Towards the development of a stock assessment for Patagonian toothfish in Division 58.4.4, SSRU C on Ob and Lena Banks

Following the advice from the CCAMLR Working Group in WG-SAM in 2012, a preliminary assessment of stock status of D. eleginoides in SSRU C in Ob-Lena Banks was made using a CASAL catch-at-length model. In this preliminary assessment, the model was started in 1990 and catch, proportions at length in the catch, and recapture data from SSRU C from 2007/08 to 2011/12 were fitted in the model. The MPD estimate of B₀ was 741 tonnes, and current biomass was 625 tonnes (84.4 % of B₀). MPD fits suggest suitable fits to the data, however, there appears to be a pattern of increasing YCS over the time, which is unlikely. Therefore, model where all of the YCS are fixed at 1 should be considered next time. MCMC plots and diagnostics suggest low mixing of the MCMC chain. Hence, the MCMC results are not yet robust. The reasons for this need to be investigated, and may require that the chain will need to be run for much longer (e.g. 10e6 iterations), and subsampled less frequently. The MCMC estimate of B₀ was 1 455 tonnes, and current biomass was 1 275 tonnes (87.6 % of B₀). An additional uncertainty in the current model is the historical IUU catch. The current model only includes the catch history since 2007/08 but there is a history of IUU catches for the whole Division starting from 1996/97. To provide a more realistic B₀ to consider the inclusion of IUU catches for this SSRU into the model. Possible approaches for this should be discussed during WG-FSA-2012.