CCAMLR

This work aims to the study of the distribution, size structure by depth, first maturity stage, weight/length, and length perimeter curve, of Patagonian toothfish (Dissostichus eleginoides) in the Argentine Sea, and atempts some explanation by a trophic analysis of such distribution.
The distribution of Patagonian toothfish extends between of 36°30'S and 55°S from 80 to 2500 m depth. Greatest concentration areas were found south and NE of the Malvinas Islands, over the slope of Buenos Aires province and between Burdwood Bank and State Island, this last place being where the studies were carried on. Bottom temperatures ranged between 2°C and 12°C. Extreme sizes were between 12 and 227 cm, and most of the adult sizes (>80cm) were found in waters at depths greater than 900 meters. The selectivity perimeter should be bigger than 46 cm. The new maturity curve range at 50% 78,27 for males, 87.06 for females and both 82,16. The length/weight curve for males was: a=5E-06; b=3.186; females: a=5E-06; b=3.161; both: a=4E-06; b=3.187.
The distribution of adults and juveniles can be explained partially by canibalism and the size of the available preys at different depth. According to the analysed data in different areas including CCAMLR area, longline or trawl operation fishing is recommended starting from 900 meters depths, in the southern area, South of 54°S and 1000 m in 38°S to 48°S and to assume a conservative position of 1000m for all the area.

The diet composition of mackerel icefish Champsocephalus gunnari caught in Subarea 48.3 in late summer of years 1994-97 was analysed using frequency of occurrence (F%) and coefficient "Q" (%) methods. Krill (Euphausia superba) followed by the amphipod hyperiid Themisto gaudichaudii was the main food item around South Georgia and Shag Rocks in the period investigated, except in 1994, when the order of importance of these two organisms in the diet was inverted. The consumption of other prey items such as mysids and Thysanoessa sp., which are known as important alternative food in years of krill scarcity and fish, was only occasional or in negligible amounts, showing interannual variation. The spatial distribution of the main preys in the diet as well as of the proportions of stomachs with/without food in the area of investigation didn't show a consistent pattern from year to year. Likewise, in the four Holmberg 1944-1997 surveys, high proportions of fish exhibited empty stomachs, phenomenon for which non consistent explanation was found. The variation of the diet with respect to the length of C. gunnari didn't show a selectivity pattern for any of the prey items. The availability of krill in the Statistical Subarea 48.3 in the period investigated can be defined as low in 1944, equivalent to those years of krill scarcity; intermediate in 1995, below the historical years of high abundance; and high in 1946 and 1997, in line with the years of krill richness. This information is well in accord with independent information from acoustic surveys and krill dependent species monitored in the CEMP.

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.