Competition for anchovy (Engraulis capensis) and sardine (Sardinops sagax) between the Cape Gannet (Morus capensis), Cape fur seal (Arctocephalus pusillus pusillus) and the purse-seine fishery on the west coast of South Africa

Abstract

Competition for pelagic fish resources was investigated by assessing the overlap in the food base of three land-based predators: Cape gannet (Morus capensis), Cape fur seal (Arctocephalus pusillus pusillus) and humans operating via a purse-seine fishery. Multivariate analysis the diet composition of gannets and seals and the catch composition of the pelagic fishery indicated that there were three ""feeding regimes"" during the 21-year study period: (1978-1998), an anchovy-dominated regime (1978-1983), an intermediate regime (1984-1990) and a sardine-abundant regime (1991-1998). It further showed that anchovy (Engraulis capensis) and sardine (Sardinops sagax) contributed substantially to the catches of all three top predators. Analysis of size-distributions of catches showed that the three predators generally caught the same sizes of anchovy and sardine, reflecting very little resource partitioning. The analysis demonstrated stronger competition between gannets and the purse-seiners than between gannets and seats, especially during seasons of poor recruitment. Furthermore, seasonal differences in the catches of the three predators are related to the recruitment, growth and migration of the prey species. The study confirmed previous observations that gannets prefer sardine to anchovy by showing gannet exploitation of sardine in the early 19905, when anchovy was still abundant. The gannets, which are species-specific feeders, may be impacted by substantial removals of pelagic fish resources by a large seal population and a large fishery.