Diet of the Cape fur seal Arctocephalus pusillus pusillus at the Robberg Peninsula, Plettenberg Bay, and implications for local fisheries

  • J Huisamen
  • SP Kirkman
  • CD van der Lingen
  • LH Watson
  • VG Cockcroft
  • R Jewell
  • PA Pistorius

Abstract

Cape fur seals Arctocephalus pusillus pusillus were harvested to extirpation on the Robberg Peninsula, Plettenberg Bay, on the south-east coast of South Africa, between the 17th and early 20th centuries. Seals returned to Robberg in small numbers during the early 1990s and their numbers subsequently increased. We studied the diet of this increasing population using faecal (scat) sampling to determine: the species composition and size of prey in the diet of Cape fur seals at Robberg; to explore temporal variation in the diet; and to investigate the potential for competition between seals and the fisheries around Plettenberg Bay. Of the 445 scats collected, 90% contained hard prey remains and 15 teleost prey species were represented in the 3 127 otoliths that could be identified. The seals’ most important prey species in terms of numerical abundance, frequency of occurrence and mass in the diet, were anchovy Engraulis encrasicolus, sardine Sardinops sagax, horse mackerel Trachurus capensis, sand tongue-fish Cynoglossus capensis and shallow-water hake Merluccius capensis  (in decreasing order of importance for numerical abundance). The proportion of anchovy in the diet increased during the study period (2003–2008), whereas the proportion of sardine decreased. The estimated average annual consumption of sardine by seals was higher than the average annual catch made by purse-seine fisheries in this area, suggesting resource competition between seals and purse-seiners, especially in the light of continuing growth of seal numbers in the area. However, direct competition between seals and linefisheries appeared to be minimal. Scat sampling of Cape fur seals holds potential to serve a useful and cost effective indicator of temporal changes in sardine abundance.

Keywords: marine protected area, pinnipeds, sardine, scat analysis, South Africa

African Journal of Marine Science 2012, 34(3): 431–441

Author Biographies

J Huisamen
School of Natural Resource Management, Nelson Mandela Metropolitan University, Private Bag X6531, George 6530, South Africa; CapeNature, 17th Avenue, Voëlklip, Hermanus 7200, South Africa
SP Kirkman
Oceans and Coasts, Department of Environmental Affairs, Private Bag X2, Rogge Bay 8012, South Africa; Animal Demography Unit, Department of Zoology, University of Cape Town, Rondebosch 7701, South Africa
CD van der Lingen
Branch: Fisheries, Department of Agriculture, Forestry and Fisheries, Private Bag X2, Rogge Bay 8012, South Africa; Marine Research Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
LH Watson
School of Natural Resource Management, Nelson Mandela Metropolitan University, Private Bag X6531, George 6530, South Africa
VG Cockcroft
Department of Zoology, Nelson Mandela Metropolitan University, PO Box 7700, Port Elizabeth 6031, South Africa; Centre for Dolphin Studies, PO Box 1856, Plettenberg Bay 6600, South Africa
R Jewell
Centre for Dolphin Studies, PO Box 1856, Plettenberg Bay 6600, South Africa; Sea Mammal Research Unit, Scottish Oceans Institute, University of St Andrews, St Andrews, Fife, KY16 8LB, Scotland
PA Pistorius
Department of Zoology, Nelson Mandela Metropolitan University, PO Box 7700, Port Elizabeth 6031, South Africa
Published
2012-10-25
Section
Articles

Journal Identifiers


eISSN: 1814-2338
print ISSN: 1814-232X