Assessing the effects of internal (trophic structure) and external (fishing and environment) forcing factors on fisheries off central Chile : basis for an ecosystem approach to management

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dc.contributor.advisorMoloney, Coleenen_ZA
dc.contributor.advisorCury, Philippeen_ZA
dc.contributor.advisorShannon, Lynneen_ZA
dc.contributor.advisorJarre, Astriden_ZA
dc.contributor.advisorChristensen, Villyen_ZA
dc.contributor.authorAlarcón, Sergio Eduardo Neiraen_ZA
dc.date.accessioned2014-08-13T14:13:53Z
dc.date.available2014-08-13T14:13:53Z
dc.date.issued2008en_ZA
dc.descriptionIncludes abstract.en_ZA
dc.descriptionIncludes bibliographical references (p. 227-253).en_ZA
dc.description.abstractHuman perception of sea fisheries has evolved from an inexhaustible resource paradigm towards a generalized concern on the degraded state of fish stocks and ecosystems. Accordingly, fisheries science and management are expanding from the traditional single-species approach towards an ecosystem approach to fisheries. Marine communities are organized as webs of interactions that are affected by external natural (climate) and anthropogenic (fishing) forcing, with their relative effects poorly known, but hypothesised to strongly depend on internal food web structure (i.e., who eats and controls whom). This thesis approaches relevant ecological considerations for an ecosystem approach to fisheries in the upwelling ecosystem off central Chile (33ºS-39ºS). The main objective is to assess the effects of internal (trophic structure) and external (fishing and environment) forcing factors at the fish stock and food web level in the study area. The methodology includes i) the construction of snapshot and dynamic food web models to test hypothesis of changes in the food web in the last century, and the relative contribution of fishing, trophic controls and bottom-up environmental variability to those changes, ii) the computation and analysis of a set of ecosystem indicators to test hypotheses of changes in different aspects of the exploited community (mean trophic level, age and length at maturity, network properties and system variability), iii) the analyses of the relationships between time series of abundance of species with known trophic interactions (Chilean hake-red squat lobster and Chilean hake-small pelagic fish) to test hypothesis of top-down and bottom-up control versus alternative hypotheses of fishing and/or environmental control in the same populations, and iv) simulation experiments to test hypotheses of ecosystem change and recovery under fishing and environmental forcing. Models and indicators are constructed using data series of abundance, catches, production, consumption and diets of the main functional groups in the study area. Snapshot and dynamic food web models are constructed and analyzed using the Ecopath with Ecosim software version 5.1 and routines therein. The observed trends in indicators and model results are in accordance with what is theoretically expected in stressed ecosystems (shift towards a food web dominated by short-lived, low trophic level and high turnover rate species), and suggest that the food web could be in a state that is more susceptible to external forcing. Fishing and the environment (bottom-up anomaly in PP) may have affected the upwelling ecosystem off central Chile both at the stock and at the food web level between 1970 and 2004. The effects of these forcing factors may have been mediated by trophic controls operating in the food web. There is also evidence to support the hypothesis that trophic controls beyond fishing, e.g., trophic (internal) and environment (external) may operate in the analysed populations and this information should be considered in their assessment and management. While target objectives are set and agreed, it is proposed that the main objective for the ecosystem approach to fisheries should be to avoid fishing-induced regime shifts, since results from simulation experiments suggest that fishing can induce ecosystem changes of lower recovery than bottom-up forcing.en_ZA
dc.identifier.apacitationAlarcón, S. E. N. (2008). <i>Assessing the effects of internal (trophic structure) and external (fishing and environment) forcing factors on fisheries off central Chile : basis for an ecosystem approach to management</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Biological Sciences. Retrieved from http://hdl.handle.net/11427/6217en_ZA
dc.identifier.chicagocitationAlarcón, Sergio Eduardo Neira. <i>"Assessing the effects of internal (trophic structure) and external (fishing and environment) forcing factors on fisheries off central Chile : basis for an ecosystem approach to management."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2008. http://hdl.handle.net/11427/6217en_ZA
dc.identifier.citationAlarcón, S. 2008. Assessing the effects of internal (trophic structure) and external (fishing and environment) forcing factors on fisheries off central Chile : basis for an ecosystem approach to management. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Alarcón, Sergio Eduardo Neira AB - Human perception of sea fisheries has evolved from an inexhaustible resource paradigm towards a generalized concern on the degraded state of fish stocks and ecosystems. Accordingly, fisheries science and management are expanding from the traditional single-species approach towards an ecosystem approach to fisheries. Marine communities are organized as webs of interactions that are affected by external natural (climate) and anthropogenic (fishing) forcing, with their relative effects poorly known, but hypothesised to strongly depend on internal food web structure (i.e., who eats and controls whom). This thesis approaches relevant ecological considerations for an ecosystem approach to fisheries in the upwelling ecosystem off central Chile (33ºS-39ºS). The main objective is to assess the effects of internal (trophic structure) and external (fishing and environment) forcing factors at the fish stock and food web level in the study area. The methodology includes i) the construction of snapshot and dynamic food web models to test hypothesis of changes in the food web in the last century, and the relative contribution of fishing, trophic controls and bottom-up environmental variability to those changes, ii) the computation and analysis of a set of ecosystem indicators to test hypotheses of changes in different aspects of the exploited community (mean trophic level, age and length at maturity, network properties and system variability), iii) the analyses of the relationships between time series of abundance of species with known trophic interactions (Chilean hake-red squat lobster and Chilean hake-small pelagic fish) to test hypothesis of top-down and bottom-up control versus alternative hypotheses of fishing and/or environmental control in the same populations, and iv) simulation experiments to test hypotheses of ecosystem change and recovery under fishing and environmental forcing. Models and indicators are constructed using data series of abundance, catches, production, consumption and diets of the main functional groups in the study area. Snapshot and dynamic food web models are constructed and analyzed using the Ecopath with Ecosim software version 5.1 and routines therein. The observed trends in indicators and model results are in accordance with what is theoretically expected in stressed ecosystems (shift towards a food web dominated by short-lived, low trophic level and high turnover rate species), and suggest that the food web could be in a state that is more susceptible to external forcing. Fishing and the environment (bottom-up anomaly in PP) may have affected the upwelling ecosystem off central Chile both at the stock and at the food web level between 1970 and 2004. The effects of these forcing factors may have been mediated by trophic controls operating in the food web. There is also evidence to support the hypothesis that trophic controls beyond fishing, e.g., trophic (internal) and environment (external) may operate in the analysed populations and this information should be considered in their assessment and management. While target objectives are set and agreed, it is proposed that the main objective for the ecosystem approach to fisheries should be to avoid fishing-induced regime shifts, since results from simulation experiments suggest that fishing can induce ecosystem changes of lower recovery than bottom-up forcing. DA - 2008 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2008 T1 - Assessing the effects of internal (trophic structure) and external (fishing and environment) forcing factors on fisheries off central Chile : basis for an ecosystem approach to management TI - Assessing the effects of internal (trophic structure) and external (fishing and environment) forcing factors on fisheries off central Chile : basis for an ecosystem approach to management UR - http://hdl.handle.net/11427/6217 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/6217
dc.identifier.vancouvercitationAlarcón SEN. Assessing the effects of internal (trophic structure) and external (fishing and environment) forcing factors on fisheries off central Chile : basis for an ecosystem approach to management. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2008 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/6217en_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.otherZoologyen_ZA
dc.titleAssessing the effects of internal (trophic structure) and external (fishing and environment) forcing factors on fisheries off central Chile : basis for an ecosystem approach to managementen_ZA
dc.typeDoctoral Thesis
dc.type.qualificationlevelDoctoral
dc.type.qualificationnamePhDen_ZA
uct.type.filetypeText
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
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