Dive behaviour and foraging effort of female Cape fur seals Arctocephalus pusillus pusillus.

S P Kirkman,D P Costa,J P Y Arnould,M Weise,J A Botha,A.-L Harrison,W H Oosthuizen,P G H Kotze

doi:10.1098/rsos.191369

Abstract

While marine top predators can play a critical role in ecosystem structure and dynamics through their effects on prey populations, how the predators function in this role is often not well understood. In the Benguela region of southern Africa, the Cape fur seal (Arctocephalus pusillus pusillus) population constitutes the largest marine top predator biomass, but little is known of its foraging ecology other than its diet and some preliminary dive records. Dive information was obtained from 32 adult females instrumented with dive recorders at the Kleinsee colony (29°34.17′ S, 16°59.80′ E) in South Africa during 2006–2008. Most dives were in the depth range of epipelagic prey species (less than 50 m deep) and at night, reflecting the reliance of Cape fur seals on small, vertically migrating, schooling prey. However, most females also performed benthic dives, and benthic diving was prevalent in some individuals. Benthic diving was significantly associated with the frequency with which females exceeded their aerobic dive limit. The greater putative costs of benthic diving highlight the potential detrimental effects to Cape fur seals of well-documented changes in the availability of epipelagic prey species in the Benguela.

Highlights

Subject Category: Biology Subject Areas: behaviour/ecology Keywords: South Africa, Benguela, pelagic, benthic, environmental change, aerobic dive limit
The current population size for the Cape fur seal has been estimated at 1.5–2 million [14,15], and its breeding range extends from the southeast coast of South Africa to southern Angola [13]
The results reveal that the majority of dives were pelagic and to depths of less than 100 m, indicating that the Cape fur seal at Kleinsee is primarily an epipelagic forager, a small proportion of individuals conducted a substantial number of benthic dives

Summary

Introduction

Subject Category: Biology (whole organism) Subject Areas: behaviour/ecology Keywords: South Africa, Benguela, pelagic, benthic, environmental change, aerobic dive limit. It is well known that top predators can play a critical role in marine ecosystem structure and dynamics through their effects on prey populations and ecosystem function [1,2] Knowledge of their foraging ecology, including trophic interactions, feeding behaviour and the factors influencing them, is key to understanding ecosystem functioning and the effects that removal, depletion, growth or range shifts of such populations may have on ecosystems. There is ever-present concern regarding competition for resources between the seal population, which removes ca 2 million tons of marine organisms per year in South Africa and Namibia [17], and commercial fisheries [21,22] This has been a primary motivation for numerous diet studies of the Cape fur seal, mostly with comparison to fisheries [17,18,22,23,24]

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Results

Conclusion