CCAMLR

The seventh cruise of the RV Atlantida was a continuation of regular AtlantNIRO research into the status of Chaenichthyidae and Nototheniidae fish in Statistical Subarea 48.3 (South Georgia, Shag Rocks and Black Rocks). The minimum biomass of commercial fish species was calculated from the results of a census trawl survey carded out in April-May 1991. This paper contains data on distribution, biology and size composition of commercial fish species.

Genetic population structure of the mackerel icefish, Champsocephalus gunnari, in waters from around South Georgia, South Orkneys, and Heard Island (Australian Antarctic) was examined using allozyme electrophoresis.
Icefish were collected from Heard Island (N = 47; November 1990), South Georgia (N = 267; January/February 1991) and the South Orkneys (N = 44; January/February 1991) and subjected to starch gel electrophoresis. sixteen putative enzyme-coding loci were examined, of which 7 were polymorphic at the 0.95 criterion: glutamate oxaloacetate transaminase, GOT-I 2.6.1.1; α - glycerophosphate dehydrogenase, α-GPDH 1.1.1.8; 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; superoxide dismutase, SOD 1.15.1.1).
Routine scoring of 358 fish at the 16 loci revealed below average levels of genetic variability, with the proportion of polymorphic loci, P = 0.175, mean heterozygosities per locus, HL = 0.058, and the effective number of alleles = 1.04 - 1.88.
The majority of alleles conformed to Hardy-Weinberg expectations, with significant deviations due exclusively to heterozygote deficiencies. Genetic differentiation was detected among samples, both within (South Georgia) r and between (South Georgia vs. Shag Rocks; South Georgia vs. South Orkneys) sea areas. Nei’s (1973) gene diversity analysis indicated that over 85% of the total gene diversity was due to the within-population component, with progressively higher between population values with increasing geographic separation. Samples from Heard Island were most genetically distinct ( I = 0.900 - 0.930; D = 0.072 - 0.106).
The apparent erosion of genetic variability may be related to abrupt changes in population size. Enhanced mobility of fish between sea areas was suggested by an overall reduction in genetic differentiation within and between South Georgia and Shag Rocks (compared with 1990), and the greater conformity to Hardy-Weinberg equilibrium. Genetic data support the notion of at least partial migration of C. gunnari between South Georgia and the South Orkneys, and possible speciation on a geographic scale.

Recently a longline fishery has developed for the Patagonian Toothfish, Dissostichus eleginoides, in the vicinity of Shag Rocks, South Georgia. Assessment of this stock is not possible by conventional methods because the bulk of the adult population may be dispersed over a wide area in deep water whereas the Shag Rocks’ shelf may be a nursery ground for juveniles. An assessment of the stock has been made by extrapolating the population parameters from trawl surveys to derive estimates for the whole stock.

Two similar methods for preparing aggregate length frequency distributions from survey data are discussed. The Mackerel Icefish, Champsocephalus gunnari at South Georgia is used as an example.

Discrepancy was noted between the age length key and modes in the length distribution of Ch. gunnari from the Hill Cove survey around South Georgia in January 1990. A revised age length key for this survey is proposed, based on data from a previous survey in the area. The revised age length key corresponds well to modes in the length distribution from the survey and to another key derived from samples collected during a subsequent survey in the same season.

Otoliths from Champsocephalus gunnari were collected during a fish stock assessment survey in sub-area 48.3 during January and February 1991. An age length key based on age readings taken from these otoliths is presented.

Criteria for the choice of a trawl for demersal fish surveys are discussed. Details of the construction and performance of the 120 Bolt Rope Bottom Trawl used during the recent fish stock assessment survey around South Georgia on the Falklands Protector are presented. It is proposed to use this trawl during future surveys in the area in order to improve continuity in the survey series. The design of the trawl is similar to the Polish P32/36 survey trawl used on the Profesor Siedlecki surveys around South Georgia between 1986/87 and 1988/89. The importance of reducing the variability of trawl performance during the survey is discussed

An assessment of the Champsocephalus gunnari fishery in sub-area 48.3 using VPA from 1976/77 to 1990/91 is presented. The analysis was tuned to abundance indices derived from fishery independent stock assessment surveys between 1986/87 and 1990/91. The results indicate a very different situation to assessments in 1989/90 with a decrease in overall biomass and recruitment over recent years. Potential TAC levels are calculated, based on the F0.1 management strategy, however, the wisdom of limiting catches in the near future on the basis of a target F is questioned