CCAMLR

The use of allozymes as a tool in stock structure analysis was examined in the commercially-exploited mackerel icefish, Champsocephalus gunnari, together with a preliminary assessment of stock separation among samples in South Georgia waters.
Four samples (116 individuals) of icefish were collected from South Georgia waters, incorporating Shag Rocks, during January 1990, and subjected to starch gel electrophoresis. Thirty-one putative enzyme-coding loci were screened for enzyme polymorphisms. Clearly resolved enzyme phenotypes were obtained at 18 loci, of which 7 exhibited polymorphism at the 0.95 criterion ( α-glycerophosphate dehydrogenase, α-GPDH 1.1.1.8; hexokinase, HK 2.7.1.1; isocitrate dehydrogenase, ICD 1.1.1.42; malate dehydrogenase, MDH-III 1.1.1.37; mannose phosphate isomerase, MPI 5.3.1.8; phosphoglucose mutase, PGM-II 2.7.5.1; pyruvate kinase, PK 2.7.1.40). Routine scoring of 116 fish at the 18 loci revealed average levels of genetic variability, with the proportion of polymorphic loci, P = 0.278, mean heterozygosities per locus, HL = 0.089, and the effective number of alleles, Ne = 1.05 to 4.72.
Allozymic analysis of icefish samples (N = 18 to 42 per sample) indicated stock separation on a local scale. Genetic differentiation of samples was supported by the markedly non-random genotypic distributions (Wright’s fixation index, FIS) detected at most loci which were due mainly to heterozygote deficiencies. There was a positive association between the extent of geographic separation and genetic divergence as estimated by Nei’s mean genetic distances (D = .007 - .057) and identities (I = 0.993 - 0.945). Individuals collected from Shag Rocks were most genetically distinct, providing preliminary evidence of stock separation from South Georgia-shelf fish. The degree of reproductive isolation among samples is as yet uncertain, and requires analysis of larger sample sizes with more comprehensive electrophoretic screening.
Genetic data indicate that localised stocks of C. gunnari exist in South Georgia waters. The questions posed by the non-equilibrium genotypic distributions and genetic differentiation on a local scale require urgent attention if the species is to be managed effectively. Proposals for an electrophoretic and biological analysis of stock structure are presented.

Whole otoliths and scales were used for a simultaneous age determination of juvenile Notothenia rossii marmorata specimens collected at Potter Cove, King George/25 de Mayo Island, South Shetland Islands. Results from the analysis of both structures agreed in 95.5%. Likewise, the examination of otolith cross sections of chosen specimens confirmed in all the cases the age readings obtained from the second whole sagittal otolith. First October instead of the commonly accepted 1 July, was taken as the birth date of the fish, which is discussed. The length-age range of the fish was 18-44 cm and 3-7 years respectively, which is well in accord with the known length-age distribution of juvenile specimens of the species in the fjord. Mean length data at age presented here were compared with published values for specimens of the same area. Our data are similar to the ones reported previously for fish of Admiralty Bay, King George/25 de Mayo I. The use of whole otoliths for age determination of juvenile N. rossii marmorata proved to be a reliable method. However, it might be not viable for adult specimens.

Chaenocephalus aceratus and Pseudochaenichthys georgianus have been exploited in Subarea 48.3 since 1977 but have never been fully assessed by the Working Group on Fish Stock Assessment because of a lack of reliable biological and catch data. This paper attempts to reconstruct the fishery for these two species by assuming that 75% of the ‘unidentified fish’ caught by the Soviet Union in the years 1977 to 1988 consisted of catches of C. aceratus and P. georgianus. Biological, age-length and length data from Polish research and commercial sources has been used with the new catch data in a VPA (Variable Population Analysis). The analyses show good agreement with historical biomass estimates from surveys, and indicate that the stock of C. aceratus has decreased from 18 000 tonnes to 6 000 tonnes, and that of P. georgianus from 40 000 tonnes to 10 000 tonnes, over the period of exploitation. Projections indicate that C. aceratus is unlikely to sustain a high level of catch even when it recovers, although P. georgianus may sustain a fishery of about 2 000 tonnes. The current TAC (Total Allowable Catch) (bycatch) limit of 300 tonnes would seem to be appropriate to allow recovery of both species in the near future.