Trends in pelagic fish populations of the Saldanha Bay region, southern Benguela upwelling system, 1980–1990: a predator's perspective

Abstract

Changes in monthly and annual prey species composition of African penguins Spheniscus demersus are described from 1 454 stomach samples collected from two breeding islands in the Saldanha Bay region of the southern Benguela upwelling system between 1980 and 1990. The diet was dominated by anchovy, although pilchard, horse mackerel, round herring and chokka squid were also taken. The relative abundance of anchovy in penguin diet was highest between 1984 and 1989, coinciding with a period of favourable environmental conditions for recruitment of anchovy and a subsequent increase in fish biomass. The more recent decrease in dietary abundance of anchovy and its partial replacement by pilchard is consistent with independent (ship-borne acoustic) evidence of a recent increase in pilchard biomass. In contrast to these congruent trends, the size of anchovy taken by penguins between 1980 and 1990 has decreased dramatically, across years of good and poor recruitment. The reasons for this remain obscure but may reflect, in part, poor recruitment of anchovy in the latter part of the decade. Monthly changes in the size frequency distribution of anchovy consumed by penguins apparently reflect seasonal patterns of fish migration.