Observer effects on pelagic fleet operations and Bayesian-based explorations of the mass balance model ECOPATH

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dc.contributor.advisorButterworth, Doug S
dc.contributor.advisorPlagányi, Éva E
dc.contributor.authorSomhlaba, Sobahle
dc.date.accessioned2014-07-31T08:10:58Z
dc.date.available2014-07-31T08:10:58Z
dc.date.issued2006en_ZA
dc.descriptionIncludes bibliographical references (leaves 105-109).
dc.description.abstractThis thesis comprises two sections. The first deals with the analysis of observer data from pelagic commercial vessels with a particular focus being to examine whether there is any indication of dumping (discarding) that is provided by these data. The second section investigates the ability of constraints associated with the ECOPATH mass-balance equations to improve knowledge on estimates of parameters in the southern Benguela ecosystem. Sardine and anchovy are the most valued and carefully managed species of small pelagic fish in South Africa. The problem of dumping in this pelagic fishery has been recognised for some time. The reasons for dumping are thought to arise, inter alia, from large bycatches of sardine (counting against a bycatch allowance) with anchovy and unsuitable sizes of sardine caught for canning purposes. As there were no data that could be used as a basis to investigate the claims, little was done to substantiate them. The Observer Programme to collect data from commercial vessels was instituted in 1999. These data are analyzed to establish whether there are any appreciable differences in catch rates between vessels with observers and those without. General Linear Models (GLMs) are used to adjust for the effects of other factors that could confound comparisons because of the non-random nature of observer placement. Central to the analyses of these data are different catch rate measures. The first measure used is catch per time at sea, which is referred to as CPUETS: the catch of the species concerned made by a vessel on a trip divided by the total number of hours it takes for the vessel to steam from port to the fishing ground, fish and then return to the port. The second measure is catch per haul, which is referred to as CPUEH. This measure divides catch by the total number of hauls made by the vessel during that trip. The CPUETS was the preferred measure due to concerns related to the CPUEH measure (essentially greater reliability of the data and less potential for bias being introduced through other effects which influence haul size). Catch rates were modelled with errors assumed to follow either a log normal or a Poisson distribution. The Poisson error model was the preferred model based on features of the standardized residuals.en_ZA
dc.identifier.apacitationSomhlaba, S. (2006). <i>Observer effects on pelagic fleet operations and Bayesian-based explorations of the mass balance model ECOPATH</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Mathematics and Applied Mathematics. Retrieved from http://hdl.handle.net/11427/4932en_ZA
dc.identifier.chicagocitationSomhlaba, Sobahle. <i>"Observer effects on pelagic fleet operations and Bayesian-based explorations of the mass balance model ECOPATH."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Mathematics and Applied Mathematics, 2006. http://hdl.handle.net/11427/4932en_ZA
dc.identifier.citationSomhlaba, S. 2006. Observer effects on pelagic fleet operations and Bayesian-based explorations of the mass balance model ECOPATH. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Somhlaba, Sobahle AB - This thesis comprises two sections. The first deals with the analysis of observer data from pelagic commercial vessels with a particular focus being to examine whether there is any indication of dumping (discarding) that is provided by these data. The second section investigates the ability of constraints associated with the ECOPATH mass-balance equations to improve knowledge on estimates of parameters in the southern Benguela ecosystem. Sardine and anchovy are the most valued and carefully managed species of small pelagic fish in South Africa. The problem of dumping in this pelagic fishery has been recognised for some time. The reasons for dumping are thought to arise, inter alia, from large bycatches of sardine (counting against a bycatch allowance) with anchovy and unsuitable sizes of sardine caught for canning purposes. As there were no data that could be used as a basis to investigate the claims, little was done to substantiate them. The Observer Programme to collect data from commercial vessels was instituted in 1999. These data are analyzed to establish whether there are any appreciable differences in catch rates between vessels with observers and those without. General Linear Models (GLMs) are used to adjust for the effects of other factors that could confound comparisons because of the non-random nature of observer placement. Central to the analyses of these data are different catch rate measures. The first measure used is catch per time at sea, which is referred to as CPUETS: the catch of the species concerned made by a vessel on a trip divided by the total number of hours it takes for the vessel to steam from port to the fishing ground, fish and then return to the port. The second measure is catch per haul, which is referred to as CPUEH. This measure divides catch by the total number of hauls made by the vessel during that trip. The CPUETS was the preferred measure due to concerns related to the CPUEH measure (essentially greater reliability of the data and less potential for bias being introduced through other effects which influence haul size). Catch rates were modelled with errors assumed to follow either a log normal or a Poisson distribution. The Poisson error model was the preferred model based on features of the standardized residuals. DA - 2006 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2006 T1 - Observer effects on pelagic fleet operations and Bayesian-based explorations of the mass balance model ECOPATH TI - Observer effects on pelagic fleet operations and Bayesian-based explorations of the mass balance model ECOPATH UR - http://hdl.handle.net/11427/4932 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/4932
dc.identifier.vancouvercitationSomhlaba S. Observer effects on pelagic fleet operations and Bayesian-based explorations of the mass balance model ECOPATH. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Mathematics and Applied Mathematics, 2006 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/4932en_ZA
dc.language.isoengen_ZA
dc.publisher.departmentDepartment of Mathematics and Applied Mathematicsen_ZA
dc.publisher.facultyFaculty of Scienceen_ZA
dc.publisher.institutionUniversity of Cape Town
dc.subject.otherMathematics and Applied Mathematicsen_ZA
dc.titleObserver effects on pelagic fleet operations and Bayesian-based explorations of the mass balance model ECOPATHen_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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