CCAMLR

In subarea 48.3 mackerel icefish (Champsocephalus gunnari) are currently assessed by a bottom trawl survey, but evidence from predators and acoustics indicates that a considerable portion of the icefish biomass is in midwater and therefore missed by the bottom trawl survey. In January 2004, a short acoustic survey on FPRV Dorada showed that mackerel icefish, of all age classes, spend time in midwater and reinforced the evidence that a bottom trawl survey significantly underestimates biomass. An acoustic survey has the potential to give a better estimate of icefish biomass, however acoustic methods need to be developed to distinguish between krill and icefish and to improve target strength estimates of icefish.

Two variants of the ASPM assessment of the Prince Edward Islands toothfish resource are examined. One allows for the possibility of changes in somatic growth rate in response to increased food availability as resource abundance drops. The other permits annual fluctuations about a deterministic stock-recruitment relationship. A fit of the first variant yields the biologically implausible result of a decrease in somatic growth rate as abundance drops. For the recruitment variability model, results remain highly sensitive to the relative weights given to CPUE and catch-at-length data in the model fitting processes. Therefore, unfortunately, neither of the variants examined appears able to resolve the discrepancy between the implications of the CPUE and the catch-at-length data regarding the current status of the resource.

Analysis of Russia HAK-4M trawl catchabilities has been made in the paper. This trawl have been used when icefish bottom surveying. An analysis of total catchability and a differential one (that is catchability in relation to fish of different length) has been realized by models of probability statistical theory for fishing trawls. Influence of trawling speed, trawl configuration and distribution patterns of fish aggregations under consideration on trawl catchability has been shown. It has been demonstrated that the bottom trawl was not a measuring system. There have been discussed matters of improving trawl survey methodology related to the bottom trawl application as a measuring system.

Revision of icefish (С.gunnari) stock estimate in South Georgia Area was caused by elaboration of the following methodical aspects of trawl-acoustic survey: the processing method improvement for target strength estimates using in situ measurement data; the combination of trawl and acoustic surveys results for icefish biomass estimates .
We discus these methodical aspects and present the new icefish target strength estimated obtained from in situ measurement data by bootstrap-method and biomass estimates and its statistical characteristics. Major sources of uncertainties in icefish stock estimates and some ways of improvement of icefish stock estimates are shown too.