Revised reports on abundance and biological information on toothfish in Division 58.4.4 a and b by Shinsei Maru No. 3 in 2010/11 season

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 March 30 to May 2, 2011. This report described the results during the survey in two SSRUs (B & C) of Ob Lena Bank. Total catch (excluding fish released alive) of Dissostichus eleginoides and other by-catch species was 37.8 tonnes in the survey area in 70 longline sets. D. eleginoides showed the highest catch of 35.4 tonnes, contributing 93.6 % to the total catch. Mean CPUEs of D. eleginoides for 56 hauls where only trotline system was adopted in the SSRUs were 8.5 indiv. / km (CV = 53.5%; 15.3 indiv. / 1,000 hooks) in terms of number and 62.3 kg / km (CV = 76.0%; 111.2 kg / 1,000 hooks) in terms of weight. The mean CPUE in terms of weight in SSRU B (49.0 kg / km (CV = 85.1%)) was significantly lower than that (93.7 kg / km (CV = 61.7%)) in the same SSRU in the 2009/10 survey, although the CPUE in terms of number was same level between the years.  This may be due to the catch of small fish in the shallow area < 500 m in the 2010/11. A total of 189 individuals of toothfish with good physical condition were tagged and released during the survey, and 4 D. eleginoides, which were tagged and released in SSRU C during the 2007/08 and 2009/10, were recaptured in the same SSRU. The overlap of size between the fish released with tag and that of overall catch was considerably high (θ=95 %). There was a significant difference in physical composition between Spanish line and trotline (χ² test, P < 0.001), with the higher percentage of dead fish for trotline. It seems that smaller fish < 70 cm are more damaged than larger ones for trotline, as in BANZARE Bank. A total of ≥130 killer whales（Orcinus orca）were observed at the 35 hauls. Most of whales observed and the predatory damaged toothfish occurred in SSRU B.