The current season (2014/15) is the final one of the first three-season research which began in 2013 in data-poor fisheries. In the present report, catch, effort and biological data recorded by all effective vessels were analyzed for the improvement of stock assessment in the research blocks. Based on the result, the next three-season (2016-18) research plan in Subarea 48.6 is also presented.
The stock sizes for five research blocks (48.6_1, 48.6_2, 48.6_3, 48.6_4 and 48.6_5) were estimated by the Petersen estimator and the CPUE x seabed analogy method using updated CCAMLR C2 data and reference area sizes. The stock size estimate using the Petersen estimator was relatively consistent with that using the CPUE method (using Division 88.2H as a reference area) for Dissostichus mawsoni in block 48.6_2. However, the estimates using the Petersen estimator were two-three times larger than those using the CPUE method in the block 48.6_3 and 48.6_4. Predicted numbers of tag recaptures from the estimated stock sizes using the both Petersen and CPUE methods (using Division 88.2H as a reference area) were relatively consistent with the observed numbers for D. mawsoni in the block 48.6_2 for 2012/13 and 2013/14 seasons. The predicted and observed numbers using either method were generally inconsistent for Dissostichus spp. in other blocks.
From the analyses of biological data, it is suggested that the northern offshore areas are spawning grounds and southern ice-shelf areas are nursery and feeding grounds for D. mawsoni, which is similar to the hypothetical life history in the Ross Sea area.
In the current season three Dissostichus mawsoni tagged and released in the previous seasons in the ice shelf block 48.6_4 were recaptured in the same block, which made it possible to estimate the stock size using the Petersen method for the block for the first time. One D. mawsoni tagged and released in the block 48.6_2 in 2013 was recaptured in the block 48.6_3 in the current season. This is the first example of very long movement of more than 1 250 km between different SSRUs in the Subarea.
Understanding of the resource structure through clarification of their life history is essential to establish stock assessment and robust stock/ fisheries management of Dissostichus spp. population(s) in data- poor exploratory fisheries. During the second three-season research, we will continue enhanced tagging program, and collection and analysis of biological data including otoliths and gonads to clarify migration route and associated life stages of the fish.
To this end, we propose to follow the current research style in the current research blocks for the second three-season research with the sample sizes re-estimated following the procedure recommended at the WG-FSA in 2013 in order to maximize the expectation of tag-recapture to the extent possible under the precautionary exploitation rate. We expect further progress of tagging research in the southern research blocks during the next three-season research because several fish tagged and released in the preceding seasons began to be recaptured.
In addition we propose to set two optional areas along the ice shelf, which would substitute 48.6_5 in case the original block is under adverse ice-conditions. This mitigation would contribute to understand the movement of D. mawsoni along the ice shelf zones or between the northern banks and southern areas, and will help development of CASAL model in the Subarea.