Here we propose the northeastward of research block of 48.6_2 along ridges to promote sufficient utilization of the catch limit, which enable us to complete planned research, evaluate the habitat model and elucidate stock structure. The distribution of densities (catch per haul in 20-minute grid) of Dissostichus mawsoni is shown in Figure 1. The research operation for the block 48.6_2 has been concentrated in its southeastern bank (south of ca. 55.2°S) with higher fish densities but scarce in the northern area with lower fish densities, and it has not taken approximately 50-70% of the annual catch limit (CL=170 tonnes) since the current research block was set in 2013. Thus, unutilized portions of catch limit were mainly attributed to low CPUE in the northern part of the block.
Meanwhile, the habitat model (WG-15/64, 2015) predicts the existence of dense population of D. mawsoni in area northeast of the research block 48.6_2 along the extended ridges. Moreover, the fishes tagged and released tended to move along the ridges running southwest and northeast in the block, which suggests the exchange of population between the research block and the northeastern area (Fig. 2).
Therefore, we propose the extension of this research block to its northeastward along ridges (Fig.2) under the current catch limits to promote completion of planned research and examination of the habitat model and the stock structure.
Abstract:
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).
Abstract:
Japan and France made the next season’s (2016/17) research plan in research blocks 58.4.4b_1 and 58.4.4b_2 using the updated CCAMLR C2 and Observer data.
The estimated median stock size in block 58.4.4b_1 and 58.4.4b_2 was 380, and 483 tonnes, respectively, in Chapman estimator using R-package tagr. The estimated median stock size in block 58.4.4b_1 and 58.4.4b_2 was 1 057, and 1 153 tonnes, respectively, in CPUE analogy method (reference area: northern area of Subarea 48.4) using the relevant information of reference area (recent median CPUE, estimated biomass and updated area size) recommended by CCAMLR Secretariat in May 2016.
Predicted numbers of tag recaptures from the estimated stock sizes using both Chapman and CPUE analogy methods were generally inconsistent with the observed numbers for each block. The numbers using CPUE analogy method were generally closer to the observed ones for both blocks.
We propose to continue the current research operation for the next fishing season with the same survey design and total sample size of 60 tonnes in order to further strengthen the stock assessments in the area.
