Assessments of the stocks of cape hakes merluccius spp. Off south africa

Abstract

A production model and an ad hoc tuned VPA are applied to the Cape hake resource off South Africa. The production model is based on an observation error estimator and is applied to survey biomass estimates and catch rates (cpue). The ad hoc tuned VPA is based on the Laurec-Shepherd tuning algorithm. “Base case” applications of the two approaches provide different appraisals of the status and productivity of the resource. The ADAPT method and an age-structured production model respectively provide results similar to those of the ad hoc tuned VPA and the production model. Whereas results from both production models are insensitive to a wide variety of assumptions, those from the ad hoc tuned VPA and ADAPT are very sensitive to the value of the instantaneous rate of natural mortality. Discrepancies between the various sets of results are shown to be a consequence of a conflict between the catch-at-age information and the cpue and biomass data. They are not primarily a result of differences in the estimation methods. Suggestions are made for research which may help resolve this conflict. © 1994 Taylor & Francis Group, LLC.