The effect of sea temperature and food availability on the spawning success of Cape anchovy Engraulis capensis in the southern Benguela

Data on the thermal structure, copepod biomass and production, and total number of eggs of the Cape anchovy Engraulis capensis were obtained from monthly surveys during the periods August 1993 - March 1994 and September 1991 - March 1995 on the western Agulhas Bank and off the South-Western Cape, South Africa. Previous work suggested that anchovy spawn on the western Agulhas Bank in temperatures between 16 and 19 degreesC, where they feed predominantly on copepods. This study shows that the western Bank is a more suitable spawning area for anchovy, having greater thermal stability, a larger area of 16-19 degreesC water and a more consistent food environment than off the South-Western Caps. Also, copepod production on the western Bank was highest in 16-19 degreesC water. To identify factors controlling the area of this water mass, a cluster analysis was used on a suite of hydrographic variables. Three periods were identified: winter (August-September), spring (October-December) and summer (January March), reflecting changes in the extent of the 16-19 degreesC water and anchovy spawning, both of which peaked during spring. Spring was further characterized by infrequent surface upwelling. During summer, upwelling frequently reached the surface and the upwelling front migrated offshore, constricting the area of 16-19 degreesC water. It is hypothesized that spawning success in anchovy is dependent upon the extent of suitable spawning habitat, both spatially (16-19 degreesC water) and temporally (spring). To put this concept into a predictive framework, the number of anchovy eggs was regressed against the area of 16-19 degreesC water; a significant, positive relationship (p < 0.001, r(2) = 0.56, n = 17) was found. An implication of the hypothesis is that the duration of spawning may be important to recruitment.