The importance of lipid-rich fish prey for Cape gannet chick growth: are fishery discards an alternative?

A recent decline in population numbers of Cape gannets (Morus capensis) breeding off the west coast of South Africa coincided with decreased availability of lipid-rich fish prey: anchovy (Engraulis encrasicolus) and sardine (Sardinops sagax). Seabirds can use fishery discards as an alternative, but the quality of this food in the Benguela ecosystem is lower than that of their natural prey species. We consider whether chick growth and survival during chick rearing co-vary with the periods of high and low availability of their lipid-rich prey species and whether fishery discards would be an alternative. The proportion of anchovy and sardine in the diet was between 66 and 84% in the years 1986-1988, but just 16-35% from 2004 to 2006. Months with large proportions of anchovy and sardine in the diet were associated with faster chick growth. No association between the proportion of fishery discards in the diet and chick growth was found. The patterns are consistent with the notion that a distributional shift of anchovy and sardine decreased their contribution to the diet of Cape gannets and slowed chick growth along with lessening chick survival at the breeding colony. The reduced survival may partially explain the decline in numbers of Cape gannets breeding in the southern Benguela.