Geographical and seasonal patterns in the diet of Cape fur seals Arctocephalus pusillus pusillus in Namibia, based on extensive scat analyses

Abstract

Namibia’s population of Cape fur seals Arctocephalus pusillus pusillus is a major constituent of top predator biomass in the northern Benguela upwelling system. Understanding their diet is key to comprehending their role in the ecosystem and potential drivers of recent population changes, such as a northward extension of the population’s range. Using prey remnants retrieved from 1 413 scat samples collected monthly at 12 Cape fur seal colonies along the Namibian coast over 28 years (1994‒2021), we assessed seasonal and geographical differences in their diet. We include the first comparison of diet between the growing colonies in the far northern area of Namibia and the colonies to the south, many of which have been in decline. Four fish species dominated in the diet. The bearded goby Sufflogobius bibarbatus, which is ubiquitous throughout the northern Benguela ecosystem, was the most abundant prey item in the diet. However, Cape fur seals appeared to show a preference for the more-nutritious fish species Cape horse mackerel Trachurus capensis and the lanternfish Lampanyctodes hectoris by switching to those prey during autumn and spring in the north and south, respectively, coinciding with the greater availability of those species during upwelling periods. The availability of Cape horse mackerel, especially during a critical period for the growth and development of pups before the onset of winter, may be key to the greater productivity of Cape fur seal colonies in the north relative to those in the south of Namibia, and warrants further investigation. Juveniles of Cape horse mackerel and Cape hake Merluccius capensis were the only fishery species of commercial importance that were relatively abundant in the seals’ diet, while the sardine Sardinops sagax, which was historically dominant, was found in negligible proportions.