Trophic investigations of Cape fur seals at the easternmost extreme of their distribution

Abstract

The diet of Cape fur seals Arctocephalus pusillus pusillus in the eastern portion of their distribution has received little attention previously, and was studied using traditional methods only. In 2013 we therefore assessed the diet of seals at the easternmost colony at Black Rocks, Algoa Bay, South Africa (33°50′ S, 26°16′ E) from both scats and the analysis of stable isotopes in blood, guard hairs and whiskers. Information from both sources indicated that seals at this site are generalist predators feeding on a mix of pelagic (mainly anchovy Engraulis encrasicolus and chokka squid Loligo reynaudii) and benthic species (tonguefish Cynoglossus spp., East Coast sole Austroglossus capensis and horsefish Congiopodus spp.). Stable isotope analysis revealed that the diet of individual females may differ consistently as a result of different individual foraging preferences or strategies. There were no differences in the diet between summer and winter. Furthermore, our results suggest that stable isotopes in three-month-old pups can be used as proxies for the diet of lactating females, using female-to-pup discrimination factors presented here. Future research should consider an extension of this approach to fully resolve ecological partitioning between individual Cape fur seals to better understand their role in ecosystem dynamics.

Keywords: Agulhas Current, Algoa Bay, Arctocephalus pusillus pusillus, carbon and nitrogen stable isotopes, scat analysis, seasonal diet, South Africa

African Journal of Marine Science 2014, 36(3): 331–344