Mortality estimates and biological reference points for the Natal stumpnose Rhabdosargus sarba (Pisces: Sparidae) in KwaZulu-Natal, South Africa

Abstract

A per-recruit stock assessment was undertaken for the sparid Rhabdosargus sarba, which is caught in estuarine and marine shore fisheries in KwaZulu-Natal. Input parameters used in the analyses included age and growth parameters and mortality estimates from catch curves. The mortality estimates from estuarine and coastal catch curves were very different. Total and fishing mortality from estuaries was much higher, because adult R. sarba emigrate out of estuaries and fishing pressure on juveniles in estuaries is substantial. As a consequence two separate models were run for estuarine and coastal fisheries. The results of the spawning biomass per-recruit model indicate that R. sarba is at 34% (in estuaries) to 63% (in coastal fisheries) of its unfished level. Because estuarine estimates of fishing mortality (F) include emigration, the results indicate that R. sarba in KwaZulu-Natal is not at present overexploited. These results were substantiated by analysing available trends in catch per unit effort (CPUE) for this species.