E. G. Lukmanov and N. V. Mukhina. Introduction

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1 Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: Some aspects of saithe (Pollachius virens L.) abundance dynamics during early life stages in relation to the effect of biotic and abiotic environmental factors during the formation of year classes E. G. Lukmanov and N. V. Mukhina Lukmanov, E. G., and Mukhina, N. V Some aspects of saithe (Pollachius virens L.) abundance dynamics during early life stages in relation to the effect of biotic and abiotic environmental factors during the formation of year classes. - Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: On the basis of a long time series, considerable variations in abundance of larval and 0-group stages of the northeast Arctic saithe population were found to be typical. Variations in environmental conditions on the edge of the reproductive area in the spawning and post-spawning periods affect the formation of year classes. Water temperatures over the Røst Bank in April in the m layer, as well as the biomass of planktonic food in the Eastern Branch of the Norwegian Current in the post-spawning period (June-July), are factors restricting year-class abundance at the larval and 0-group stages. Larval saithe distribution is under the influence of the Eastern Branch of the Norwegian Current and Coastal Branch of the North Cape Current, but no quantitative estimates of relationships between velocities of currents and abundance of larvae were made. The number of fish at age 1 may be determined by the abundance of larvae, biomass of planktonic food and velocity of the Coastal Branch of the North Cape Current. No relationship between the spawning stock biomass and indices of saithe abundance at early stages of ontogeny was observed. E. G. Lukmanov and N. V. Mukhina: Polar Research Institute o f Marine Fisheries and Oceanography (PINRO ), 6 Knipovich Street, , Murmansk, USSR. Introduction Year-class strength is mainly estimated during the first year of life and depends on the conditions for fish reproduction and survival during early stages of ontogeny (Hjort, 1914; Dekhnik et al., 1985). A wellmarked relationship between stock and recruitment, which is some evidence for recruitment dependence on environmental factors, is not observed for the northeast Arctic saithe. At present the nature of those factors is poorly studied. According to Garrod and Colebrook (1978) the similarity of variations of relative strengths of year classes from different stocks of northeast Atlantic saithe may serve as an index of the total climatic effect on commercial fish stocks. Jakobsen (1987) suggests that the abundance of saithe year classes is less affected by environmental fluctuations compared with other Gadidae because of a more stable nutritive base (various fishes and crustaceans forming dense concentrations make up the main food for saithe). Temperature of water is one of the main environmental factors in the life cycle of saithe because saithe is a more warm-requiring species than cod. In the Barents Sea, saithe are distributed along the branches of the warm current (Mironova, 1960), the main concentrations usually being observed within the coastal front zone, where the Atlantic and coastal waters are interacting. Depending on their areas of distribution, saithe spawn at various temperatures. Thus, spawning off the northwestern and northern coast of Norway was observed at surface temperatures of C (Lukmanov et al., 1975), that in the North Sea at C (Berenbeim and Golubyatnikova, 1984). Duration of the embryonic period depends on water temperature. Development of saithe eggs at temperatures of 4~5 C lasts for 19 d and at 7-8 C for 13 d (Ehrenbaum, 1936). A number of investigators noted that the water temperature during spawning has a so-called inertia of influence that enhances survival, not only in the spawning period but also during the first years of life (Berenbeim et ai, 1983). Velocity and direction of currents have a significant influence upon distribution, migration, and density of 319

2 saithe concentrations. North Sea saithe are usually distributed in the direction of main flows, keeping closer to the edge of the North Sea shelf and using the flows for horizontal swimming (Chuksin et al., 1984). In the Lofoten area, on the Fugløy and Malangen banks, where the Atlantic waters circulate clockwise forming a half-closed system of currents, immature fish form over-wintering concentrations (Lukmanov et al., 1975). The aim of this paper is to analyse the influence of environmental conditions, in particular water temperature, planktonic food biomass, velocity of the Norwegian and North Cape Currents during spawning and post-spawning periods, and also to study how parent stock size affects the formation of northeast Arctic saithe year-class abundance. Materials and methods Reports of the ICES Arctic Fisheries Working Group (Anon., 1982,1987) are the main source of information on abundance and biomass of saithe stock. Data on world catches from Divisions I and Ha (the Barents and Norwegian Seas) for (Anon., 1982, 1987) were used as an indirect index of saithe total stock size. Data on larval stock assessment from the ichthyoplankton surveys conducted in April-July in the northeastern Norwegian and southwestern Barents Seas (Fig. 1), the results of the Barents Sea international 0- group fish surveys for (Anon., 1985), and materials of the ICES Working Group concerning the absolute abundance of saithe at age 1 estimated by VPA (Anon., 1982, 1987) were used as criteria for a yearclass abundance assessment. Quantitative estimates of population abundance indices are expressed in common logarithms. Using the dynamic method (Zubov and Mamaev, 1956; Mukhina et al., 1987) relative velocities in the 0-10 m layer were calculated to study the intensity of the Eastern Branch of the Norwegian and North Cape Currents. Mean values of water temperature in the Eastern Branch of the Norwegian Current in the m layer in April (along section 7-C on the Røst Bank) were used as environmental predictors. Data on biomass of planktonic food in the Eastern Branch of the Norwegian Current in the 0-50 m layer (Sections 7- C, 8-C, 9-C, 10-C, 11-C) in June-July were used to estimate availability of food for saithe larvae. Correlation analysis and multiple regression methods were applied to determine relationships between indices of year-class abundance and environmental factors. Reliability of the relationship was determined by the Fisher criterion. Results and discussion Fluctuations in fish population abundance take place at all stages of ontogeny, but the highest variations are observed during the embryonic and larval periods. Fish survival at early stages of ontogeny is known to comprise only hundredths to thousandths parts of one per cent of the initial number of eggs (Dekhnik et al., 1985). The highest fluctuations in northeast Arctic saithe abundance are observed at larval and 0-group stages (Fig. 2). Maximum catches of larvae exceed the minimum by nine times. Variability of the abundance value is highest at the stages mentioned where the coefficient of variation is 63%. Variability of abundance sharply / 74 «35 9-C IV» 0.8 c: «e 0.2 * * 30' Figure 1. Positions of sections and stations during the ichthyoplankton survey. Roman letters indicate Divisions. 1 - standard stations; O 2 - non-standard stations;-----* 3 - Eastern Branch of the Norwegian Current (w arm ); *4 - Bear Island-Spitsbergen Current (cold) year Figure 2. Saithe abundance in different periods of ontogeny. 1 - Larvae in April-July (specimens); 2-0-group (specimens h ' '); 3 - Fish at age 1 (million specimens); 4 - Fish at age 3 (million specimens); 5 - Spawning stock biomass (000 t); 6 - World catch (0001) in Divisions I and lia in the Barents and Norwegian Seas (north of 62 N). 320

3 Table 1. Correlation coefficient (r) between abundance indices for northeast Arctic saithe. Indices 0-group Fish abundance at ages 1-3 (VPA) 1 + VPA 2 + VPA Spawning stock biomass (0001) World catch (Divisions I and lia) (0001) Larvae group VPA VPA Spawning stock biomass 0.54 decreases at subsequent stages. At ages 1 and 3 the coefficient of variation makes up in total 2.3%; abundance and biomass of spawning stock vary within 4.9 and 3.7% respectively, and size of catches is 2.9%. No clear and logical relationship between the estimates of saithe year-class abundance at different stages of ontogeny was found (Table 1). There was also no relationship between spawning stock biomass and indices of abundance of saithe at early stages of development. According to the data obtained by Norwegian scientists (Jakobsen, 1987) saithe abundance at age 1 may be taken as an index of year-class strength. We assume that the formation of abundance of each yearclass of northeast Arctic saithe to be completed by age 1 and for that reason our investigations are limited to analysing the effects of environmental factors on yearclass abundance at stages from larva to age 1. Variability of saithe abundance at the larval stage off the Lofoten Shallows takes place under the influence of several factors. This area is the northern edge of the northeast Arctic saithe reproduction area (Baranenkova, 1957) where, as a rule, sharp variations of environmental conditions and abundance of spawning fish are observed. According to our observations the main concentrations of larvae up to 6 mm long were found on the Røst and Malangen banks (Table 2). Dynamics of surface currents affect the distribution of larvae. In spring, the Lofoten Shallows is an area of increased Table 2. Abundance (specimens) and length of larval saithe off the Lofoten Shallows and in the Barents Sea according to ichthyoplankton surveys data. April-May June-July Area Size Mean length Number of specimens Size Mean length Number of specimens Røst Bank Vesterålen Bank Andøy Bank Malangen Bank Fugløy Bank Søre Bank Kopytov Bank Norwegian Deep Nordkyn Bank Finnmarken Bank Rybachya Bank Kildin Bank Southern slope of Bear Island Bank Western Coast (up to 37 43'E) Central Plateau Demidov Bank (up to 74 30'N) Motovsky Bay Deep waters (71 10'N, 12 10'E)

4 velocity of the Eastern Branch of the Norwegian Current. Cyclonic gyres which, in their turn, promote formation of larval concentrations in sites of larval hatching, were observed on the Røst, Vesterålen and Malangen banks (Mukhina et al., 1987). From Table 2 it may be seen that the area of larval saithe distribution is rather large in April-July; thus, the eastern boundary of larval distribution is observed along 37 43'E and the western one along 12 10'E. During this period larvae are transported northward to 74 30'N; this is undoubtedly because of surface currents. In spite of the fact that a quantitative estimate of the relationship between larval abundance and velocity of the Norwegian and North Cape Currents during the spawning period off the Lofoten Shallows is lacking, this relationship is poorly pronounced at subsequent stages of development and makes up 0.27 for 0-group and 0.30 for age 1, although statistically it is not significant. Temperature conditions off the Lofoten Islands are determined mainly by the Atlantic Current; they are characterized by considerable heat inertia (Mukhin and Dvinina, 1982). The influence of water temperature during the spawning period lasts for North Sea saithe up to the age 3 (Berenbeim and Golubyatnikova, 1986). According to our data the value of the relationship between water temperature in the m layer on the Røst Bank in April and the abundance of larvae in April-July makes up 0.45 at a significance level of The relationship for the 0-group is also pronounced (r = 0.40) but it is not significant statistically; no relationship is observed during the spawning period for ages 1 and 3. Thus, temperature conditions during the spawning period on the edge of the area of reproduction only affect the abundance of larvae. Biomass of planktonic food is a relatively reliable index of the status of prey for fish larvae (Degtereva, 1979), and in the Lofoten area it is, as a rule, determined by development of Calanus finmarchicus. The Norwegian Current waters are most productive in April-July (the Røst and Malangen banks areas). Values of the relationship between biomass of planktonic food in the 0-50 m layer in the Eastern Branch of the Norwegian Current and abundance of saithe at different stages of early ontogeny indicate that the mean actual relationship (r = 0.47 at n = 23, P = 0.05) is observed only at the larval stage. At subsequent stages of development, the relationship between 0-group fish and availability of food vanishes, but the inertia of influence of the factor mentioned up to age 1 (0-group: r = 0.34, n = 16, P = 0.05; age 1: r = 0.32, n = 23, P = 0.05) is maintained. It should be noted that the food factor is more significant than temperature, and food availability for larvae determines opportunities for an organism during its development. It is especially pronounced when determining factors responsible for the abundance of saithe in the period from larva to oneyear-old fish. Abundance of saithe year classes is therefore determined during their first year of life under the influence of environmental factors characteristic for each period of the early life cycle; thus, from knowledge of water temperatures and biomass of planktonic food during spawning and post-spawning periods, we are able to determine the relative abundance of larval saithe in April-July. The dependence between these indices can be expressed by the equation: yi = X] 1.4 x2 (1) (R = 0.63, P = 0.05, n = 23) where yi is a common logarithm of larval saithe abundance (in individuals) in April-July; Xi is water temperature ( C) in the m layer in April along the section 7-C (Røst Bank); and x2 is a common logarithm of food plankton biomass in the 0-50 m layer in the Eastern Branch of the Norwegian Current in June-July (mg n r 3). At the 0-group stage (August-September), abundance of saithe depends on both water temperature and the biomass of planktonic food and relative abundance of larvae in April-July. Close connection between these predictors is determined from: y2 = Xj +1.9 x x3 (2) (R = 0.65, P = 0.05, n = 16) where y2 is a common logarithm of the 0-group saithe abundance (in individuals h ' 1) in August-September; and x3 = y^ Up to the end of the first year of life of a year class, environmental conditions during the spawning period still have a significant influence on year-class abundance. The abundance of one-year-old saithe may be determined by the number of their larvae, biomass of planktonic food and velocity of the Coastal Branch of the North Cape Current during the post-spawning period: y3 = x, x x4 (3) (R = 0.66, P = 0.05, n = 16) where y3 is absolute abundance of one-year-old saithe calculated by VPA; and x4 is relative velocity in the 0-10 m layer of the Coastal Branch of the North Cape Current in April in cm sec^1. References Anon Report of the saithe (coalfish) Working Group. ICES CM 1982/Assess: 9, p. 96. Anon Preliminary Report of the International 0-group Fish Survey in the Barents Sea and Adjacent Waters in August-September ICES CM 1985/G: 75: 27 pp. Anon Report of the Arctic Fisheries Working Group. ICES CM 1987/Assess:

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