An assessment of the current status and potential yield of Namibia's northern West Coast steenbras Lithognathus aureti population

Abstract

Two distinct populations of West Coast steenbras Lithognathus aureti are found in Namibian waters; one, the northern population, is exploited recreationally and commercially. Data collected between 1995 and 1999 were used to evaluate its current state and potential yield, using two yieldper-recruit approaches. A Beverton and Holt model was used to study the effect of different fishing mortalities, natural mortality and age-at-first-capture schedules on yield-per-recruit, and also to derive fishing mortality levels for prescribed biological reference points. A Thompson and Bell model was used to test the performance of some fisheries regulations, e.g. the effects that implementation of a minimum size could have on the sustainable exploitation of the stock, within safe biological limits. Results from the Thompson and Bell model estimated maximum sustainable yield for the northern population of Lithognathus aureti at between 109 and 173 tons. The best estimate was some 134 tons, a value also proposed as the limit reference point. Current depletion is 42%, and a minimum size limit of 40 cm for West Coast steenbras off Namibia is proposed. This restriction would only slightly increase spawner stock biomass, but the spawning potential of the population would increase considerably.

Keywords: biological reference points, management, per-recruit models, West Coast steenbras

African Journal of Marine Science 2001, 23: 157–168