Food habits of an endangered seabird indicate recent poor forage fish availability off western South Africa

Abstract

Large recent decreases of Cape gannets Morus capensis, Cape cormorants Phalacrocorax capensis and African penguins Spheniscus demersus in South Africa resulted in their being listed as Endangered. These seabirds, endemic to the Benguela upwelling system (BUS), primarily rely on anchovy Engraulis encrasicolus and sardine Sardinops sagax for food, yet decreased during periods of abundance of these prey species. In order to investigate this dichotomy, we examined long-term dietary characteristics for gannets in the region in relation to acoustically-derived biomass of prey. Principal component (PC) analysis of diet composition indicated an alternation in the use of anchovy and sardine (PC1), as well as a marked decrease, after the early 2000s, in the availability of these preferred forage resources (PC2). PC2, which we term the Forage Availability Index, was positively related to numbers of gannets and cormorants breeding each year and to estimates of survival of adult penguins at their two largest colonies in northwest South Africa. This indicates that recent availability of anchovy and sardine was insufficient to support these bird populations. Our results emphasize the need to account not only for overall abundance but also local availability of forage resources, when applying an ecosystem approach to managing fisheries for seabirds.