South Africa and Japan made the next season’s (2016/17) research plan in Subarea 48.6 using the updated CCAMLR C2 and Observer data.
We have firstly checked data quality and found some abnormal data for the two vessels in block 48.6_2, therefore we removed the data for the current analysis.
We investigated the biological features of D. mawsoni using 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. Life history seems to be related to the Weddell gyre system from the location, but the larvae dispersion model previously reported shows that larvae are driven further eastward. We cannot identify an appropriate stock unit currently.
The stock sizes for five 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. In addition, we considered that only recaptures for 1 year at liberty is effective to estimate the precautionary biomass for D. mawsoni in block 48.6_2 and 48.6_3 because of recaptures with short period at liberty.
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. The predicted numbers were generally inconsistent for Dissostichus spp. in other blocks.
We estimated predicted numbers of recaptures in the next three years using present catch limit and precautionary exploit rate of 4 % based on Chapman estimator and CPUE analogy method.
Japan proposes the northeastward extension of the current research block 48.6_2 in order to elucidate the stock structure. Details are described in a separated document (Namba et al., WG-SAM-16xx, 2016).