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.
Antarctic krill (Euphausia superba) is an abundant fishery resource, the harvest levels of which are expected to increase. However, many of the length classes of krill can escape through commonly used commercial trawl mesh sizes. A vital component of the overall management of a fishery is to estimate the total fishing mortality and quantify the mortality rate of individuals that escape from fishing gear. The methods for determining fishing mortality in krill are still poorly developed. We used a covered codend sampling technique followed by onboard observations made in holding tanks to monitor mortality rates of escaped krill. Haul duration, hydrological conditions, maximum fishing depth and catch composition all had no significant effect on mortality of krill escaping 16 mm mesh size nets, nor was any further mortality associated with the holding tank conditions. A non- parametric Kaplan-Meier analysis was used to model the relationship between mortality rates of escapees and time. There was a weak tendency, though not significant, for smaller individuals to suffer higher mortality than larger individuals. The mortality of krill escaping the trawl nets in our study was 4.4 ± 4.4 %, suggesting that krill are fairly tolerant of the capture-and-escape process in trawls.
