This paper presents an updated assessment for the Patagonian toothfish (Dissostichus eleginoides) fishery at Heard Island and McDonald Islands (HIMI) in CCAMLR Division 58.5.2 with catch until the end of 2019 and observations until the end of 2018. The updated assessment model is based on the best available estimates of model parameters, the use of abundance estimates from a random stratified trawl survey (RSTS), longline tag-release data from 2012-2017 and longline tag-recapture data from 2013-2018, and auxiliary commercial composition data to aid with the estimation of year class strength and selectivity functions of the trawl, longline and trap sub-fisheries.
Compared to the 2017 assessment that was accepted by WG-FSA-17 to be used for management advice, this assessment takes into account (1) update the model with catch data to 2019 and observations to the end of 2018 including new ageing data from the RSTS and commercial fishery from 2017-2018, (2) inclusion of fishing-induced mortality from longline gear loss, (3) updated growth parameters, (4) updated length-weight relationship parameters, (5) updated maturity-at-age parameters, and (6) a simplification of the longline selectivity functions. All model runs were conducted with the CASAL version 2.30-2012-03-21 rev. 4648 that was agreed on by WG-SAM-14.
The updated assessment model leads to a smaller estimate of the virgin spawning stock biomass B0 than that obtained in 2017, with an MCMC estimate of 70 519 tonnes (95% CI: 65 634 - 76 626 tonnes). The estimated SSB status at the end of 2019 was 0.51 (95% CI: 0.49-0.53). The smaller biomass meant that the catch limit that satisfies the CCAMLR decision rules decreased from 3525 tonnes to 3030 tonnes.
Over the course of the projection period the median SSB status reaches a minimum of 40% before increasing to the target level at the end of the 35-year projection period, a pattern that is driven by the switch of the fishery from trawl to longline and below-average year class strength since 1998. The level of the predicted drop in SSB status by 2021, the time of the next stock assessment, was largely independent of the YCS period chosen as reference for the projections. With a comprehensive monitoring program of the fishery until then which include annual trawl surveys and extensive fish ageing to consolidate and estimate recent trends in YCS, the 2021 assessment will inform any decision whether further catch reductions will be necessary.
As the result of this assessment, we recommend a reduction of the catch limit from currently 3525 tonnes to 3030 tonnes for the Patagonian toothfish fishery in Division 58.5.2.