Robust estimation of the natural mortality rate in a completed tagging experiment with variable fishing intensity

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Abstract
A method is described for estimating the instantaneous natural mortality rate, assumed constant, in a fish population, using data from a completed tag-recapture experiment, that is, one in which the population of tagged fish is subject to fishing until no live tagged fish remain. Data from different experiments may be pooled in order to make a more accurate estimate of the natural mortality rate. Jackknife methods, utilizing an efficient computational algorithm, are used to reduce the bias of the estimate of the natural mortality rate and to provide a standard error for the estimate. The method is shown, by simulation studies, to be more robust to departures from constancy in fishing intensity than that of Gulland (1955; Biometrika, 42: 269–270), which assumes such constancy. A modification of the method which requires an additional assumption about the fishing intensity is also developed so that a preliminary estimate of the natural mortality rate can be obtained before the completion of an experiment. Southern bluefin tuna ( Thunnus maccoyii ) tag-recapture data are analysed to illustrate the use of the method in practice.

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