CCAMLR

Estimates of natural mortality are currently unavailable for Patagonian toothfish. Such estimates are needed for estimating recruitments and for use in assessing yield. This paper proposes a method for jointly estimating natural mortality and cohort strength at age 4 using a negative log-likelihood function based on one or more cohorts for which their abundance has been estimated in two or more years. A worked example is provided in an appendix.

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.