Andrzej Kompowski XX Polar Symposium Department of Biological Marine Resources Lublin, 1993 Agricultural College Szczecin, Poland THE STATE OF THE BLACKFIN ICEFISH (CHAENOCEPHALUS ACERATUS, (LÓNNBERG, 1906), (PISCES, CHANNICHTHYIDAE) STOCK OFF SOUTH GEORGIA (ANTARCTIC) IN 1988-1992 INTRODUCTION The changes in the Antarctic marine environment and resulting fluctuations of fish year-class abundance are of great importance to the fishery. The fluctuations of year-class strength are observed among others in blackfin icefish. The monitoring of the length-and-age structure of the South Georgia blackfin icefish stock has been continued since 1976. The objective of this paper was to present the results of blackfin icefish stock monitoring made during the past four fishing seasons. The fish samples were collected at random from the catches made with a bottom trawl during the research cruises of r. v. „Profesor Siedlecki" (1988/89), m. t. „Hill Cove" (1989/90) and m. t. „Falkland Protector" (1990/91 and 1991/92). The total length of 9305 fishes was measured and the age of 1478 individuals was read from otoliths. RESULTS In the fishing season 1988/89 the bulk of the catches was constituted of small immature specimens, belonging to a few clearly visible length-classes: 15-20; 24-28 and — particularly abundant, 36-45 cm (the modal values were 17,26 and 39 cm respectively). Age determinations from otolith revealed that these length-classes corresponded to I, II and IV age — groups, respectively. The IV group constituted as much as 39.4% caught specimens. The catches were also composed of age — groups V to XII (Fig. 1.). In the next season 1989/90 immature fishes also predominated in the catches, representing 39.4% of the whole fish lot. Two length-groups were easily distinguishable, with modal lengths 18 and 27 cm (Fig. 1.). According to otolith analyses, these specimens belonged to age — groups I and II. In addition, fishes of 42-52 cm in length, belonging to the strong 1984 year-class, were fairly 199 abundant. The specimens of this year-class, which in previous season 1988/89 were immature, in the described season (in January) had their gonads of II or III maturity stage (acc. to five — degree Everson's scale) — i. e. they were matured. In 1990/91 young immature specimens represented about a half of all collected fishes. They belonged to three length-classes (modal values: 17, 26 and 32 cm, Fig. 1.) corresponding to I, II and III age-groups respectively. The percentage of the abundant year-class 1984, beloging now to VI age-group, was 18.4. This age-group was composed of completely matured specimens, length of which ranged from 45 to 57 cm (modal value 49 cm, Fig. 1.). In the last described season, 1991/92, young immature specimens dominated in a higher degree than previously — their percentage was about 70% of ail measured fish. These fish belonge d to I, II and III age-group—corresponding to the year-classes born in 1990, 1989 and 1988, respectively (Fig. 1.). The 1984 year-class — now belonging to VII age-group, showed a substantial decline in number in comparison with the precedings years. It comprised exclusively pretty large (48-57; moda 52 cm) specimens. DISCUSSION The demographic structure of blackfin icefish stock in the South Georgia area has been observed regularly since 1975/76 catch season. It allows to distinguish some periods differed in composition of the stock. In 1975/76-1977/78, big, matured fish dominated. In 1978/79 and 1979/80, strong year classes of 1976, 1977 and 1978 reinforced the stock. In the period of 1980/81-1985/86, the inflow of young specimens was very small and gradual ageing of the stock was recorded. In catches from 1986/87 a very numerous year-class born in 1984 had appeared and was then observed till 1992. And finally, in the last four described seasons 1988/89-1991/92 a very abundant recruitment to the stock took place. The causes of the strong fluctuations of blackfin icefish are, as yet, not completely cleared up. Juvenile blackfin icefish about 13 cm in total length live in pelagial and feed on krill (Kompowski, 1980)*. It can be, thus, possible that the fluctuations of the krill biomass effects, to some extent, blackfin icefish year-class strength. Address of the author: prof, dr hab. Andrzej Kompowski, Department of Biological Marine Resources, Agricultural College, Kazimierza Królewicza 4, 71-550 Szczecin, Poland * Kompowski A., 1980: Studies on juvenile Chaenocephalus aceratus (Lonnberg, 1906) (Pisces, Chaenichthyidae) from off South Georgia. Acta Ichth. et Piscat., 10. 1, 1980: 45-53. 200 STAN STADA BORELA (CHAENOCEPHALUS ACERATUS, LÓNNBERG, 1906, PISCES, CHANNICHTHYIDAE) W REJONIE POŁUDNIOWEJ GEORGII (ANTARKTYKA) W LATACH 1988-1992 Streszczenie Wyniki monitoringu struktury długościowej i wiekowej borela w ostatnich czterech sezonach połowowych: 1988/89-1991/92 — wykazały bardzo obfite uzupełnienie stada przez osobniki młodociane, które stanowiły zawsze ponad połowę liczby poławianych ryb. Uzyskane wyniki potwierdzają uprzednie obserwacje o silnych fluktuacjach liczebności pokoleń borela. Być może, powstawanie silnych i słabych liczebnie pokoleń powiązane jest z wahaniami biomasy kryla, gdyż skorupiak ten jest głównym pokarmem pelagicznego narybku borela. 201 L e n g t h % Age 30 1988/89 20 n= 1114 288 10 п И О 20 10 319 о П Г"! Д 20 10 401 О Д п 40 30 20 п = 470 10 О ПпПп 30 40 50 i ii ш iv vvivnvmix х XI XII Total length cm] Age groups Fig. 1. Length and age of blackfin icefish, Chaenocephalus aceratus, in the South Georgia shelf waters in 1988/89 - 1991/92 seasons. .