Multiple factors affecting South African anchovy recruitment in the spawning, transport and nursery areas

Abstract

Despite high primary productivity, the yield of pelagic fish in the southern Benguela is relatively low compared to that in the Humboldt system. Part of the constraint may be the ability of pelagic fish to reproduce successfully in a strongly pulsed upwelling environment, where enrichment, retention and concentration mechanisms are less compatible than in Peru-Chile. Anchovy Engraulis capensis spawn upstream of the main upwelling centres on the food-poor, thermally stratified western Agulhas Bank, over a protracted summer season (October-February) when high wind speeds of 7-8 m.s-1 are prevalent. Eggs spawned farther east, on the central or eastern Agulhas Bank, may be subject to increased cannibalism and advective losses, whereas those spawned farther west could be susceptible to heavy advective losses offshore during periods of strong southerly winds. Copepod concentrations are negatively correlated with spawner biomass on the western Bank and are inversely linked to high rates of gonad atresia in anchovy and reduced subsequent recruitment. There is a restricted area of suitable spawning temperatures (16-19°C) on the western Bank, and fish outside that range appear to have less successful spawning, affecting recruitment; a reduction of the duration of spawning appears to decrease the chances of good recruitment. Oil content of pelagic catches on the west coast of South Africa suggests that good feeding conditions for adult fish are also favourable for growth of young fish in the nursery area. An inverse correlation between the numbers of recruits estimated in June and the strength of upwelling-favourable winds over the previous summer indicates the importance of advective loss of eggs and larvae in the transport and nursery areas. The presence of Agulhas Rings close to the shelfedge, predators on juvenile anchovy, and the strength of upwelling on the West Coast could also be important factors affecting recruitment success. There seem to have been changes in the inshore-offshore distributions of spawning anchovy and/or their eggs over the past three decades, resulting in different recruitment responses to south-easterly winds.