Review of some assumptions for modelling Patagonian toothfish dynamics at CCAMLR Subarea 48.3

During the year 2005 meeting of the WG-FSA two different methods for assessing Patagonian toothfish stock in CCAMLR Subarea 48.3 were available to the Working Group: CASAL and ASPM (WG-FSA-05/16; WG-FSA-05/73). Although the underlying basic age-structure population dynamics models in both cases were similar, there were considerable differences in assumptions and implementations of the two methods. In this paper, we discuss the related effects considering constant h parameter and deterministic recruitment over time vs. considering a fitted h parameter and variable recruitment. The first alternative produces a more stable population structure and would introduce bias in the biomass and spawning stock biomass estimated. In the second case (variable recruitment and fitted h), the model have more flexibility to fit the changes in population structure and abundance. The parameter r from the stock recruitment relationship is also analyzed, in relation with the current underlying assumption of fish stock resilience.