CCAMLR

The diet composition of the icefish Champsocephalus gunnari caught in the Subarea 48.3 in February/March 1994 was analyzed using the frequency of occurrence (%) method. Krill (Euphausia superba) followed by the amphipod hyperiid Themisto gaudichaudii were the main prey items at South Georgia and Shag Rocks; fish were found in the stomachs more frequently than in past seasons around South Georgia, in parallel with the virtual absence of mysids and Thysanoessa spp.. Krill was available for fish in common levels, equivalent to those of previous years (e.g. 1985, 1992). However, high proportions of empty stomachs were found at South Georgia and Shag Rocks, close to the values reported for the 1991 year of krill "shortage". Besides, the tentative analysis of part of the samples using the coefficient Q (%) showed a greater importance of T. gaudichaudii with respect to krill. It is possible that the inconsistencies found between the results obtained by both methods are due to a particular spatial distribution of krill in the area in the period of study, but no previous data are available for comparison. No krill aggregations were found in the region by acoustic methods in the 1994 summer season; a fact that might be related with a significant change in surface temperature observed during the cruise.

At the 1993 meeting of the Working Group on Fish Stock Assessment, members requested that work be undertaken to evaluate certain aspects of the experimental management strategy for the Antarctic crab fishery. A simulation model is proposed for conducting this work. Length frequency and catch rate data from the 1991/92 crab fishery are used to motivate the construction of an ontogenetic migration model for describing crab distribution, movement, and recruitment during the simulation. The fishery simulation is spatially explicit, and the dynamics of crab abundance are described on a daily basis. The simulation includes advective and diffusive movement rates, a lagged Beverton-Holt stock recruitment relationship, and a model for catch that contains a random normal deviate. Finally, an algorithm is developed for describing the spatial and temporal distribution of fishing effort during the simulation. This algorithm is based on the idea that each fishing vessel constructs a map of expected catch rates across the fishing grounds. Each vessel updates this expectation map on a daily basis and uses it to determine the distribution of targeted fishing effort and searching effort.

From the end of March to the beginning of May. 1993, during fishing operations for krill conducted by the m/t Lepus on the fishing grounds off South Orkney Islands: and South Georgia, investigations were• carried out to determine biological/fisheries characteristics of the exploited stocks. One of their goals was to determine the proportion of juvenile fish forms in krill concentrations. The present study gives the results of these observations. It appears from the investigations that the by-catch of juvenile fish in the krill fishery was small. and off South Orkney Islands there was no such by-catch at all in the samples collected. In both areas a small by-catch from the family Myctophidae was observed.

Methods for the calculation of the horizontal opening (swept area) of a bottom trawl are discussed. Details of the Scanmar trawl monitoring equipment used for making in situ measurements of horizontal opening and headline height of the FP-1 20 trawl during the 1993/94 UK fish stock assessment survey around South Georgia on the MV Cordella are provided. Empirical equations are derived for the calculation of horizontal opening from other, more easily obtained measurements, such as tow speed and trawI depth. A new multiple regression equation is compared to that used previously for the estimation of biomass parameters of the major fish species around South Georgia. The resulting difference in biomass estimates is of the order of a 1% to 5% increase, depending upon species.

A long lining stock depletion trial was undertaken by the Korean longliner Ihn Sung 66 in January 1994 in accordance with CCAMLR Conservation Measure 69/XII and the Experimental Protocol circulated by the Secretariat in COMM CIRC 93/50. The results of this experiment are presented. Leslie and DeLury methods are tested using simulations to determine the most appropriate method. The Leslie method is applied using a specialized Catch and Effort Data Analysis computer package (CEDA) to calculate local abundance. Various alternative error models are investigated to estimate confidence intervals for the local stock estimate. The initial population size was estimated to be 2914 fish. Assuming a fishable area of 50 nm2 this represented a density of 0.5kg/nm2 Suggestions are made for the design of future depletion experiments.

Traditional methods for estimating yield per recruit as a function of fishing mortality can result in inappropriate high estimates of yield that could lead to depletion of stocks because of widely varying recruitment patterns, overestimation of the abundance of the stock and age of the most recent survey. These problems are addressed in this paper by using stock projections to assess the probabilities of the stock of Electrona carlsbergi becoming depleted over a 20 year period. These projections have been undertaken using a generalised version of the CCAMLR Krill Yield Model. This is appropriate because this species has a number of similar attributes to krill, including population dynamics, behaviour and its importance as prey in the Antarctic ecosystem. The decision rules for determining yield adopted by Working Group on Krill were adopted for this analysis (where yield = γ.median pre-exploitation biomass): (i) choose γ1, so that the probability of the spawning biomass dropping below 20% of its pre-exploitation median level over a 20-year harvesting period is 10%; (ii) choose γ2, so that the median escapement over a 20-year period is 75%; (iii) select the lower of γ1 and γ2 as the level of γ for calculation of yield. The estimate of γ for a fishery targetting juvenile fish was 0.091.

The progress of the fishery for Dissostichus eleginoides in Subarea 48.3 in the 1993/94 season is described. Fishing took place around Shag rocks and South Georgia, being concentrated in 4 of the 5 experimental areas. CPUE for 1994 appears similar to that from previous years except for Chilean vessels.

As requested by the WG last year, the Secretariat circulated a list of software supported at the Secretariat with the Agendas and invitations to attend the working group. This document gives additional information on software and platforms available.