CCAMLR

Acoustic surveys carried out in summer and autumn periods of 1987-1989 estimated the biomass of myctophids in the Atlantic sector of the Southern Ocean over the region between 48° and 56°S and from 8° to 48W at about 1.7 mill.t. Over the larger part of the surveyed area the fish were sparse, their dense concentrations composed predominantly of one myctophid species Electrona carlsbergi occurred over only restricted areas. The bulk of the biomass has been reported from the region of the Antarctic Convergence.

In the summer-autumn period E. carlsbergi fed mainly on copepods. Daily food intake measured using various methods ranged from 3.7 to 5.6 % of fish body weight. The amount of food consumed by E. carlsbergi during the year in the southern regions of the Antarctic Convergence is about 15 times its own weight.

Catches of Champsocephalus gunnari from the Kerguelen Shelf and Skiff Bank stocks were re-calculated from those used by the Working Group on Fish Stock Assessment in 1989 using length-frequency data for the years 1981 to 1990. Cohort analysis yielded a Stock Biomass of 22 711 tonnes in 1988 for the Age 2 Kerguelen Shelf stock, an order of magnitude lower than that calculated from a biomass survey in 1988 by the USSR. This biomass is consistent with that from previous cohorts and indicates a possible declining trend in the abundance of this stock.

Based on a tentative adjustment of catches of Notothenia gibberifrons previously reported as ‘Pisces nei’ an attempt is made to reconstruct the fishery on that species between 1977/78 and 1985/86. Stock size until the mid 1980s seemed to be more reduced than previously assumed. The current state of the stock is unknown.

A declining trend observed in the abundance of fjord fish of the species Notothenia rossii marmorata and Notothenia gibberifrons from two localities of South Shetland Islands, is analyzed by a nested ANOVA and demonstrated as significant. The material studied was obtained with trammel nets at Potter Cove, King George/25 de Mayo I., from 1983 to 1990 and in waters around Half-Moon I., Moon Bay, in 1989. The analysis was based on the proportion of catches of the mentioned species in relation to Notothenia neglecta, a species with similar ecological habits in the fjords. Comparison with analogous published data obtained at close localities in the 60’s and 70’s, also shows a marked declination. This phenomenon in the area might be explained as a consequence of the depletion of the stocks due to commercial exploitation.

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.