Interactions of the South African fur seal and fisheries in the Benguela ecosystem

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dc.contributor.advisorField, John Gen_ZA
dc.contributor.advisorShelton, Peter Aen_ZA
dc.contributor.authorWickens, Patricia Anneen_ZA
dc.date.accessioned2016-11-16T13:25:07Z
dc.date.available2016-11-16T13:25:07Z
dc.date.issued1989en_ZA
dc.descriptionBibliography: pages 168-180.en_ZA
dc.description.abstractThis thesis adopts an holistic approach to evaluation of the "seal problem", which encompasses the operational and biological interactions between the South African fur seal and fisheries in the Benguela Ecosystem. The operational interactions are examined using a qualitative assessment, and speculative, single-species simulation models are developed to improve understanding of seal population dynamics, present options for seal management and explore seal-fishery interactions. The models show that the probability of a seal living longer than 20 years is 14%; that the population has increased at a rate of 5.1 % p.a. since the last major census in 1983; that in 1988 there were an estimated 1.5 million seals that consumed over 2 million tons of fish; that although seal population size is usually estimated by multiplying the number of pups by a factor of 4, this factor is very variable and under different culling regimes is usually greater than 4; that historically, the mean population : pup ratio is 4.55. The most effective means of reducing the population is to cull both pups and bulls (the population decrease resulting mainly from reduced pup production caused by disturbance during bull culling). Culling pups is the least effective means of population control. Culling cows alters the population sex ratio drastically and is considered undesirable. To achieve a small or negative population growth rate, more bulls and pups need to be culled than have been taken historically. Each of the fisheries is found to be subject to operational interactions with seals, this being most severe for the purse-seine fishery. The per capita consumption of fish by seals is similar under any management regime so it is not justified to adopt a particular culling strategy to reduce overall consumption. When either seal predation or fishing mortality is reduced, the model simulates an increase in fishery yield of Cape hakes. But there is inadequate understanding of hake population dynamics at present, and depending on the interactions between species, decreased predation may result in increased or decreased fishery yields. This study shows that operational interactions are a real problem and this biases the opinion of fishermen towards seals as competitors for fish resources.en_ZA
dc.identifier.apacitationWickens, P. A. (1989). <i>Interactions of the South African fur seal and fisheries in the Benguela ecosystem</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Biological Sciences. Retrieved from http://hdl.handle.net/11427/22566en_ZA
dc.identifier.chicagocitationWickens, Patricia Anne. <i>"Interactions of the South African fur seal and fisheries in the Benguela ecosystem."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 1989. http://hdl.handle.net/11427/22566en_ZA
dc.identifier.citationWickens, P. 1989. Interactions of the South African fur seal and fisheries in the Benguela ecosystem. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Wickens, Patricia Anne AB - This thesis adopts an holistic approach to evaluation of the "seal problem", which encompasses the operational and biological interactions between the South African fur seal and fisheries in the Benguela Ecosystem. The operational interactions are examined using a qualitative assessment, and speculative, single-species simulation models are developed to improve understanding of seal population dynamics, present options for seal management and explore seal-fishery interactions. The models show that the probability of a seal living longer than 20 years is 14%; that the population has increased at a rate of 5.1 % p.a. since the last major census in 1983; that in 1988 there were an estimated 1.5 million seals that consumed over 2 million tons of fish; that although seal population size is usually estimated by multiplying the number of pups by a factor of 4, this factor is very variable and under different culling regimes is usually greater than 4; that historically, the mean population : pup ratio is 4.55. The most effective means of reducing the population is to cull both pups and bulls (the population decrease resulting mainly from reduced pup production caused by disturbance during bull culling). Culling pups is the least effective means of population control. Culling cows alters the population sex ratio drastically and is considered undesirable. To achieve a small or negative population growth rate, more bulls and pups need to be culled than have been taken historically. Each of the fisheries is found to be subject to operational interactions with seals, this being most severe for the purse-seine fishery. The per capita consumption of fish by seals is similar under any management regime so it is not justified to adopt a particular culling strategy to reduce overall consumption. When either seal predation or fishing mortality is reduced, the model simulates an increase in fishery yield of Cape hakes. But there is inadequate understanding of hake population dynamics at present, and depending on the interactions between species, decreased predation may result in increased or decreased fishery yields. This study shows that operational interactions are a real problem and this biases the opinion of fishermen towards seals as competitors for fish resources. DA - 1989 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 1989 T1 - Interactions of the South African fur seal and fisheries in the Benguela ecosystem TI - Interactions of the South African fur seal and fisheries in the Benguela ecosystem UR - http://hdl.handle.net/11427/22566 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/22566
dc.identifier.vancouvercitationWickens PA. Interactions of the South African fur seal and fisheries in the Benguela ecosystem. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 1989 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/22566en_ZA
dc.language.isoengen_ZA
dc.publisher.departmentDepartment of Biological Sciencesen_ZA
dc.publisher.facultyFaculty of Scienceen_ZA
dc.publisher.institutionUniversity of Cape Town
dc.subject.otherCape fur seal - Ecologyen_ZA
dc.subject.otherSealing - South Africa - Environmental aspectsen_ZA
dc.subject.otherFisheries - Environmental aspects - South Africaen_ZA
dc.titleInteractions of the South African fur seal and fisheries in the Benguela ecosystemen_ZA
dc.typeDoctoral Thesis
dc.type.qualificationlevelDoctoral
dc.type.qualificationnamePhDen_ZA
uct.type.filetypeText
uct.type.filetypeImage
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
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