A survey was conducted in order to collect information for the stock status and various biological information on toothfish in Division 58.4.4a & b (Ob Lena Bank) by using a commercial bottom longline vessel, Shinsei Maru No. 3, from April 15 to May 19, 2012. This report described the results during the survey in two SSRUs (B & C) of Ob Lena Bank. Total catch (excluding tagged and released fish) of Dissostichus eleginoides was 28.3 tonnes in the survey area in 70 longline sets, contributing 91.3 % to the total fish catch (31 tonnes). Mean CPUEs of D. eleginoides was in the SSRUs were 6.7 indiv. / km (CV = 70.0%) in terms of number and 48.3 kg / km (CV = 79.1%) in terms of weight. The mean CPUE in terms of weight in SSRU B (32.1 kg / km) was rather lower than that (50.5 kg / km) in the same SSRU in 2010/11. A total of 172 individuals of toothfish with good physical condition and single-hooked were tagged and released during the survey, and 3 D. eleginoides, which were tagged and released in SSRUs B and C from the 2007/08 to 2010/11, were recaptured in the same SSRUs, respectively. The overlap of size between the fish released with tag and that of overall catch was high (θ=81 %). Fish with good physical condition and single-hooked contributed 50 % to the total among fish caught by Spanish line system, while this kind of fish contributed 14 % to the total among fish caught by trotline system. A total of 133 killer whales (Orcinus orca) were observed at the 30 hauls. All of whales observed and the predatory damaged toothfish occurred in SSRU B.
The vessel tried to avoid killer whales as in the previous surveys but could not escape from them effectively in SSRU B. The depredation may lead to underestimate of nominal CPUE and rather small number of recaptured fish until now in this SSRU.