An investigation of integrated stock assessment methods for the Patagonian toothfish (Dissostichus eleginoides) in Division 58.5.2 using CASAL

This paper follows preliminary work in 2005 in developing an integrated assessment for Patagonian toothfish in Division 58.5.2. It focuses on developing a “base-case” assessment, which is primarily based on the 2005 assessment implemented in the Generalised Yield Model (GYM) using survey data of the abundances of juvenile fish but adding fishery catch-at-length data, including a standardised catch-per-unit-effort (CPUE) series. The base-case scenario was then extended to include estimates of IUU catch and mark-recapture data. Key sensitivity trials were also used to explore the tension between estimating natural mortality and selectivity for the different fisheries. The results of this work show that an integrated assessment for Dissostichus eleginoides in Division 58.5.2 seems tractable. The recruitment series derived from surveys has similarities with the recruitment series estimated by Welsford et al. (2006), but with a recognition that some age classes are only partially selected in these surveys. Although computer processing time is substantially increased, the inclusion of the catch-at-length and the mark-recapture data generally improves the confidence in the estimates of B0 and the recruitment series. Despite these results, there is considerable tension in the estimation model between natural mortality and fishing selectivities. The uncertainty arising from this tension will need to be explored further in order to appropriately accommodate this uncertainty in the estimate of long-term annual yield. Further discussion is needed on the choices being made in how the datasets are used in this assessment and the approaches being used to estimate the key parameters. An important stage in the development of this assessment will be to evaluate the robustness of these assessments to the uncertainties identified here as well as uncertainties in the spatial dynamics of the stock and the fisheries.