An exploratory method for the assessment of multispecies fisheries

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Abstract
Different species of fish interact by competing for common food resources, and by eating one another. The construction of simplified mathematical models of such interactive systems is quite straightforward. The principal problems are not mathematical or computational, but ignorance of the true nature and magnitude of the interactions in real life, and inability to digest and comprehend the results of the calculations which rapidly become rather voluminous. A simple steady-state model of a multispecies system of fisheries has been developed. It consists of a set of conventional single-species age-structured yield models, linked by mixed fishery and predator-prey interactions. The predatory (natural) mortality exerted by each predator is for simplicity assumed to be proportional to its biomass. Size and species preferences of predators are taken into account, but competition, and the consequences for the predators, are not. The model has deliberately been kept as close as possible to standard assessment models, and requires only a succinct representation of the predatory interactions to be supplied. It is designed to permit the consequences of a wide range of fishing strategies to be explored. A simple illustrative example involving two species and two fisheries is presented. The results confirm that the choice of stock-recruitment relationship remains very important even when predatory interactions are included, and suggest that the results of such modelling may not be very sensitive to all the details of the predatory interaction. It is suggested that this model, and those of a similar degree of elaboration, may be useful tools for exploratory studies of the management of multispecies systems.

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