Research plan for toothfish in Divisions 58.4.4a and 58.4.4b by Shinsei Maru No. 3 in 2012/13

A tagging survey in SSRUs B and C in Division 58.4.4 a & b was conducted in the 2011/12 fishing season with the aims of providing the data required for assessments of the population structure, size, movement and growth of Dissostichus spp. in the centred area of Division 58.4.4 a & b. An approximate stock biomass level of Dissostichus spp. in the survey area was tentatively estimated using simple Petersen and CPUE comparison methods. The medial estimate using simple Petersen (3 062 ton) was larger than that (1 928 tonnes) estimated in the WG-FSA meeting in 2011 (SC-CAMLR XXX, Annext 4, paragraphs 5.19-21), although the 95 % confidence intervals were wide due to small numbers of recapture. CVs of 20 % achievable from Petersen biomass estimates may be achieved within three years with an annual research catch of approximately 55 tonnes. The estimate (1 086 tonnes) using CPUE comparison method was a third smaller than that using simple Petersen. The catch biomass level was calculated as 153 tonnes using simple Petersen and 54 tonnes using CPUE comparison at 5 % of exploitation rate. From these results the proposed sample size as existing 70 tonnes for the next 2012/13 survey seems to be appropriate. However, it should be noted that one of the reasons for small number of recaptured fish and decreased CPUE in 2011/12 may be due to depredation by killer whales (see details in the simultaneously submitted WG-SAM-12 report on the results). Thus, the catch biomass estimate using simple Petersen may be overestimated, while that using CPUE comparison may be underestimated. We propose to continue the mark-recapture experiment in SSRUs B and C in the 2012/13 season with slightly modified designs, considering security of operation, effective capture and high tagging performance. To apply mark-and recapture studies, tagging rate of ≥ 5 fish / ton will be conducted. The comparison test with the experimental gear, which consists of three segments of Trot line system and Spanish line system respectively, will be conducted at 14 hauls, due to understanding of differences in the physical and hooking conditions of tagged fish between those caught on Spanish lines and those caught on trotlines.