Errors of perception in stock and recruitment studies due to wrong choices of natural mortality rate in Virtual Population Analysis

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Abstract
Virtual Population Analysis (VPA) is commonly used to obtain estimates of fish stock sizes. The necessary input of the rate of natural mortality-at-age (M t ) is often chosen subjectively and seldom revised. Equations describing the errors introduced by the selection of the value of M t have been derived. My analysis demonstrates that if the rate of exploitation and fishing mortality remains stable over the period covered by the catch-at-age data, relative changes in fish stocks will be correctly identified by the VPA independently of the chosen values for M t . The choices of M t values will affect the perceived changes of a fish stock and the perceived stock-recruitment relationship when the fishing intensity changes significantly or the rate of natural mortality fluctuates. A reduction of the errors requires unbiased estimates of the mean M t values and basic information on their fluctuations

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