Recruitment of the barents sea plaice (Pleuronectes platessa L.)

229

Netherlands Journal of Sea Research 34 (1-3): 229-235 (1995)

RECRUITMENT OF THE BARENTS SEA PLAICE (PLEURONECTES PLATESSA L.)

M.V. KOVTSOVA and V.D. BOITSOV Polar Research Institute of Marine Fisheries and Oceanography (PINRO), 6 Knipovich Street, 183763 Murmansk, Russia

ABSTRACT To study recruitment of Barents Sea plaice, we used a series of annual data obtained during the 19721990 period on the abundance of eggs and larvae and strength of year classes entering the fishery. We calculated annual values for population fecundity and survival. To show the most important factors determining plaice recruitment, a multifactorial regression analysis was used. Year-to-year variations in plaice recruitment were significantly correlated with fluctuations in population fecundity and larval abundance. This suggests that year-class strength of Barents Sea plaice is determined before the end of the pelagic phase.

Key words: plaice, recruitment, interannual variation, Barents Sea

1. INTRODUCTION

2. MATERIAL AND METHODS

One of the main conditions for the successful assessment of a fish stock is the correct estimation of recruitment. In general, year-class strength is already determined at an early age, long before recruitment to the adult population occurs (Kender, 1944; Kislyakov, 1961; Izhevsky, 1960, 1961; Lisivnenko,1961; Ponomarenko, 1984; Templeman, 1965, 1972; Ryland & Nichols, 1967; Harding et aL, 1978; Grauman, 1972; Bowers, 1974; Erm, 1976; Leggett, 1986; Van der Veer, 1986; Leggett & DeBIois, 1994). Despite extremely high fecundity of the majority of commercial species, the survival of the early life-history stages is very low, in the order of hundreds or thousands of a percent. Strong variations in these survival rates result in considerable differences in year-class strength. The aim of this paper is not to review the whole list of literature on plaice survival but to analyse longterm observations on the Barents Sea plaice and to study the role of biotic and abiotic factors influencing survival of some year classes. Besides, the importance of the population fecundity and size of breeding stock in determining plaice recruitment is not yet known. In this paper we study the role of various factors in the survival of Barents Sea plaice during early life and in the determination of year-class strength.

2.1. ICHTYOPLANKTON SURVEYS Plankton surveys were carried out in the southern Barents Sea annually between April and July over the period 1972 to 1991 (Fig. 1). Also a number of stations were sampled in the Motovsky Bay, an area where mass spawning of plaice occurs. Each year, 35 stations were visited and a total of about 105 hauls were taken. Simultaneously, hydrographic observations were made. At all stations sampling was done by an IKS-80 net made of gauze N-140 with 80 cm diameter of opening, a length of 2 m and a mesh size of 0.5 mm. At every station a sample was taken in the surface layer and in addition oblique fishing was performed in 0-10 m, 0-25 m and 0-bottom layers at a fishing speed of 2.5 kn and with a duration of 10 min. To determine the vertical distribution of the larvae, along some stations a set of catches were taken from different depths from the bottom to surface. The precise volume of the catches was not measured but amounted roughly to 300 m 3. All catches were sorted out fresh on board and all eggs and larvae were collected manually. Flatfish eggs (plaice, long rough dab, yellowtail flounder, fluke) were visually identified under an MBC-2 binoc-

230

M.V. KOVTSOVA & V.D. BOITSOV

Barents Sea --° 300--, J

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Fig. 1. Ichthyoplankton sampling stations (o) in the Barents Sea. Main spawning area of plaice is indicated (shaded), together with the location where water level was measured and with dashed depth contours. ular according to Rass (1946). Mean numbers of eggs and larvae of the catch were taken as index of abundance. Mean lengths of larvae were measured in June-July.

veys between December and March during the migrations of the adult fish to the spawning areas. Population fecundity of plaice was calculated by the

2.2. DEMERSAL SURVEYS

TABLE 1 Mean individual fecundity (103 eggs) of Barents Sea female plaice, together with standard error (SE) and number of females analysed (N).

Abundance estimates of plaice were obtained from trawl surveys carried out in the area along the coast from 30 ° to 45°E annually during June - July of the 1972 - 1989 period. A grid of sampling stations was spread over the area with regular intervals of 20 miles. At each station fishing was done with a 7-m bottom trawl with a length of 49 m and one tickler chain in front of the net. The mesh size of the net was 12.5 x 12.5 cm and the cod end had a mesh size of 3 x 3 cm. Fishing speed was on average 3 kn and the duration of each haul was 1 h, All plaice were sorted out fresh and of each specimen total length (cm) and wet weight (g) were determined. Otoliths were removed for ageing under a light microscope in the laboratory. Material on fecundity was collected from trawl sur-

age group 7 8 9 10 11 12 13 14 15 16 20 21 23

103 eggs

SE

N

23 24 75 81 125 133 196 210 228 287 344 230 165

11.4 11.5 12.6 11.3 49.8 50.7 61.4 22.5 57.1 75.0 81.0 15.6 58.0

5 5 10 25 20 20 10 10 15 10 10 5 5

RECRUITMENT OF BARENTS SEA PLAICE

400-

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~ 10-

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0 1675

1680 1 3 8 5 1690 1695

1970

1675

1680

1985

1690

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Fig. 2. Mean number of plaice eggs per ichthyoplankton catch for each of the years of the 1972 - 1991 period,

Fig. 3. Mean number of plaice larvae per ichthyoplankton catch for each of the years of the 1972 - 1991 period.

method of Serebryakov e t al. (1984). The total fecundity of the population (F; eggs) was estimated as:

recruitment were estimated according to: Ni

P ' = Y. C i

S i = ~-~ pl

where i is the age group from the age of recruitment onwards (4 years). Cz is the contribution of the mature plaice from age group i to the population fecundity. C i was determined by the formula: C i ~ Fi'N'mi'R

i

where F i is the mean fecundity of a fish from group i (Table 1); N i is the number of fish of group i; m i is the fraction of mature fish from group i, and R i is the portion of females, i.e. the ratio in age group i. Indices of survival rates between spawning

age age age sex and

where Si is the survival rate of year class i, Ni is the abundance of year class i at recruitment (age 4) and F tW is the . population fecundity of year class i. In a ~ . . . s~mflar way survival rates dunng the vanous stages of early life history were calculated. 2.3. STATISTICAL ANALYSIS Multifactorial linear regression models were used to assess the most important factors determining plaice recruitment. The reliability of the parameters of the models was estimated by Students t-test and 10

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numberofeggs Fig. 4. Relationship between mean number of eggs and subsequent mean number of larvae per ichthyoplankton catch for each of the years of the 1972 - 1990 period.

Fig. 5. Mean length (mm) of the plaice larvae in June-July in the ichthyoplankton catches for each of the years of the 1972 - 1990 period.

232

M.V. KOVTSOVA & V.D. BOITSOV

TABLE 2 Estimates of the abundance of plaice (specimens per mean weighted catch per 1h. trawling) at different ages from the yearclasses 1972-1989 (data from trawl survey). sampling year

1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989

age (years) 4

44 12 37.5 19 2 4.5 2 10 6 8 4.5 15.5 15 20.5 27 40.6 25.5 20

5

43 12.2 32.5 18.5 1.9 4 1.6 13 4 7 3.6 12 14.7 24.4 24.3 37.3 30 32

6

7

8

67 231.8 180 105.4 117.6 64.2 59.7 4 7 . 3 133 12.5 94 132.8 27 77.2 108 17.3 87 59 53 38 57.6 15 37.8 27.3 14.6 19.6 8.8 5.1 4.3 9 1.8 1 4 . 7 28.1 14.7 33.1 17.6 32.1 18.8 21.2 26.5 20.6 20.2 50.6 45 45.5 80.8 80 50.5

9

69.1 95 126 120 63 91.9 60.3 12.6 12.7 22.8 8 8.9 20 20.5

10

58 74.6 84 59 36.6 68.5 13.9 10.6 21.3 4.3 12.8 7 15

assessed using the statistics. 3. RESULTS 3.1. ICHTYOPLANKTON SURVEYS The mean number of eggs per catch showed considerable variation over the years (Fig. 2). In most years the mean number per haul was lower than 100 eggs. In some years higher mean numbers were found, with an extreme observation of 360 eggs per catch in 1972. The mean number of larvae per catch also showed considerable variability over the years (Fig. 3). In most years less than 10 larvae were found per haul, but in 1972 and in 1985 much higher numbers were observed of 30 and 20 larvae, respectively. No significant relationship was found between the mean number of eggs and the mean number of larvae per haul (Fig. 4). The mean length of the larvae fluctuated between 4.5 and 10 mm (Fig. 5). No significant relationship was found between mean larval abundance and mean larval length (Fig. 6).

11

45.7 54 55 96.6 80.8 16.3 13.8 21.3 3.8 1.3 10.5 7

12

13

14

15

16

17

30 40 78.6 76.2 17.4 16.7 29.6 7 2 2 8

18 41.9 67 9 12.2 17.3 3.2 0.6 1 1.5

27.4 53.6 4.2 7.8 6.3 2.3 0.6 0.8 0.5

27.8 1.7 4 4.8 0.8 0.5 0.5 0.5

0.7 0.7 1.5 0.3 0.5 0.3 0.3

0.4 0.7 0.5

started to decrease again. The annual fecundity of the mature population fluctuated between 2.2 and 44.7-1012 eggs (Fig. 8). Lowest population fecundity was observed at the end of the 70s. Hereafter a steady increase in population fecundity occurred up to a level of about 40.1012 eggs in the late 80s. 3.3. SURVIVAL RATES Survival rates were calculated at different stages from early ontogenesis until age 4, when the recruits enter the commercial stock (Table 3). The survival rates of the Barents Sea plaice at different stages of ontogeny varied from year to year particularly at the early stages of development. Maximum values of the survival rate from eggs (stages 1-4) to larvae exceeded the minimum ones 230 times, and for larvae to recruitment at age 4 the difference was 14.5 times. This suggests that plaice mortality decreased gradually with age. 3.4. STATISTICAL ANALYSIS

3.2. DEMERSAL SURVEYS Recruitment of plaice was defined as the abundance of age group 4 and is shown in Table 2, first column. It fluctuated considerably over the years (Fig. 7). After a period of high numbers at the beginning of the 1970s, recruitment dropped to low numbers in the period 1976 to 1982. Hereafter an increase was observed to the same level as at the beginning of the 1970s. In the last years, 1988 and 1989, recruitment

In the multifactorial linear regression model, a number of biotic and abiotic parameters were used as factors. The following parameters were taken: population fecundity; spawning stock abundance; index for egg abundance (log transformed); index of larval abundance (log transformed); mean length of plaice larvae and the survival rate between the various life stages (egg, larvae, recruitment (log transformed)). Furthermore, the abundances of the two most com-

RECRUITMENT OF BARENTS SEA PLAICE

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number of larvae per haul Fig. 6. Relationship between mean length and mean number of larvae in the ichthyoplankton catches for each of the years of the 1972 - 1990 period. mon zooplankton species were incorporated, because plaice larvae mainly feed on them: the abundance of Calanus nauplii in the Barents Sea coastal waters and an index for young euphausiid abundance in the coastal waters of Murmansk (log transformed). The abiotic factors were related to water temperature and considered to reflect water movements in the area: the water temperature in the 0 to 50 m layer in the Murmansk coastal zone in April-June, the water temperature in the 50 to 200 m layer in the coastal zone of Murmansk in the second half of the year; the water level in the Ekaterininskaya harbour in MayJuly and the atmospheric pressure off Vardoe in MayJuly. Statistical analysis revealed significant correlations between a number of these factors. An inverse correlation was found between survival rate from larvae to age 4 and atmospheric pressure along the coastal zone west of Murmansk in May - July (r=-0.66). This 50 cn ob

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m Q_ O CZ_

201001970

1075

1980

1085

1990

Fig. 8. Population fecundity of Barents Sea plaice (in 1012 eggs) for each of the years of the 1971-1990 period.

Fig. 7. Recuitment of Barents Sea plaice, expressed as abundance (n.h-1) of 4-year-old individuals, from VPA estimates for the period 1972-1990. relationship probably resulted from the wind regime impact upon larval redistribution and, as a consequence, upon their survival. When the atmospheric pressure decreases the northern and northwestern winds increase and this promotes transport of larvae into the coastal inlets and bays where survival conditions may be better than in offshore areas. A strong relationship existed between recruitment and population fecundity (r=+0.69). The rich year classes appeared most frequently in the years when the population fecundity was high. Besides population fecundity, also larval abundance correlated positively with plaice recruitment (r=+0.72). These three factors, population fecundity, larval abundance and larval surTABLE 3 Indices of survival rates of plaice year-classes from eggs to the age of 4+. year of survival rate estimation of abundance mentrecruit- 4+/PF larvae~eggs 4+~larvae of recruitment 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88

0.0000873 0.0000700 0.0000294 0.0000150 0.0000004 0.0000085 0.0000149 0.0002157 0.0003571 0.0001380 0.0001622 0.0000404 0.0000284 0.0000495 0.0000546 0.0000413 0.0000315

0.92 1.48 2.00 0.18 0.30 0.11 0.01 0.40 1.22 0.63 1.60 1.60 1.82 2.30 1.30 1.70 1.48

1.47 7.37 12.43 4.75 1.00 2.25 1.80 13.00 1.20 2.67 2.10 0.88 7.50 1.38 10.00 4.10 3.33

rich middle rich middle poor poor poor poor poor poor poor middle middle middle rich rich rich

234

M.V. KOVTSOVA & V.D. BOITSOV

vival, could explain 84% of the interannual variation in recruitment. 4. DISCUSSION The life cycle of Barents Sea plaice corresponds to that found in other areas, such as the North Sea. As in the North Sea (Talbot, 1976, 1978) specific spawning areas occur. A main location for Barents Sea plaice is the Motovsky Bay and the coastal zone west of Murmansk. The highest larval concentrations are found in the 0 to 25 m layer. Below 25 m only few eggs were caught. Larvae were also mainly distributed in the 0 to 25 m layer. After spawning the plaice eggs and larvae are transported by currents towards coastal waters along the Murmansk coast, where metamorphosis takes place and the larvae immigrate through inlets and bays and settle on the bottom. This mechanism appears to be similar to the one described for North Sea plaice (Bannister et aL, 1974; Kuipers, 1977; Creutzberg et aL, 1978; Zijlstra et aL, 1982). Plaice spend their first three years of life in places where their larvae have been transported to, as they do in nursery areas around the North Sea, such as the Wadden Sea (Zijlstra, 1972). The multiple linear regression models explaining recruitment revealed significant relationships with some biotic and abiotic factors. However, these results are especially dependent upon the factors selected. The hydrodynamic conditions clearly determine egg and larval transport. Unfortunately no direct measurements were available, so indirect indices for water dynamics and air circulation (sea level, atmospheric pressure and water temperature) had to be used. On the coast of Murmansk, sea level is an indication of water advection to the Barents Sea from the west. Food availability and predation are two other important factors for the survival of juvenile plaice (for review see: Leggett, 1986; Bailey, 1994; Leggett & DeBIois, 1994). In the North Sea plaice larvae mainly feed on Oikopleura dioica (Shelbourne, 1953; Wyatt, 1974; Last, 1980). In the Barents Sea, the main food objects are copepods, and Oikopleura is of less importance. In the zooplankton, Calanus finmarchicus and Pseudocalanus elongatus predominate during the spring period. Therefore, the number of Calanus nauplii in the 0 to 50 m layer was considered to be a useful index of zooplankton abundance. No information is available on predators and predation rates during the successive life stages of juvenile plaice in the Barents Sea. Instead, indirect indices were estimated, based on the survival rates for the various life stages. The significant positive correlation between recruitment and potential egg production in Barents Sea plaice contrasts with the general lack of such a relationship in other flatfish populations (lies, 1994). This might suggest that the variability in mortality during

the early life history is insufficient to override the variation in population fecundity. However, in this case the positive relationship between recruitment and population fecundity appears to be due to the coinciding time trends in recent years. Any conclusion, therefore, has to be tentative. More research is required to obtain insight into the underlying mechanisms. 5. REFERENCES Bailey, K.M., 1994. Predation on juvenile flatfish and recruitment variability.--Neth. J. Sea Res. 32: 175-189. Bannister, R.C.A., D. Harding & S.J. Lockwood, 1974. Larval mortality and subsequent yearclass strength in the plaice (Pleuronectes platessa L.). In: J.H.S. Blaxter. The early life history of fish. Springer-Verlag, Berlin: 21-37. Bowers, A.B., 1974. Marine fish culture in Britain. Growth of cultured plaice to marketable in the laboratory.--& Cons. perm. int. Explor. Mer 35: 149-157. Creutzberg, F., A.Th.G.W. Eltink & G.J. Van Noort, 1978. The migration of plaice larvae Pleuronectes platessa into the western Wadden Sea. In: D.S. McLusky & A.J. Berry. Physiology and behaviour of marine organisms. Proc. 12th EMBS. Pergamon Press, New York: 243251. Erm, V.A., 1976. On factors determining the abundance of pike-perch in the Pyarny harbour.--Trudy BALTNIR© 12:51-58 (in Russian). Grauman, G.B., 1972. Causes of fluctuations of abundance of cod and sprat in the Baltic Sea.--Trudy VNIRO 87: 249-269 (in Russian). Harding, D.H., J.H. Nichols & D.S. Tungate, 1978. The spawning of plaice (Pleuronectes platessa L.) in the southern North Sea and English Channel.--Rapp. P.-v. Reun. Cons. int. Explor. Mer 172:102-113. lies, T.C., 1994. A review of stock-recruitment relationships with reference to flatfish populations.--Neth. J. Sea Res. 32: 399-420. Izhevsky, G.K., 1960. On natural laws of formation of commercial productivity of seas. In: G. Izhevski. Problems of commercial productivity of seas. Pitschepromizdat, Moscow: 5-15 (in Russian). - - , 1961. Oceanographic grounds of formation of commercial productivity of seas. Pitschepromizdat, Moscow: 1-216 (in Russian). Kender, R., 1944. Untersuchungen LJber den Baltischen Dorsch w&hrend der Forschungsfahrten mit dem F.D. 'Poseidon' in den Jahren 1925-1938.--Ber. Dt. Wiss. Komiss. Meerforsch. 5 (1): 24-45. Kislyakov, A.G., 1961. Connection between the hydrographic conditions and fluctuations of abundance of cod year-classes.--Trudy sovetschaniya po dinamike chislennosti ryb. 13:260-264 (in Russian). Kuipers, B.R., 1977. On the ecology of juvenile plaice on a tidal flat in the Wadden Sea.--Neth. J. Sea Res. 11: 56-91. Last, J.M., 1980. The food of twenty species of fish larvae in the west-central North Sea.--MAFF, Fish. Res. Tech. Rep. 60: 1-44. Leggett, W.C., 1986. The dependence of fish larval survival on food and predator densities. In: S. Skreslet. The role of freshwater outflow in coastal marine ecosystems. NATO ASl Series Vol. G7, Springer-Verlag, Berlin:

RECRUITMENT OF BARENTS SEA PLAICE

117-137. Leggett, W.C. & E. Deblois, 1994. Recruitment in marine fish species: is it regulated by starvation and predation in the egg and larval stages?--Neth. J. Sea Res. 32: 119-134. Lisivnenko, L.N., 1961. Plankton and feeding of larvae in the Riga Bay.--Trudy Latv. otd. VNIRO 3:105-108 (in Russian). Ponomarenko, I.Ya., 1984. Survival of bottom-dwelling young cod in the Barents Sea and the factors determining it. Proc Soviet-Norwegian Symp. Reproduction and Recruitment of Arctic Cod. Leningrad, 26-30 Sept. 1983, Inst. of Mar. Res., Bergen, Norway: 213-229. Rass, T.S., 1946. Stages of ontogenesis of teleost fish.-Zool. magazin. 25:127-148 (in Russian). Ryland, J.S. & J.H. Nichols, 1967. Effect of temperature on the efficiency of growth of plaice prolarvae.--Nature 214: 529-530. Serabryakov, V.P., V.M. Borisov & V.K. Aldonov, 1984. Population fecundity of yearclasses of Arcto-Norwegian cod. Proc. Soviet-Norwegian Symp. Reproduction and recruitment of Arctic cod. Leningrad, 26-30 Sept. 1983. Inst. Mar. Res., Bergen, Norway: 36-71. Shelbourne, J.E., 1953. The feeding habits of plaice postlarvae in the Southern Bight.~. mar. biol. Ass. U.K. 32" 149-160. Talbot, J.W., 1976. The dispersal of plaice eggs and larvae in the Southern Bight of the North Sea.--& Cons.

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perm. int. Explor. Mer 37." 221-248. - - . , 1978. Changes in the plaice larval dispersal in the last fifteen years.--Rapp. P.-v. Reun. Cons. perm. int. Explor. Mer 172:114-123. Templeman, W., 1965. Relation of periods of successful yearclasses of the Grand Bank to periods of success of yearclasses for cod, haddock and herring in areas to the North and East.--ICNAF Spec. Publ. 5= 523-533. --, 1972. Year-class success of some North Atlantic stocks of cod and haddock.--ICNAF Spec. Publ. 8: 223-235. Van der Veer, H.W., 1986. Immigration setlement and density-dependent mortality of a newly settled 0-group plaice (Pleuronectes platessa) population in the western Wadden Sea.--Mar. Ecol. Prog. Ser. 29: 223-236. Wyatt, T., 1974. The feeding of plaice and sand-eel larvae in the Southern Bight in relation to the distribution of their food organisms. In: J.H.S. Blaxter. The early life history of fish. Springer-Verlag, Berlin: 245-251. Zijlstra, J.J., 1972. On the importance of the Wadden Sea as a nursery area in relation to the conservation of the southern North Sea fishery resources.--Symp, zool. Soc. London 29: 233-258. Zijlstra, J.J., R. Dapper & J.IJ. Witte, 1982. Settlement, growth and mortality of post-larval plaice (Pleuronectes platessa L.) in the western Wadden Sea.--Neth. J. Sea Res. 15: 250-272.