Preliminary assessment of the Falklands Patagonian toothfish (Dissostichus eleginoides) population: Use of recruitment indices and the estimation of unreported catches

Longline fishing for Patagonian toothfish (Dissostichus eleginoides) has taken place in Falkland Island waters for over 10 years. This species was previously only caught as bycatch in the Loligo gahi and finfish trawl fishery. This paper presents a preliminary assessment of the Falkland Islands population of Dissostichus eleginoides using an age-structured production model (ASPM). Two models were investigated; one using a Beverton-Holt stock recruitment relationship and another using trawler CPUE based estimates of toothfish abundance to estimate yearly recruitment. A stock recruitment relationship was not included within the model that used abundance estimates, as recruitment to the population was estimated directly from the index of relative recruit abundance calculated from trawler CPUE. The model was fitted to longline standardised CPUE and to the catch-at-length data. The models produced estimates that provided similar declines in the toothfish population although the start and end biomass estimates varied slightly. The models provided estimates of between 13,000 and 26,000 tonnes of current spawning stock biomass and showed current biomass to be between 38 and 46% of virgin biomass. The fit to CPUE was poor between 1994 and 1996, which we hypothesised could have been due to unreported catches or changes in q or M. This was a time when there was considerable IUU fishing in the southwest Atlantic. When the model was allowed to estimate a level of extra catch the fit was improved and 5000 tonnes of extra catch was estimated. Two further models were briefly examined to investigate the possibility of changes in M and q but both required large changes in those parameters in order to fit. Estimates of MSY varied widely, from 912 to almost 3000 tonnes.