Are closed fishing seasons an effective means of increasing reproductive output?. A per-recruit simulation using the limpet Cymbula granatina as a case history

Closed fishing seasons are sometimes imposed during the breeding period of harvested species in the belief that this will achieve greater annual reproductive output, but no studies have modelled the validity of this. We specifically tested this by simulating reproductive output and yield when closed seasons are imposed at different times of the year, using the limpet Cymbula granatina as a base case for a per-recruit model because (a) the reproduction of individuals that are not harvested is not disturbed by the act of harvesting and (b) the individuals do not aggregate to reproduce, an act that would make them more prone to capture. Model outputs showed that implementation of a closed fishing season during the breeding period does not increase the reproductive output of the population and has only marginal effects on yield if total annual fishing effort remains unchanged. This outcome is, however, dependent on the particular characteristics of the species, particularly whether the species is prone to disturbance or aggregates to breed. When the model was tuned to simulate species that are disturbed by fishing or do become vulnerable to fishing because they aggregate to breed, the reproductive output and yield were substantially increased by the imposition of a closed season during the breeding period. Thus, closure of fishing during the breeding period is effective only for species that conform to either of these conditions, or as a means of reducing annual fishing mortality. The frequent application of closed seasons to species that are not disturbed by harvesting and do not aggregate to breed is therefore generally invalid, despite wide belief that it will improve reproductive success.