Previous retrospective analysis showed that removal of tagging data had a strong influence on the estimation of biomass and recruitment which, potentially, could be a result of changes in the spatial distribution of fishing and tagging effort. This work presents an update of the analysis of description of spatial changes in the Subarea 48.3 toothfish fishery presented to WG-SAM-2024 and the effects that these changes have on estimates of biomass from Chapman indicators and Casal2 stock assessments.
We conclude that:
- Changes to the depth range of the fishery have occurred and are likely to lead to a significant underestimate of the stock. In particular, prior to 2010, 19% of the stock was estimated to be shallower than 750 m. Since then, very few tags have been released and recaptured at these depths, and so the current estimate of vulnerable biomass largely excludes this part of the stock. Using aggregated tag data may underestimate the current vulnerable biomass by over 5,000 tonnes.
- The contraction of the fishery from management area 48.3A and uncertainty about the magnitude and location of IUU is unlikely to have a large effect on the management of the stock.
- Changes to the spatial footprint indicate that the tag recaptures sample a decreasing proportion of the historic footprint.
As a result, the current stock assessment is likely to underestimate the stock size and underestimate the stock status. Further work is needed to incorporate this analysis into the integrated stock assessment, and a workplan is proposed to address this.