AN AGE-STRUCTURED OPEN-ACCESS FISHERY MODEL

A dynamic model for open-access fisheries is presented. In addition to density dependence in recruitment and fishing effort changing in prop ortion to the level of profit fishermen earn which characterizes previ ous open-access models, it incorporates full age structure for the fis h stock, lognormal environmental recruitment variability, and gear sel ectivity. The predator-prey cycling solution of the original Schaefer dynamic model, and subsequent open-access models, persists for these m odel extensions. Density dependence in recruitment induces greater glo bal stability. Environmental recruitment variability, common in marine populations, is destabilizing in the neighborhood of the open-access equilibrium. These two influences, combined in the open-access fishery model, generate robust long-lasting irregular cycles of stock and eff ort. Volterra proved for the original Lotka-Volterra model that the ti me averages of the variables over one cycle were exactly equal to thei r equilibrium steady states. This is shown to extend as a good approxi mation for the model presented here. Approximating model steady states of effort and catch by the corresponding averages from data time seri es underlies a new algorithm of parameter evaluation, applied here to an open-access model of the Georges Bank sea scallop (Placopecten mage llanicus) fishery.