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 four surveys were completed in February 2012, 2013, 2014, and 2015. In this paper we provide results of the fifth survey in the time series. The objectives of this survey were: (1) to continue a time series of longline surveys to monitor 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 (2) to monitor trends in larger (large sub-adult and adult) toothfish abundance in two areas (both situated in SSRU 881.M) of importance to predators: McMurdo Sound in 2016 and Terra Nova Bay in 2017.
The 2016 survey was successful in completing all of the planned stations and objectives. Standardised catch rates of sub-adult (<110 cm) toothfish for the core strata showed an increase to the highest point observed in the time series. Age frequency data from the surveys have shown the progression of a cohort from age seven in 2012 to age nine in 2014 and to age ten for females in 2015. In 2016, the survey length frequency distributions indicate than another stronger year class has entered the population with a mode at 75 cm, and that the older year class has 100 cm. The decline in the catch rates from 2012–2015 was correlated with the decline in the abundance of the older cohort over time through mortality and the movement of older fish out of the core survey area, as well as the relatively weaker subsequent year classes. The increase in 2016 is also reflective of the introduction of a new, stronger year class into the population. These results suggest that the surveys are indexing local abundance and are providing a method for monitoring recruitment and estimating recruitment variability. We notify here that the survey will be conducted in 2016-17 as endorsed by the Commission.