At its 2011 meeting, the Scientific Committee agreed that a time series of relative abundance from a well-designed survey could be a useful input into the Ross Sea stock assessment model. The first three surveys were completed in February 2012, 2013, and 2014. In this paper we provide results of the fourth survey in the time series. The objectives of this survey were: (1) To carry out a longline survey to monitor sub-adult toothfish in the southern Ross Sea (strata A–C) using standardised gear in a standardised manner; and (2) To sample additional experimental stations in an adjacent area to identify areas of high sub-adult abundance that could be included as strata in future annual surveys.
The 2015 survey was successful in completing all of the planned stations. Standardised catch rates of sub-adult (<110 cm) toothfish for the core strata showed a significant decline in this fourth year of the survey series. Age frequency data from the surveys have shown the progression of a cohort from age 7 in 2012 to age 9 in 2014 and to age 10 for females in 2015. The decline in the catch rates during the survey is consistent with the decline in the abundance of this cohort through time through mortality and the movement of older fish out of the core survey area, as well as the relatively weaker subsequent year classes. These results suggest that the surveys are indexing local abundance and will provide a reliable means of monitoring recruitment and estimating recruitment variability. Stations in the experimental stratum in Terra Nova Bay also had high catch rates of slightly larger sub-adult and adult fish, and warrant future monitoring. We recommend the survey be continued to provide information on year class strength and recruitment variability, and an index of local abundance to be incorporated in the stock assessment.