South Africa and Japan have revised the next season’s (2016/17) research plan in Subarea 48.6 using the latest CCAMLR C2 and Observer data, following several advices during WG-SAM meeting in 2016.
We have used extracted data provided by CCAMLR Secretariat in this late August for the current analysis, but not used the cleaning data provided by them in this early September due to tight schedule toward the submission.
We investigated the biological features of D. mawsoni in the Subarea and adjacent data- poor fisheries using latest Observer data. The geographical distribution of sizes, GSI and Fulton’s condition factors for D. mawsoni in Subarea 48.6 suggest that northward spawning migration and southward feeding migration as Ross Sea stock as suggested in the previous reports. Life history seems to be related to the Weddell gyre system from the location, but the larvae dispersion model previously reported does not support this hypothesis. However, some immature and mature fish from around Subarea 48.4 and juveniles from around west of Prydz Bay regions are speculated to migrate to Subarea 48.6 and be important resources for the population. The survey using the satellite tags along with analog tags is necessary in order to identify an appropriate stock unit in the future.
The stock sizes for research blocks were estimated using the Chapman estimator, the CPUE analogy method and preliminary CASAL models. We divided the two areas for block 48.6_2 by latitude of 55.2°S to estimate the biomasses of D. mawsoni because the recaptures were concentrated in the southern area. We considered that recaptures for 3 years at liberty is effective to estimate the appropriate biomass for D. mawsoni in the blocks following the recommendation in WG-SAM meeting in 2016.
Predicted numbers of tag recaptures from the estimated stock sizes using Chapman were relatively consistent with the observed numbers for D. mawsoni in the southern area of block 48.6_2, and blocks 48.6_3 and 48.6_4. However, the predicted numbers using the CPUE analogy method were generally inconsistent.
In WG-FSA meeting, we propose the sample sizes in each current research block for the next season generally based on precautionary exploit rate of 4 % of the Chapman estimates. In addition, Japan will present the preliminary proposals on appropriate shifting of the current research block 48.6_2 and will examine suitable shelf area to elucidate the stock structure under the optimum sea-ice conditions in the Subarea and adjacent waters from the 2017/18 season survey.