At its 2011 meeting, the Scientific Committee agreed that a time series of relative recruitments from a well-designed survey could be a useful input into the Ross Sea stock assessment model and endorsed a proposal to carry out this work once the fishery had closed at the end of the 2011/12 season. The survey had two main objectives: (i) To establish the feasibility of developing a time series of longline surveys to monitor pre-recruit (<100 cm TL) toothfish in the south of SSRUs 881.J and 881.L in the southern Ross Sea using standardised gear in a standardised manner; and (ii) To carry out experimental depth-stratified fishing in 400–600 m depth adjacent to the survey boundaries to establish the most appropriate depth strata for future annual surveys.
The survey demonstrated it is possible to carry out a longline survey of pre-recruit toothfish from a commercial fishing vessel in Ross Sea using standardised gear in a standardised manner. Based on a total of 45 sets in the three core strata (A12-C12) the survey biomass estimate had an overall c.v. of 9%, which met the target c.v. of 10%. The survey caught mainly 70–110 cm TL, 5–10 year old, Antarctic toothfish. It provided new data on the depth distribution of pre-recruit fish in this area. Catch rates of pre-recruit toothfish were highest between 500 and 900 m and very low shallower than 450 m. The size distribution of toothfish was very similar between the four main strata suggesting no depth preference between 400 and 900 m. It is recommended that future surveys in this area should focus on depths of 500–900 m. The survey also demonstrated the feasibility of collecting samples for wider ecosystem monitoring.
Although the main aim of the 2012 survey was to monitor pre-recruit toothfish, the survey provided the opportunity to compare survey catch rates in 2012 with historical commercial catch rates by San Aotea II and its sister ship Janas in 1999, 2001, and 2004. The results of this analysis suggest that there has been no significant change in catch rates in the southern Ross Sea over the past decade.