Calculating Mortality Rates and Optimum Yields from Length Samples

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Abstract
An equation is derived for yield per recruit of a fishery (or other exploited animal population) as a function of fishing intensity and age of first capture. The equation has the advantage that it does not require explicit estimates of natural mortality or individual growth rate parameters. Linear length growth is assumed until maximum size is reached, and mortality parameters are expressed relative to growth rate. Mortality parameters are estimated from average length samples of separate populations experiencing different fishing efforts in the same fishery. The equation may be used to compare existing fishing efforts and age of first capture with optimal values. Samples of the catfish Bagrus docmac from Lake Victoria (East Africa) are used to illustrate the method. Key words: yield equation, Beverton–Holt, fish lengths, Lake Victoria, Bagrus docmac, fishing effort, recruitment age

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