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. In this paper we provide results of the sixth annual survey in the time series. The objectives of this survey included monitoring sub-adult (≤ 110 cm TL) toothfish in the south of SSRUs 881.J and 881.L in the southern Ross Sea (Strata A–C) using standardised gear in a standardised manner; and monitoring trends in larger (large sub-adult and adult) toothfish in two areas (both situated in SSRU 881.M) of importance to predators: Terra Nova Bay in 2017, with McMurdo Sound surveyed in 2016.
The estimated relative biomass index of toothfish for the core strata showed an increase to the maximum observed in the time series. Scaled length frequencies for the core strata indicate the progression of a strong year class that appears in 2012 progressing through each year, with the appearance of a second strong cohort in 2015. The decline in relative biomass during the survey time series is consistent with the decline in abundance of this 2012 cohort through time, through mortality and the movement of older fish out of the survey core area, as well as the weaker subsequent year classes. A second, strong cohort is evident entering the population in 2015, with a subsequent increase in biomass in 2016 and 2017. These results suggest the surveys are indexing local abundance and age structure, and are providing a reliable means of monitoring recruitment, estimating recruitment availability and year class strength.
Higher than expected catch rates early in the survey necessitated the reduction of station density in the core strata to avoid exceeding the catch limit. As a result of sampling fewer stations in the core strata, the variance of survey estimates was higher than in previous surveys. The higher than expected catches required an adaptive survey design to maintain a scientifically robust and comparable survey whilst constraining catch to below the catch limit. Principles to follow while adapting the survey to unexpected circumstances may be useful for others conducting longline surveys in CCAMLR fisheries.