CCAMLR

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

Samp1es of fjord fish of the species N. rossii and N. neglecta collected at Potter Cove, King George/25 de Mayo Island, South Shetland Islands, from 1983 to 1991, allowed to study the previously reported abundance changes as a size-related process. The data were analyzed using a Nested ANOVA design to obtain variance components and expected values. An increase and then a decrease in the expected mean sizes of pre-recruit N. rossii was clearly observed along the whole sampling period. This was interpreted as a consequence of a significant reduction in the influx of the youngest age classes into inshore waters followed by the departure of the older (and more abundant) specimens offshore. These changes are strongly associated with the last significant catches of fin-fish in the Subarea 48.1 in 1979/80 and could also be related to the still undetermined incidental mortality of larvae and juvenile in the krill fishery. Not surprisingly, the analogous treatment of data of N. neglecta, a species with similar ecological habits in the fjords but not commercially fished, showed variations around the expected overall mean size without any significant trend. In addition, the 1991 abundance data of fjord N. rossii and N. gibberifrons relative to N. neglecta indicate low levels, similar to those already reported for the last three years in the same area.