The trophic dynamics of the broadnose sevengill shark (Notorynchus cepedianus) in False Bay, South Africa, using multiple tissue stable isotope analysis

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dc.contributor.advisorO'Riain, M Justinen_ZA
dc.contributor.advisorKock, Alisonen_ZA
dc.contributor.advisorLewis, Matthew Charlesen_ZA
dc.contributor.advisorBarnett, Adamen_ZA
dc.contributor.authorDe Necker, Leighen_ZA
dc.date.accessioned2017-09-01T14:20:45Z
dc.date.available2017-09-01T14:20:45Z
dc.date.issued2017en_ZA
dc.description.abstractDespite their important ecological role, there is limited quantitative information on the trophic ecology of large, apex predator sharks. This is largely a consequence of their occupying naturally low population densities, being highly mobile and elusive and ranging over large distances. Stable isotopes provide a low cost, non-lethal method for investigating the short and long-term diet of a predator, which when combined with prey data can be used to understand the trophic interactions and the potential regulatory effects they may have on the structure and function of marine ecosystems. In this study, I used non-lethal stable isotope analysis to investigate the trophic dynamics of sevengill sharks, Notorynchus cepedianus, within False Bay, South Africa. A total of 39 muscle biopsies (33 female, six male), and 28 blood plasma samples (25 female, three male) were collected from sevengill sharks. These were analysed together with 161 prey samples from 32 different species, including cephalopods, crustaceans, teleosts, chondrichthyans and marine mammals. In addition, seven white shark, Carcharodon carcharias, muscle samples were collected from False Bay and analysed for comparison with those of sevengills. Sevengills in False Bay had the highest δ15N values of all species sampled in this study (including white sharks) and appear to feed predominantly on a variety of coastal prey species from various functional groups. A stable isotope mixing model revealed that inshore chondrichthyans were their most important prey, with Cape fur seals and inshore teleost species also being important prey groups. There was no apparent seasonal shift in the diet of sevengills, despite clear seasonal aggregation in coastal kelp forests during the summer months. δ15N decreased significantly with sevengill size, with immature females having higher δ15N values than mature females, while δ13C increased with sevengill size. These trends are hypothesised to be linked to dietary shifts associated with the relative use of different habitat types when individuals become sexually mature. There is a need to combine stable isotope data with movement patterns and habitat use to better understand the relationship between isotope ratios and habitat use. Ongoing research on sevengill and white shark movement patterns in False Bay, together with the results presented in this study, will provide important information on the trophic and ecological role that two top predatory sharks play in False Bay. Keywords: Notorynchus cepedianus, sevengill shark, stable isotopes, trophic ecologyen_ZA
dc.identifier.apacitationDe Necker, L. (2017). <i>The trophic dynamics of the broadnose sevengill shark (Notorynchus cepedianus) in False Bay, South Africa, using multiple tissue stable isotope analysis</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Biological Sciences. Retrieved from http://hdl.handle.net/11427/25025en_ZA
dc.identifier.chicagocitationDe Necker, Leigh. <i>"The trophic dynamics of the broadnose sevengill shark (Notorynchus cepedianus) in False Bay, South Africa, using multiple tissue stable isotope analysis."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2017. http://hdl.handle.net/11427/25025en_ZA
dc.identifier.citationDe Necker, L. 2017. The trophic dynamics of the broadnose sevengill shark (Notorynchus cepedianus) in False Bay, South Africa, using multiple tissue stable isotope analysis. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - De Necker, Leigh AB - Despite their important ecological role, there is limited quantitative information on the trophic ecology of large, apex predator sharks. This is largely a consequence of their occupying naturally low population densities, being highly mobile and elusive and ranging over large distances. Stable isotopes provide a low cost, non-lethal method for investigating the short and long-term diet of a predator, which when combined with prey data can be used to understand the trophic interactions and the potential regulatory effects they may have on the structure and function of marine ecosystems. In this study, I used non-lethal stable isotope analysis to investigate the trophic dynamics of sevengill sharks, Notorynchus cepedianus, within False Bay, South Africa. A total of 39 muscle biopsies (33 female, six male), and 28 blood plasma samples (25 female, three male) were collected from sevengill sharks. These were analysed together with 161 prey samples from 32 different species, including cephalopods, crustaceans, teleosts, chondrichthyans and marine mammals. In addition, seven white shark, Carcharodon carcharias, muscle samples were collected from False Bay and analysed for comparison with those of sevengills. Sevengills in False Bay had the highest δ15N values of all species sampled in this study (including white sharks) and appear to feed predominantly on a variety of coastal prey species from various functional groups. A stable isotope mixing model revealed that inshore chondrichthyans were their most important prey, with Cape fur seals and inshore teleost species also being important prey groups. There was no apparent seasonal shift in the diet of sevengills, despite clear seasonal aggregation in coastal kelp forests during the summer months. δ15N decreased significantly with sevengill size, with immature females having higher δ15N values than mature females, while δ13C increased with sevengill size. These trends are hypothesised to be linked to dietary shifts associated with the relative use of different habitat types when individuals become sexually mature. There is a need to combine stable isotope data with movement patterns and habitat use to better understand the relationship between isotope ratios and habitat use. Ongoing research on sevengill and white shark movement patterns in False Bay, together with the results presented in this study, will provide important information on the trophic and ecological role that two top predatory sharks play in False Bay. Keywords: Notorynchus cepedianus, sevengill shark, stable isotopes, trophic ecology DA - 2017 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2017 T1 - The trophic dynamics of the broadnose sevengill shark (Notorynchus cepedianus) in False Bay, South Africa, using multiple tissue stable isotope analysis TI - The trophic dynamics of the broadnose sevengill shark (Notorynchus cepedianus) in False Bay, South Africa, using multiple tissue stable isotope analysis UR - http://hdl.handle.net/11427/25025 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/25025
dc.identifier.vancouvercitationDe Necker L. The trophic dynamics of the broadnose sevengill shark (Notorynchus cepedianus) in False Bay, South Africa, using multiple tissue stable isotope analysis. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2017 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/25025en_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.otherBiological Sciencesen_ZA
dc.titleThe trophic dynamics of the broadnose sevengill shark (Notorynchus cepedianus) in False Bay, South Africa, using multiple tissue stable isotope analysisen_ZA
dc.typeMaster Thesis
dc.type.qualificationlevelMasters
dc.type.qualificationnameMScen_ZA
uct.type.filetypeText
uct.type.filetypeImage
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
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