CCAMLR

The survey comprised 51 bottom trawl hauls taken in subarea 48.3. Of these, one was discarded due to net damage. About 80% of the hauls were taken in the shelf of South Georgia Islands (41) and the remaining in the Shag Rocks area (9); 64% of the hauls were intended as replicates of the position of hauls in the previous cruises. Mean densities were obtained considering the spatial aggregation of stations in a nested ANOVA model. No significant changes in the mean densities of several species, including Champsocephalus gunnari at the shelf of the South Georgia Islands. The age and size structure of this species in the whole subarea 48.3, is dominated by younger fish (less than two years). Four years and below: 99.07% at the South Georgia Islands and 98.57 % at Shag Rocks.

The results of four fish surveys (EDUARDO L HOLMBERG 1994 to 1997) are analyzed to detect associations between the sizes of Champsocephalus gunnari (also age classes) , Gobionotothen gibberifrons, Notothenia rossii, Pseudochaeinichthys georgianus, Chaenocephalus aceratus and Dissostichus eleginoides and depth of catch, as well as among C. gunnari age classes. Except D. eleginoides median sizes of all species resulted significantly associated with depth, positively in the case of C. gunnari and N. rossii and negatively for the remaining species. Less association with depth is observed in C. gunnari as the fish grow older, indicating that the spatial separation is controlled by depth. The accumulated catches of age I- present maximum difference with the older age classes at about 150 meters a result which might be used to fish different age fractions of the populations, thus regulating the age composition of the catches.

The stock assesment of Dissostichus eleginoides from the subarea 48.3, between the split years 1992 to 1997, was estimated using the Sequential Population Analysis (SPA) model based in the catch at age data. The model was tunned by the standarized CPUE. For this last process GLlM was used and for the aplication of SPA the non-lineal function of Solver in Excel was used. The results show descent trends in Total, Fishable and the Spawning Biomass. This last part of the population is actually (1997) very close to the 20% of the initial (1992) spawning biomass. An aproximation of the future TAC was made using the Thompson and Bell model of yield per recruits.

The technical information used in the stock assessment of the Patagonian toothfish (Dissostichus eleginoides) in diferent parts of the world, have been summarized in this document in order to facilitate the future research and management works of this species.

Long-lining operations from a single vessel fishing for Dissostichus eleginoides off South Georgia (Sub-area 48.3) were assessed over 48 days during April and May. All lines were set at night using the Spanish double-line method with extra weights on the line and all deck-lights extinguished to mitigate against bird mortality. Data were recorded during hauling using a randomised cluster sampling method; the known number of coils laid during a night's setting operations provided the sampling frame, and the sampling units consisted of lengths of line between marked connecting lines. Previous catch rates were used to set daily sampling effort, and accurately achieved a CCAMLR sampling target of 60 fish/day. The method was further developed to integrate different, and varying, levels of effort at two observation stations into one daily sampling schedule. Data for total length, maturity and sex were taken. Mean total length of females caught was higher than males, and length at first maturity was also larger. The proportion of mature fish in the male population sampled was 52.0%, but was only 24% in the female population. The mean daily loss rate of toothfish observed at the hauling point was only 0.8% of the mean daily catch rate; total by-catch by numbers was less than 5%. Consistent CPUE by numbers between catches in 1994 and 1997 despite reduction in CPUE by weight indicated that exclusion effects may be important in determining the catch taken by longlines. Continuing decreases in size of fish caught relative to the length at first maturity of females indicate that reproductive over-exploitation may become a threat.

The management of Champsocephalus gunnari at South Georgia is complicated by the likelihood of substantial periodic variation in natural mortality rates. These may be associated with increased consumption of icefish by Antarctic fur seals in years of poor krill availability. Thus natural mortality of icefish may, in some years, increase by a large factor (assumed here to be 4) declining to normal levels again when krill return. A scheme is elaborated that would use information from studies on krill and predators undertaken as part of CEMP to interpret or modify information from commercial fisheries and research surveys leading to estimates of stock biomass. An extension of this scheme would use predictions of coming periods of krill scarcity as early warnings of increased natural mortality on icefish. This scheme would require greater quantitative knowledge of food web dynamics within the South Georgia ecosystem than we have at present. There is therefore a need for an interim approach to setting precautionary catch limits for this fishery. An approach based on the general yield model is described, which can be used to calculate an appropriate proportion of an estimate of Bo which could be taken by fishing, under conditions where M is assumed to be highly variable from year to year.