CCAMLR

The results of mackerel icefish (Champsocephalus gunnari) trawling survey carried out by Russia at R/V Atlantida in February 2000 in Subarea 48.3 are presented. During that period C.gunnari were feeding. In all parts of the Subarea, except the North-Western one, C. gunnari of 22-28 cm in length and 2-3 years old predominated. Fish biomass distributed near the bottom estimated with different methods amounted to 47-87 thous.t. The estimation also was presented taking into account catchability coefficient. The objective evidences are submitted to prove that a certain part of C.gunnari stock may stay in the pelagic layer in the feeding period. Therefore, the bottom survey estimates only the population proportion distributing near the bottom at that moment. It is proposed to review the survey method. Arrangement of regular trawling-acoustic surveys in the area may improve the accuracy of this important commercial species abundance estimates and will answer some questions which currently have no grounded solutions.

In February 2000 the bottom trawling survey was carried out in Subdivision of South Georgia (48.3) to assess the main commercial and promising near-bottom and bottom fish species. The data on distribution, biology, minimal (relative) biomass and abundance of fishes were obtained.

The unusually effective fishery on mackerel icefish (C.gunnari) in the South Georgia Island subarea (48.3), when during 1.5 months two fishing vessels (Russia and Chile) caught more than 4 thousand tons, and the followed bottom trawl surveys by Russia and Great Britain in December 1999 - February 2000 showed clearly a necessity to revise the methods of icefish (which is near-bottom and pelagic fish species) biomass assessment based on the results of bottom trawl surveys.
A question on unreliability of bottom trawl surveys conducted by research and small-tonnage fishery vessels and used for the icefish stock assessment was repeatedly raised on the meetings of Working Group on Fish Stock Assessment (WGFSA) both during the period of the icefish fishery in 1980s and after its ceasing in 1990s.
In all cases, a comparison of results of stock assessment based on bottom trawl surveys (1984-1997) with the total catch, catch per unit effort (CPUE), and calculations by VPA showed a clear underestimation of the stock (Table 1).
Now, having obtained once again good fishery and scientific data for comparison, it became possible to define reasons of underestimation of icefish biomass by bottom trawlings:
- vertical opening of bottom trawls used by research vessels does not exceed 15 m, while a vertical extent of even near-bottom icefish concentrations reaches 40-50 m;
- at scientific trawlings the use of trawls with mesh less than 80 mm is allowable that results in selectivity and capturing of a high number of small fish;
- catchability of a "slower" small-mesh bottom trawl with the best will of researches is not 1.0;
- as a rule, surveys conducted during a month do not include a sufficient number of trawlings and their results do not reflect the real irregular distribution of icefish concentration;
- high catches in some places are considered as statistically insignificant and are not taken into account (Soviet and British surveys in 1990).
As a result, over more than 10 years bottom surveys pervert the real state of near-bottom and pelagic Nototheniidae stocks not only in the South Georgia subarea (48.3) but in subareas of South Orkney (48.2) and South Shetland (48.1) Islands. According to their unreliable results either very low TACs are set or the fishery is completely closed.
Results of successful fishery and surveys of 1999-2000 dictate a necessity to revise the assessment methods based only on bottom trawlings. The complex investigations including the regulated fishery and trawl-acoustic surveys by midwater trawl are needed to catch icefish concentrations both in pelagial and near-bottom layer.

This paper presents an assessment of the harvested population of Patagonian toothfish at Macquarie Island based on data from a tag-recapture experiment initiated during the 1995/96 fishing season. Population models that include dynamics of tagged and un-tagged fish, daily releases, catches, recaptures, natural mortality, and annual net recruitment are used to assess the population of one of the main fishing regions of Macquarie Island: Aurora Trough. The pre-tagging abundance is estimated by incorporating a Petersen approach in a novel semi-parametric model using maximum likelihood methods. The model (the "base-case model") assumes the recaptures are Poisson distributed, and the recapture expectations are conditional on catch numbers and previous recaptures. A second model (the "selectivity model") attempts to account for apparent decreasing availability with length. It assumes a maximum age or length above which a fish is no longer available to the gear. Each released fish is assigned a length of time in the available population according to its estimated release age. Once this time has elapsed, the fish is removed from the analysis. The base-case model estimated that pre-tagging abundance was between 1.0 and 1.5 million fish, depending on assumed mixing levels between tagged and un-tagged fish. Estimates of net recruitment were occasionally negative, suggesting that emigration may have exceeded immigration. The estimated percentage of the pre-tagging available abundance remaining in Aurora Trough was about 35% in the base-case model. In general, the selectivity model estimated a lower available abundance, and a lower percent remaining, than the base-case model. The substantial decline in available abundance predicted by both models provides some evidence of large-scale emigration, and could not be due to fishing alone.

The recent fish survey at in the vicinity of Heard Island in 2000 has provided estimates of recruitment for at least one new cohort (1996) and another estimate of the 1995 cohort. The older cohorts are likely to be distributed more widely than the shallow part of the Heard Plateau, consistent with the observations from 1999. A reanalysis of all survey data from the region has shown that toothfish smaller than 450 mm are likely to be found predominantly in the shallow part of the Heard Plateau. Surveys aimed at providing a time series of recruitments could focus on this area. However, if estimates of mortality are to be obtained for this species then surveys over the deeper strata of the plateau will be required.