An assessment of the long-term changes in chondrichthyan abundance on the inshore trawl grounds of the Agulhas Bank, South Africa

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dc.contributor.advisorAttwood, Colinen_ZA
dc.contributor.authorMussgnug,Robert Sebastianen_ZA
dc.date.accessioned2015-09-15T10:32:56Z
dc.date.available2015-09-15T10:32:56Z
dc.date.issued2013en_ZA
dc.description.abstractTrawl fisheries have been operating in South African waters for roughly 110 years. In contrast to other trawl fisheries, the South African fishery was opened by government-funded scientific trawl surveys beginning in 1898. Detailed records of survey trawls undertaken immediately prior to and during the beginning of commercial trawling activity provided a rare opportunity to examine longterm changes in fish abundance. This dissertation focuses on the chondrichthyans, which are believed to be the group of fishes most at risk from intense exploitation. Despite some problems associated with changes in taxonomy and the efficiency of gear, this analysis was able to compare two distinct periods at three taxonomic levels: The periods were taken to represent baseline values prior to the opening of trawl fisheries and contemporary data, separated by roughly 80 years of intense, trawling activity. Three historically important trawl grounds were identified as having sufficient samples from each period. Between the surveys, trawl velocity did not deviate much from contemporary trawl velocities, although all surveys were found to have a significantly different velocity. In respect to the catch composition, significant changes were found in the relative abundance of the total catch composition, as a general shift from large, long-lived species towards smaller species was found. This applied both for teleosts as well as chondrichthyans, with the entire chondrichthyan catch composition showing a decrease, with the exception of Callorhinchus capensis, which increased significantly. Changes in chondrichthyan swept-area density were found to follow those trends found in the relative abundance: An increase was found in Callorhinchus capensis, whereas all other species decreased. All chondrichthyans were classed in low productivity categories and were assigned a high risk factor to overfishing. Reasons for the declines in the chondrichthyan assemblage were low fecundity, slow maturation and the inability due to these factors to adapt to an environment altered by trawling. Only in the case of Callorhinchus capensis were secondary effects of trawling such as the removal of competitor species likely to have caused the increase in relative abundance and swept area density as well as Callorhinchus capensis having a relatively high fecundity among chondrichthyans. In general, chondrichthyan decreases exceeded those of teleosts, and this work provides broad empirical support for the hypothesis that the low fecundity and slow growth of chondrichthyan species places this group at higher risk than teleosts.en_ZA
dc.identifier.apacitation (2013). <i>An assessment of the long-term changes in chondrichthyan abundance on the inshore trawl grounds of the Agulhas Bank, South Africa</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Biological Sciences. Retrieved from http://hdl.handle.net/11427/14017en_ZA
dc.identifier.chicagocitation. <i>"An assessment of the long-term changes in chondrichthyan abundance on the inshore trawl grounds of the Agulhas Bank, South Africa."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2013. http://hdl.handle.net/11427/14017en_ZA
dc.identifier.citation 2013. An assessment of the long-term changes in chondrichthyan abundance on the inshore trawl grounds of the Agulhas Bank, South Africa. Thesis. University of Cape Town ,Faculty of Science ,Department of Biological Sciences. http://hdl.handle.net/11427/14017en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Mussgnug,Robert Sebastian AB - Trawl fisheries have been operating in South African waters for roughly 110 years. In contrast to other trawl fisheries, the South African fishery was opened by government-funded scientific trawl surveys beginning in 1898. Detailed records of survey trawls undertaken immediately prior to and during the beginning of commercial trawling activity provided a rare opportunity to examine longterm changes in fish abundance. This dissertation focuses on the chondrichthyans, which are believed to be the group of fishes most at risk from intense exploitation. Despite some problems associated with changes in taxonomy and the efficiency of gear, this analysis was able to compare two distinct periods at three taxonomic levels: The periods were taken to represent baseline values prior to the opening of trawl fisheries and contemporary data, separated by roughly 80 years of intense, trawling activity. Three historically important trawl grounds were identified as having sufficient samples from each period. Between the surveys, trawl velocity did not deviate much from contemporary trawl velocities, although all surveys were found to have a significantly different velocity. In respect to the catch composition, significant changes were found in the relative abundance of the total catch composition, as a general shift from large, long-lived species towards smaller species was found. This applied both for teleosts as well as chondrichthyans, with the entire chondrichthyan catch composition showing a decrease, with the exception of Callorhinchus capensis, which increased significantly. Changes in chondrichthyan swept-area density were found to follow those trends found in the relative abundance: An increase was found in Callorhinchus capensis, whereas all other species decreased. All chondrichthyans were classed in low productivity categories and were assigned a high risk factor to overfishing. Reasons for the declines in the chondrichthyan assemblage were low fecundity, slow maturation and the inability due to these factors to adapt to an environment altered by trawling. Only in the case of Callorhinchus capensis were secondary effects of trawling such as the removal of competitor species likely to have caused the increase in relative abundance and swept area density as well as Callorhinchus capensis having a relatively high fecundity among chondrichthyans. In general, chondrichthyan decreases exceeded those of teleosts, and this work provides broad empirical support for the hypothesis that the low fecundity and slow growth of chondrichthyan species places this group at higher risk than teleosts. DA - 2013 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2013 T1 - An assessment of the long-term changes in chondrichthyan abundance on the inshore trawl grounds of the Agulhas Bank, South Africa TI - An assessment of the long-term changes in chondrichthyan abundance on the inshore trawl grounds of the Agulhas Bank, South Africa UR - http://hdl.handle.net/11427/14017 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/14017
dc.identifier.vancouvercitation. An assessment of the long-term changes in chondrichthyan abundance on the inshore trawl grounds of the Agulhas Bank, South Africa. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2013 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/14017en_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.titleAn assessment of the long-term changes in chondrichthyan abundance on the inshore trawl grounds of the Agulhas Bank, South Africaen_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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