o f t h e o t h e r

149 species. Most studies had single species as targets and very few have been comparative. Indeed, it has proven rather difficult to perform an even superficial comparative analysis. One of the objectives of a comparative analysis would be to determine how these large stocks of species with, at least superficially, similar life history strategies can co-exist. Obviously, physiological and ecological strategies must have evolved which minimize the effects of predation and competition and permit the observed degree of spatial and temporal overlap. We believe that for the large gadids some of the more significant mechanisms have been indicated. The level of distributional overlap may seldom be very high due to differences in habitat or temperature preferences or timing of events like migration and spawning. Moreover, at the times overlap is high, e.g., during the pelagic 0-group phase and on juvenile feeding grounds, competition or predation may be limited by factors such as differences in feeding habits or preferred prey, high abundance of common prey or the size structure of the predator populations. Differences in developmental patterns or rates, including growth and reproductive strategy (age and size at maturity, fecundity) are probably also significant, although it is difficult to see how at this stage. If the impression is correct that interaction strengths are low among the gadids, it may be more fruitful to look closer at intra-specific regulatory mechanisms such as cannibalism and inter- and intra-year class competition. However, an extensive collection of basic biological and ecological data seems to be a prerequisite for an assessment of the relative importance of inter- and intraspecific mechanisms. The gaps in our knowledge of the biology of the gadids may be small compared with our lack of understanding of the gadids as elements of the Barents Sea ecosystem. The reason is the virtual absence of appropriate quantitative data on feeding and predation. Figures 11 and 12 summarize time series of abundance estimates and landings of non-gadid fish, shrimp and mammal stocks of the Barents Sea. They illustrate quite dramatic changes in abundance of single stocks like herring and capelin, which are the most abundant pelagic species and important gadid prey. Herring is present in the Barents Sea as juveniles, but was virtually absent from the late sixties until 1983-1984, following a severe depletion of the'stock (Dragesund et al., 1980). The capelin stock most probably increased appreciably in the same period, although there are no historical abundance estimates from this stock. Moreover, in the same period as the gadid stocks were reduced to a very low level, a completely new and extensive Pandalus fishery started on previously undetected grounds (Teigsmark, 1983). This may indicate increasing abundance of the stock in the absence of an important predator, although it cannot be confirmed. The redfish stocks (Sebastes menteUa and S. marinus) and the greenland halibut (Reinhardtius hippoglossoides) stock are heavily exploited. The stock of large baleen whales are severly depleted and the only abundant mammals at present

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are the minke whale and the harp seal. The harp seal stock is probably increasing from a present level of ~ 1 million animals ( Benjaminsen, 1979 ), while no reliable estimate exists for the Minke whale which utilizes the Barents Sea as a feeding area in summer. Figures 11 and 12 illustrate the impact of modern exploitation patterns and indicate that major structural changes occur in the ecosystem, either as a direct result of or as a response to exploitation. It is obvious that the gadids, due to their high abundance, wide distribution and central position in the food web, play a rather crucial role in the ecosystem. Speculations on effects of inter-specific links, be it cod-capelin, cod-shrimp, seal-cod or even whale-cod are frequent and should be met with appreciable increase in basic ecological research. Recent efforts in this direction have been described by Mehl et al. (1986). It remains our belief, however, that single-species studies are still rather important both as such and as input to ecosystem or multispecies models. A combination of the two approaches may provide the only sound basis for future management of the gadid stocks and the Barents Sea ecosystem. ACKNOWLEDGEMENTS

We should like to thank Dr. G. N~evdal and O. Nakken for helpful corrections and suggestions. We also acknowledge the assistance of the technical staff. The first two authors will submit the paper as partial fulfilment of the requirements for the Dr. scient.-degree at the University of Bergen.

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152 Anonymous, 1984a. Report of the Arctic fisheries working group. Counc. Meet. Int. Counc. Explor. Sea, 1984 (Assess:3): 1-66 ( Mimeogr. ). Anonymous, 1984b. Report of the saithe (coalfish) working group. Counc. Meet. Int. Counc. Explor. Sea, 1984(Assess:7): 1-93 (Mimeogr.). Anonymous, 1984c. Ressursoversikt for 1984. Fisken Havet, 1984 (S~ernummer 1): 1-69 (in Norwegian). Anonymous, 1985. Ressursoversikt for 1985 og tilstanden i havmiljoet for noen ~r fram til 1985. Fisken Havet, 1985 (Saernummer 1): 1-84 (in Norwegian). Antipova, T.V. and Baranova, Z.P., 1982. Feeding of the Barents Sea haddock fingerlings in autumn/winter 1969-78. Counc. Meet. Int. Counc. Explor. Sea, 1982 (G:17). Bailey, R.S., 1982. The population biology of Blue whiting in the North Atlantic. Adv. Mar. Biol., 19: 257-355. Bakken, E., Lahn-Johannesen, J. and Gjos~eter, J., 1975. Bunnfisk p~ den norske kontinentalskr~ning. Fiskets Gang, 34 (1975): 557-565. Baranenkova, A.S. and Khokhlina, N.S., 1964. On the condition of formation of the Arcto-Norwegian cod stock in the 1956, 1960 and 1961 year classes during the first year of their life. Trud~ polyar, nauchno-issled. Inst. morsk, r~b. Khoz. Okeanogr., Vyp. 16: 195-214. Baranenkova, A.S. and Khokhlina, N.S., 1968. Distribution of eggs, larvae and adults of the Norway pout off north-western Norway and in the Barents Sea. Rapp. P.-v. R~un. Cons. Int. Explor. Mer, 158: 90-100. Baranenkova, A.S., Sorokina, G.B. and Khokhlina, N.S., 1973. Distribution and abundance of eggs and larvae of main commercial fishes of the Barents Sea on drift routes from spawning areas in April-July 1969. Trud~ polyar, nauchno-issled. Inst. morsk, r~b. Khoz. Okeanogr., Vyp. 16: 195-214. Benjaminsen, T., 1979. Pup production and sustainable yield of White Sea Harp seals. FiskDir. Skr. Ser. HavUnders., 16: 551-559. Benko, Y.K., Dragesund, O., Hognestad, P., Jones, B.W., Monstad, T., Nozovtsev, G.P., Olsen, S. and Seliverstov, A.S., 1970. Distribution and abundance of 0-group fish in the Barents Sea in August-September 1965-1968, pp. 35-53. In: O. Dragesund (Editor), International 0-Group Fish Surveys in the Barents Sea 1965-1968. ICES Coop. Res. Rep., Ser. A, No. 18, 80 pp. Beverton, R.J.H. and Lee, A.J., 1965. The influence of hydrographic and other factors on the distribution of cod on the Spitsbergen shell ICNAF Spec. Publ., 6: 225-244. Bigelow, H.B. and Schroeder, W.C., 1953. Fishes of the Gulf of Maine. Fish. Bull. US Fish. Wildl. Serv., 53,577 pp. Bjorge, A., Christensen, I. and Oritsland, T., 1981. Current problems and research related to interaction between marine mammals and fisheries in Norwegian coastal and adjacent waters. Counc. Meet. Int. Counc. Explor. Sea, 1981(N: 18): 1-10 (Mimeogr.). Bjorke, H., 1984. Distribution of eggs and larvae of gadoid fishes from Stad to Lofoten during April 1976-1982. In: E. Dahl, D.S. Danielsen, E. Moksness and P. Solemdal (Editors), The Propagation of Cod Gadus morhua L. Flodevigen rapportser., 1, 1984: 365-394. Bjorke, H. and Sundby, S., 1984. Distribution and abundance of post larval Northeast Arctic cod. In: O.R. Godo and S. Tilseth (Editors), Reproduction and Recruitment of Arctic Cod. Proceedings of the Soviet-Norwegian Symposium, 26-30 September 1983, at Leningrad, Inst. Mar. Res., Bergen, pp. 72-98. Blacker, R.W., 1971. Synopsis of biological data on haddock Melanogrammus aeglefinus (L., 1758). FAO Fish. Synop. No. 84. Blindheim, J. and Loeng, H., 1981. On the variability of Atlantic influence in the Norwegian and Barents Sea. FiskDir. Skr. Set. HavUnders., 17: 161-189. Blindheim, J. and Nakken, O., 1971. Abundance estimation of the spawning Lofoten cod 1971. Counc. Meet. Int. Counc. Explor. Sea, 1971 (B:15): 1-5 (Mimeogr.).

153 Borkin, I.V., 1983. Distribution and length composition of Polar cod larvae in the Barents Sea in 1980. Ann. Biol., 37: 161. Borking, I.V., 1984. Soviet investigation on Polar cod in the Barents Sea in 1981. Ann. Biol., 38: 134-135. Borkin, I.V. and Shleinik, V.N., 1981. Results of investigations on the Barents Sea Polar cod in 1977-1980. Counc. Meet. Int. Counc. Explor. Sea, 1981 (G:24): 1-13 (Mimeogr.). Brown, W.W. and Cheng, C., 1946. Investigation into the food of the cod (Gadus callarias L. ) off Bear Island, and of the cod and haddock (G. aeglefinus L. ) off Iceland and the Murman coast. Hull Bull. Mar. Ecol., 3: 35-71. Corlett, J., 1958. Distribution of larval cod in the western Barents Sea. ICNAF Spec. Publ. 1: 281-288. Dalen, J., R~rvik, C.J. and Smedstad, O.M., 1977. Bunnfiskunders~kelser ved Bj~rnoya og VestSpitsbergen h~sten 1976. (Investigations on demersal fish at Bear Island and West-Spitsbergen in autumn 1976). Fisken Havet, 1977 (3): 29-51 ( in Norwegian). Dalen, J. and Smedstad, O.M., 1978. Bunnfiskundersokelser ved Bj~rnoya og Vest-Spitsbergen hosten 1977. ( Investigations on demersal fish at Bear Island and West-Spitsbergen in autumn 1977). Fisken Havet, 1978 (3) : 1-14 (in Norwegian). Dalen, J., Hylen, A., Jakobsen, T., Nakken, O. and Randa, K., 1984. Preliminary report of the Norwegian investigations on young cod and haddock in the Barents Sea during the winter 1984. Counc. Meet. Int. Counc. Explor. Sea, 1984 (G:44): 1-26 ( Mimeogr. ). Damas, D., 1909. Contribution h la Biologie des Gadides. Rapp. P.-v. R~un. Cons. Int. Explor. Mer, 10:1-277 (in French). Dannevig, G., 1895. The influence of temperature on the development of the eggs of fishes. 13th Annu. Rep. Fish. Board Scot., Pt. III, Sci. Invest., 147-152. Dannevig, G., 1953. The feeding grounds of the Lofoten cod. Rapp. P.-v. R~un. Cons. Int. Explor. Met, 136: Appendix B, 2 pp. Dementyeva, T.F. and Mankevich, E.M., 1965. Changes in the growth rate of the Barents Sea cod as affected by environmental factors. ICNAF Spec. Publ., 6: 571-577. Dommasnes, A. and Rottingen, I., 1977. Loddeunders~kelser i Barentshavet i september-oktober 1976. (Capelin investigations in the Barents Sea in September-October 1976). Fisken Havet, 1977(2): 47-59 (in Norwegian). Dragesund, O, and Hognestad, P.T., 1966. Forekomst av egg og yngel av risk i vest- og nordnorske kyst- og bankfarvann v~ren 1965. Fiskets Gang, 52:467-472 (in Norwegian). Dragesund, O. and Olsen, S., 1965. On the possibility of estimating yearclass strength by measuring echo-abundance of 0-group fish. FiskDir. Skr. Ser. HavUnders., 13: 48-75. Dragesund, 0., Midttun, L. and Olsen, S., 1970. Methods for estimating distribution and abundance of 0-group fish, 25-31. In: O. Dragesund (Editor), International 0-group Fish Surveys in the Barents Sea 1965-1968. ICES Coop. Res. Rep., Ser. A, No. 18, 80 pp. Dragesund, 0., Hamre, J. and Ulltang, O., 1980. Biology and population dynamics of the Norwegian spring-spawning herring. Rapp. P.-v. R~un., Cons. Int. Explor. Mer, 177: 43-71. Eggvin, J., 1932. Vannlagene p~ fiskefeltet. Aarsberet. Vedkommende Norges Fiskerier, 2 (1932) : 90-95 (in Norwegian). Eggvin, J., 1938. Oceanographical conditions in North-Norway connected with the cod fisheries. FiskDir. Skr. Set. HavUnders., 5: 23-47. Ekman, S., 1967. Zoogeography of the Sea. Sidgewick and Jackson, London. Ellertsen, B., Moksness, E., Solemdal, P., Sundby, S., Tilseth, S., Westg~rd, T. and Oiestad, V., 1977. Vertical distribution and feeding of cod larvae in relation to occurrence and size of prey organisms. Counc. Meet. Int. Counc. Explor. Sea, 1977(L:33) : 1-31 (Mimeogr.). Ellertsen, B., Solemdal, P., Str~mme, T., Tilseth, S. and Westg~rd, T., 1980. Some biological aspects of cod larvae (Gadus morhua L. ). FiskDir. Skr. Set. HavUnders., 17: 29-47. Ellertsen, B., Solemdal, P., Str~mme, T., Sundby, S., Tilseth, S., Westg~rd, T. and Oiestad, V.,

154

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