On an approach for comparing the implications of alternative fish stock assessments, with application to the stock of Cape hake Merluccius spp. off northern Namibia

Abstract

The 1989 meeting of an ad hoc Working Group on Stock Assessment Methodology at the International Commission for the Southeast Atlantic Fisheries (ICSEAF) suggested a "risk analysis" approach to contrast the implications of the different estimates of Total Allowable Catch (TAC) provided when different assessment methodologies are applied to a stock. The details of such an approach are developed. The approach involves deterministic forward projection of the biomass trajectory implied by each assessment for a period (e.g. 10 years) under the TACs or target levels of effort indicated by that and the other assessments. Within the framework developed, it is possible to allow for the effects of "learning" (i.e. updating parameter estimates as more data become available). The approach is relatively simple computationally and allows for quick and sometimes revealing comparisons between assessment/management procedures to be made. It is applied to the variety of assessments available for the stock of Cape hake off northern Namibia. The implication of the analyses conducted is that application of the f _0.1 harvesting strategy in combination with either the Spanish assessment procedure and the ICSEAF "revised" data set, or the South African assessment procedure using the Spanish OTB-8 data series, could lead to rapid resource depletion, so that use of these procedures would seem to be inappropriate for this stock.