We provide an update of the Bayesian sex and age structured population stock assessment for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region (Subareas 88.1 and SSRUs 88.2A–B), using catch, catch-at-age, and tag-recapture data from 1998 to 2017 and including the results from the Ross Sea Shelf survey from 2012 to 2017. The model estimates of unfished biomass and current status were higher than that from the 2015 assessment. This was likely driven by the revised estimates of the effective tagging survival and tag detection rates, both of which have decreased in the recent years. Sensitivity runs showed that the results were relatively insensitive to the use of a logistic ogive rather than a domed ogive for fishing selectivity.
Model runs showed that the data from the Ross Sea Shelf survey were required to reliably estimate relative year class strengths. Year class strengths were estimated from 2003 to 2011 and showed two strong year classes and two weak year classes.
Overall, model fits to the data were adequate, and, as in previous assessments, the mark-recapture data provided the most information on stock abundance. Markov chain Monte-Carlo (MCMC) diagnostics suggested no evidence of non-convergence in the key biomass parameters.
The precautionary yield, using the CCAMLR decision rules and two potential catch splits between the Shelf, Slope, and North areas of the Ross Sea region consistent with previous fishing activities and with the Ross Sea region MPA, was either 3234 t or 3258 t. This yield is higher than that specified for 2018 in CM 91-05, which states that “the total catch limit shall be fixed at a level within the range of 2583 to 3157 tonnes per fishing season, based on advice from the Scientific Committee in 2017, 2018 and 2019” (CM 91-05 paragraph 28 (i)).
As the precautionary yield estimate is higher than the range specified in CM 91-05 for 2018, we therefore recommend that the catch limit be set at 3 157 tonnes for the 2018 and 2019 fishing seasons as per CM 91-05.