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A global analysis of apparent trends in abundance and recruitment of large tunas and billfishes inferred from Japanese longline catch and effort data

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Title: A global analysis of apparent trends in abundance and recruitment of large tunas and billfishes inferred from Japanese longline catch and effort data
Author: Ahrens, Robert Norman Matthew
Degree Doctor of Philosophy - PhD
Program Zoology
Copyright Date: 2010
Publicly Available in cIRcle 2010-03-10
Abstract: There has been substantial debate in recent years about the extent to which industrialized fishing has affected tunas and other large pelagic predator populations and altered the pelagic community. Variations in the type of data incorporated into assessments, statistical treatment of catch rate information, and different assessment methodologies have lead to diverging interpretations of stock levels and the sustainability of current large-scale industrialized fisheries. Simple nominal catch rates derived from Japanese longline catch and effort data paint a biased picture of the impact of industrialized fishing on the large pelagic tuna and billfish community, suggesting that abundance as of 2002 was only 10% of pre-1950 levels. Methods that correct for the spatial expansion, shift in distribution, and change in species targeting of the Japanese fleet, by averaging catch rates over spatial areas while imputing missing catch rate values, indicate a less severe decline with tuna and billfish stock reduced to an average of 50% of pre 1950 levels. For the majority of stocks, simple assessment methods indicate these relative abundance trends may still be biased and additional information sources are necessary to constrain assessments and evaluate the status of the stocks. With the incorporation of prior information on current fishing mortality rates and in some instances stock productivity, assessments indicate that a number of stocks are over-fished and experiencing over-fishing. Optimization models based on the same catch and effort data, aimed a redistributing fishing effort to maximize profits subject to fishing mortality constraints, suggest economic efficiencies can be gained in the long term if effort reductions are coupled with closed areas. Areas open to fishing should be placed where potential value and recruitments into the fishery are high. Fisheries are complex adaptive systems and it is not necessarily apparent how data resulting from fishing activities relate to the states of the assemblage of species captured. Careful consideration must be given to the nature of the sampling processes that give rise to these data. Without such consideration or alternative sources of information, inferences about impacts of fisheries on natural systems can be severely distorted.
URI: http://hdl.handle.net/2429/21742
Scholarly Level: Graduate

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