No abstract

No abstract

East coast geelbek exist as a single stock comprising three distinct age/size structured subpopulations. The subadults (1-4 years) occur in the SW Cape, South Africa. They feed principally on anchovy Engruulis cupensis, which they follow inshore in summer and offshore in winter. The adults (5-9+ years) undergo a seasonal inshore and longshore migration to Natal where they spawn offshore in spring. During this migration they feed mainly on the pilchard Surdinops sugax, which also move north-eastwards during this time, facilitated by regional oceanographic conditions. ' Adolescent ' fish (415 years) appear to undergo limited gonadal growth and partially accompany the spawning stock on their migration. It is suggested that the eggs, larvae and early juveniles are transported southwards by the peripheral inshore waters of the Agulhas Current. Juveniles ( 1 2 years) use the SE Cape region as a nursery area for about 1 year, initially feeding on mysids and then switching to anchovy, before moving southwestwards to join the subadults in the SW Cape region. 'C 1995 The Fisheries Socirty of the British lslcs

The dusky kob Argyrosomus japonicus is an important linefish species that is abundant in South African waters between Cape Agulhas and the M~ambique border. The juveniles «I 070 mm) occur inshore, where they are targeted by surf-zone and estuarine anglers, and the adults are caught mainly beyond the surf by recreational and commercial linefishermen. Yield-per-recruit (YIR) and spawner biomass-per-recruit (SBIR) models were applied to the South African dusky kob stock, with accommodation for different inshore (Fj"'ho",) and offshore (FoJf'hm,,) fishing mortality. It is shown that, as a result of high juvenile mortality (Fi'uho •.•.= 4-6-fold FSB2S and 6-IO-fold FSB40)' dusky kob were exploited far beyond optimal (FSB40) and threshold (FSB25) fishing limits, and that SBIR was 1.0-4.5% of the pristine value. This is indicative of severe stock depletion and of recruitment overfishing. Based on the bag frequencies of 16 367 angler outings, it is clear that the current recreational bag limit of 10 fish•angler-I.day-I does not limit fishing effort. Three alternative management scenarios (combinations of Fi"'ho •.•.and minimum size at first capture) are discussed, which elevate SBIR above 25%SBIR",=o, increase YIR beyond current levels, and equitably apportion YIRinsho •.•. and YIRojJ,',o •.•.. Whereas it would take about 41 years for any of the alternative scenarios to attain maximum SBIR, a j-fold increase is expected after 9 years and a 6.0-6.4-fold increase after 20 years. The increase in spawner biomass associated with the alternative strategies is expected to increase both inshore and offshore yield dramatically (as a result of higher recruitment), even though YIR'n,ho'" would be 69-70% of its current value. It is concluded that, because of the susceptibility of A. japonicus to recruitment overfishing, the species may have been overexploited in fisheries throughout its wide distributional range, and that accurate life history data and assessment-based regulations are essential for successful long-term management.

The southern African coastline is dominated by strong currents. Along the eastern seaboard, the warm western boundary Agulhas Current sweeps close inshore along the shelf edge before diverging from the coast on the Agulhas Bank and retroflecting back into the Indian Ocean. On the western seaboard, strong jet currents develop in the southern Benguela, associated with the strong thermal gradients induced by upwelling and Agulhas Current intrusions and eddies. There is, in general, northward drift of surface waters in the Benguela Current with strong offshore losses in the vicinity of an exceptionally active upwelling region off Lüderitz. Several potent mechanisms exist for offshore dispersal and loss from the productive shelf waters, such as eddies, filaments, retroflections and offshore Ekman drift, which pose special problems for successful retention of planktonic eggs and larvae from broadcast spawners. Most fish species in southern Africa have evolved highly selective reproductive patterns, which ensure that sufficient progeny are retained or can enter the nursery grounds along the coastline. Four important reproductive habitats, comprising spawning areas, transport mechanisms and nursery grounds, occur between Moçambique and Angola. These are used by a wide variety of pelagic, demersal and inshore-dwelling fish species.