A Tag–Release/Recovery Method for Predicting the Effect of Changing the Catch of One Component of a Fishery Upon the Remaining Components

Abstract

We describe (and illustrate, using data on southern bluefin tuna, Thunnus maccoyii) a tag–release/recovery experiment designed to predict the effect of changing the catch of one component of a fishery upon the remaining components for which fishing patterns and intensities are fixed. Formulae are developed for estimating the predicted changes and their standard errors. We also demonstrate how to determine the number of fish that should be tagged to achieve a desired accuracy of results from such an experiment. The proposed approach requires neither comprehensive knowledge of mechanisms governing the fish population and fishery being considered nor extensive historical catch and fishing effort data, but it will reflect all aspects of the dynamics of the system if the tag–release/recovery experiment is properly implemented. Consequently, it can be applied to many complex fisheries systems for which this knowledge is lacking. However, the proposed approach does not enable predictions to be made when the change in the catch significantly alters the number of recruits to the fishable stock, changes the fish behaviour, or changes the rates of natural mortality, migration, or growth.