CCAMLR

Following on from the apparent failure to satisfactorily assess the status of the C. gunnari population in Subarea 48.3 at WG-FSA-91 using VPA, attempts were made to re-work the analysis using Laurec-Shepherd and ADAPT tuning techniques, from 1991 back to 1977. The predicted age structure, dominated in recent years by the 1987 year class (1 year olds in 1988), was quite robust, despite the use of various combinations of survey and cpue indices for tuning. According to the VPA the population in 1991/92 was composed of a large proportion of 5 year olds, which was not observed during the survey on Falklands Protector in January 1992. Breakdown in the credibility of the VPA results in most recent years is attributed to the invalid assumption of constant M and contradictions in the input data. A conservative approach to management for 1992/93 is recommended, based on the results of surveys by Falklands Protector in 1990/91 and 1991/92.

The diet composition and feeding intensity of mackerel icefish (Champsocephalus gunnari) around Shag Rocks and the mainland of South Georgia was investigated in January/February 1985, January/February 1991 and January 1992 by analyzing appr. 9,000 stomachs. Main prey items were krill (Euphausia superba), the hyperiid Themisto gaudichaudii, mysids and in 1985 also Thysanoessa species. The proportion of krill and Themisto in the diet items varied considerably between the three seasons, whereas the proportion of mysids in the diet remained fairly constant. krill is obviously the prefered diet. In years of krill shortage, such as in 1991, krill is replaced by Themisto gaudichaudii and sometimes Thysanoessa. Variation in food composition between sampling sites was high. This high variation can be primarily attributed to differences in prey availability, but appears to be much less influenced by differences in length composition of the fish between sampling stations. Feeding intensity varied also considerably between seasons. It was highest in 1992.

This study analyses fisheries activities carried out by the Chilean longline fishery targeting the Patagonian toothfish (Dissostichus eleginoides) in CCAMLR Statistical Subarea 48.3 (South Georgia Island). From the end of December 1991 until the closing of the fishery on the 10th March 1992, the fleet caught around 80% of the TAC set for this subarea. The catch and effort data and the biological sampling carried out on board the vessels operating in the region were used for the analysis.
From the location of the hauls two separate fishing grounds could be clearly distinguished: the first one at the north of South Georgia Island, including Black and Shag rocks, and the second one at the South of the island, hauls being made at depths between 1 100 and 1 450 m. The effort applied by the Chilean fleet totalled 2.71 million hooks, 2 883 tonnes were caught at an average CPUE of 1.06 k/hook; this yield was higher to those recorded in previous seasons.
During this period the standard length of 8 340 specimens was measured at 5 cm intervals, using established sorting procedures, and also by converting the standard length to total length, where the length frequency distribution by sex was determined. The catch lengths distributions ranged between 40 and 145 cm for males and 40 and 200 cm for females. The average length was estimated at 96.7 cm for males and 108.8 cm for females. The total catch was estimated at 255 139 specimens, with a ratio of 59% males and 41% females.
The stock assessment was made by using two methodological approaches: pseudo-cohort analysis and DeLury’s method. The total catch recorded for the subarea was used, while the total effort was estimated by projecting the results of the Chilean fleet to all vessels that operated during the season. According to the biomass obtained by each method, TACs were established, using different combinations of F0.1, total length at first catch and natural mortality which were assumed to be most likely for this species. Thus, according to DeLury’s method, TACs ranged between 1 085 and 5 768 tonnes, whereas the cohort analysis gave TACs fluctuating between 7 832 and 8 210 tonnes. On the basis of these results, the proposed TAC for the 1992/93 season in Subarea 48.3 is 6 000 tonnes.

The Antarctic silver fish (Pleuragramma antarcticum) is one of the most numerous species in the outer waters of Antarctica. In the Indian sector of the Southern Ocean there are at least three relatively isolated groups. Fish aged from 0+ to 3+ years live in the epi- mesopelagic zone where there are macro aggregations of plankton, while fish aged from 3+ to 12+ years form near-bottom, pelagic schools. Fish belonging to various spatial groups have different growth characteristics, Females mature when they reach 13-16 cm in length and 4-6 years of age. Males reach maturity at 12-18 cm in length and 4-7 years of age. The potential absolute and relative fertility of this species is one of the highest for high-latitude Antarctic fish. Spawning occurs in the winter-spring period. P. antarcticum is a plankton-eater and is itself an important part of the diet of species higher up the food-chain. P. antarcticum has a fairly high level of instantaneous natural mortality, mainly due to its position on the food chain. It would probably be unwise to commence exploratory commercial fishing of this species in areas of international monitoring.

Studies of by-catch of juvenile fish in krill catches by research vessels were conducted during 1985-1988 in the Kosmonavtov and Sodruzhestva seas (Antarctic Indian Ocean sector). Isaacs-Kidd trawl with an opening area of 6.0 m2 was used. Trawls were taken in the 0-200 m surface layer over the bottom depth range from 100 to 5000 m. In 1988, studies included catches taken by a commercial krill trawl with an opening area in a small meshed section of the trawl beg of 100 m2. Catches were mainly taken over depth ranges from 300 to 500 m and from 2 500 to 4 200 m. Most frequently observed species of fish in krill catches were Pleuragramma antarcticum, Electrona antarctica and Trematomus eulepidotus. Other species were observed in catches only occasionally and in small numbers. By-catch of juvenile fish was highest in Prydz Bay and Fram Bank areas. By-catch of juvenile fish in Isaacs-Kidd trawl was usually less than 50 fish per 30 min trawling. By-catch in commercial trawls was from 114 to 1 x 106 fish per tonne of krill caught. Small by-catch of fish was usually observed when krill catches were higher than 2 tonnes (2-12 tonnes) and over the bottom depth less than 1 000 m. Maximum by-catch (up to 1 x 106 fish per tonne of krill) was observed when krill catches were less than 2 tonnes and over the bottom depth from 1 500 to 4 500 m. It was found that areas of maximum concentrations of juvenile fish and krill did not overlap. Therefore, high fish by-catch corresponds to low krill catch and vice versa. It was concluded that krill fishery should be best conducted over the bottom depth more than 1 200 m and on high-density krill concentrations. It was also suggested that in order do avoid large by-catch of juvenile fish, krill fishing should not be conducted in shallow waters over the continental slope

Biological data and fishery statistics collected in 1984-1992 were used for stock size and TAC estimation of the Patagonian toothfish, Dissostichus eleginoides, from the Kerguelen shelf area. Method of R. Jones (1964) was used for estimating population size and TAC. Calculations were done for the 1984/85 and 1991/92 seasons where directed fishery for the species took place. Population size of D. eleginoides for these seasons was estimated at 43-50 x 103 tonnes. TAC was estimated at 7.3-7.5 x 103 tonnes.

Results of length-age composition studies of the Patagonian toothfish, Dissostichus eleginoides, from various fishing grounds of the shelf and continental slope areas of the Kerguelen Islands are presented and discussed in the paper. Detailed results of these studies are compiled in tables such as age-length keys, mean fish length by age, length and age frequency distributions in catches. Soviet vessels had first carded out fishery for the species in the 1984/85 season. Fishing grounds during this period were located in the west shelf area (48°30’ - 49°30’S and 66°50’ - 67°20’E). Total catch was about 6.7 x 103 tonnes. Length composition of catches taken within 300 - 500 m depth range did not fluctuate considerably, mean fish length was 76.0 - 86.6 cm. Fish of two to 14 years old age groups were most abundant in catches, mean age was 8.2 - 9.8 years. In the 1991/92 season fishing for the species was concentrated in the north shelf area (47°10’ - 47°30’S and 66°50’ - 67°20’E). Total catch was about 5.2 x 103 tonnes. In other seasons between 1984 and 1992 catches of the species was taken only occasionally and annual catches were from 0.5 to 3.1 x 103 tonnes. Mean length of fish taken at 300 - 500 m depth was 58.1 cm, mean age was 5.8 years. Mean length of fish taken at 540 - 600 m depth was 85.8, mean age was 9.7 years. Results presented in the paper are in agreement with results of other studies conducted in the past by French and Russian scientists.

There is a lack of information regarding the abundance of exploitable finfish in Subarea 48.1. A comprehensive, bottom trawl survey offish stocks in Subarea 48.1 would make it easier for the Working Group on Fish Stock Assessment to assess the stocks in this region and assist the Scientific Committee in giving management advice to the Commission. A stratified random survey is proposed; the strata correspond to depth ranges (stratum 1: 50-150 m; stratum 2: 151-250 m; stratum 3: 251-500 m). A rule is derived from catch rate data obtained during previous stock assessment surveys around Elephant Island to allocate sampling effort among the three depth strata. One hundred and seventy six trawl stations are randomly selected by gridding the survey area. The survey is designed to cover all of Subarea 48.1.

AtlantNIRO had conducted in 1982-1990 a series of juvenile fish trawl surveys of abundant species around South Georgia Is. Results of these survey were summarised and compared with published data. Juveniles of Champsocephalus gunnari were found to be distributed from the surface to 500 m depth with a maximum density observed at depth between 100 and 200 m. High catches of C. gunnari juveniles were regularly taken in the south, south-west and south-east parts of the shelf. Juveniles of C. gunnari were usually found along the periphery of krill concentrations but not inside these concentrations. Estimated mortality of C. gunnari juveniles is relatively small and was estimated that 0.3 to 0.5% of fish survived up to an age of one year.