Optimal mesh size for tropical multispecies trawl fisheries

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Abstract
Existing methods of determining the best mesh size for tropical multispecies trawl fisheries are highly simplistic and do not reliably estimate the mesh size providing maximum yield in weight or value. A method of estimating the mesh size providing greatest yield by weight or value is presented, based on the Beverton and Holt (1957) single-species model. Yield-per-recruit analysis is inapplicable in a multispecies fishery, and calculation of optimal mesh size requires estimates of relative recruitment to the populations comprising the fishery. Methods of estimating absolute or relative recruitment from commercial catch and effort time series data, average commercial catch data, and research vessel survey data are given. The methods of estimating optimum mesh size and recruitment are applied to data from the northwest shelf of Australia. The estimates of relative recruitment provided by all methods were consistent. The (stretched) mesh size optimizing total catch for the NW shelf fishery was stable over a wide range of input parameter values, including fishing mortality and recruitment. In principle, total yield can be estimated if estimates of absolute recruitment are available, but sensitivity analysis indicates the precision of yield estimated to be very low.

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