Age and growth of two populations of West Coast steenbras Lithognathus aureti in Namibian waters, based on otolith readings and mark-recapture data

Age and growth of West Coast steenbras Lithognathus aureti, sampled from two separate populations (northern and southern) along Namibia's coast, was determined using sectioned otoliths, and the results were validated using mark-recapture data. For both populations, the special three-parameter Von Bertalanffy growth model described growth adequately. Growth of the southern population was described by the equation Lt = 73.556 (1– e-0.065(t+3.92)) cm and for the northern population by Lt = 84.601(1– e-0.088(t+2.756) cm. Environmental conditions, such as difference in sea surface temperature, density-dependent competition for food, or biochemical genetic variations between the two populations, are possible reasons for the geographic differences found in the growth rates and length-at-age. Slow growth and longevity are characteristics of West Coast steenbras that make it extremely susceptible to overfishing; careful management of the resource is therefore essential. Keywords: age, growth, mark-recapture, otoliths, West Coast steenbrasAfrican Journal of Marine Science 2001, 23: 169–17

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

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 steenbrasAfrican Journal of Marine Science 2001, 23: 157–16

Observations on the linefish resources of Namibia, 1990–2000, with special reference to West Coast steenbras and silver kob

The Namibian linefishery is a multisector fishery, with recreational and commercial sectors argeting overlapping species. Prior to 1990, catch data were recorded for the boat sector, but little research was done on the population dynamics of any of the species. Hence, none of the stocks were assessed owing to the lack of parameter estimates for fisheries modelling. Current management regulations for the Namibian linefishery are therefore not based on scientific investigations but, for historical reasons, were adapted from South African legislation for similar species. From 1994, linefish research in Namibia focused on the life history and population dynamics of silver kob Argyrosomus inodorus and West Coast steenbras Lithognathus aureti, important species in both commercial and recreational sectors. A roving-roving creel survey was used to determine effort and catches of recreational shore-anglers. Both species were assessed using yield-per-recruit models. Based on the results, reduced daily bag limits and the implementation of size limits for the recreational sector were proposed. For the commercial sector, the introduction of a total allowable catch would seem to be the best option. Results from an economic survey indicated that the recreational sector earns six times more for the country annually than the commercial sector. Keywords: creel survey, economics, genetics, migration, silver kob, West Coast steenbras, yield-per-recruitAfrican Journal of Marine Science 2001, 23: 135–14