CCAMLR

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.

A recent 2000 survey of mackerel icefish in the vicinity of Heard Island enabled a revised estimate of yield for the coming 2 seasons. While the biomass of icefish appears lower than in previous years, it is dominated by a strong age 2 year class. Estimates of yield for the Heard Island Plateau population using the short-term assessment are 1150 tonnes for 2000/01 and 1000 tonnes for 2001/02 fishing seasons. The results of the 2000 survey confirm the need to retain a closure to fishing on Shell Bank. The paper reviews current catch controls and suggests that some consideration needs to be given to the implementation of closed periods during the year to provide safeguards to predators as well as to ensure spawning aggregations are not disrupted. Also, some consideration may need to be given to altering the minimum size regulations at CCAMLR in 2001 to account for the potential for unassessed cohorts to enter the fishery prior to assessments occurring.

A random stratified trawl survey was conducted in May 2000 to assess the abundance of icefish, Champsocephalus gunnari, and juvenile pre-recruit toothfish, Dissostichus eleginoides, on the Heard Island Plateau and Shell Bank. Abundances and length density plots of these species plus two principal bycatch species Lepidonotothen squamifrons and Channichthys rhinoceratus are given for each stratum. A total of seven surveys have been conducted in the Heard Island and McDonald Islands region since 1990 and results from each survey are presented so that inter-annual comparisons can be made. Abundance of D. eleginoides, L. squamifrons and C. rhinoceratus in each stratum are relatively steady, varying by a factor of 2-3 between years, but C. gunnari abundance can vary by a factor of about 20 between years. This is the result of occasional strong year classes entering the fishery and dominating the population for about 2 years before disappearing, as has been documented in other locations.

For the last 3 years, WG-FSA has accorded high priority to development of a method to integrate different indicators of stock status into assessments using the generalised yield model (GYM). In this paper, we propose a method, based on use of the sampling/importance-resampling algorithm, for incorporating information on trends in standardised CPUEs into GYM assessments. The use of the method is illustrated using data for D. eleginoides in Subarea 48.3. As the method requires only very small adjustments to the computer program implementing the GYM assessments, we propose that this method should be adopted for use during the 2000 meeting of WG-FSA.