Mixed-Species Yield-per-Recruitment Analyses Accounting for Technological Interactions

Multipecies fishery assessment models may be divided into two general categories: those involving analyses of the trophic interdependence of species (biological interactions) and those addressing the question of co-occurrence in fishing gear (technological interactions). Certainly, both categories of interactions may occur simultaneously. Nevertheless, the latter class of models is probably adequate if major species of interest exhibit a high degree of mutually exclusive food resource partitioning, and interspecific competition for food is low. In this study, multispecies analogs of the Thompson–Bell yield-per-recruit model are developed for alternative situations in which species groups (assemblages) are exploited solely by one fishery, and when several fisheries (defined by gear type, seasonal changes in species mix, etc), exploit the same species concurrently and/or sequentially. Species included in the analyses are assumed to have negligible trophic dependence. Equilibrium fishery yields (in aggregate and for individual species) are computed as functions of standardized effort levels and retention characteristics of gear (e.g. mesh size). The multiple fishery model allows for variable harvest strategies with respect to gear selectivity and effort levels among component fisheries. Simultation output includes total and individual equilibrium species yields, exploitation rates, and mean fish weights in the catch for all fisheries combined, and each separately. Applications of the models are illustrated for mixed-species otter trawl fisheries exploiting primarily Atlantic cod, Gadus morhua, haddock, Melanogrammus aeglefinus, yellowtail flounder, Limanda ferruginea, and winter flounder, Pseudopleuronectes americanus, in the Georges Bank region off the northeastern United States. Results emphasize the potential for growth underfishing or overfishing of individual species/stocks when total system yield is the optimization criterion