A Simulation Model for Evaluating Seasonal Closures in Australia's Multispecies Northern Prawn Fishery

Abstract

In Australia's northern prawn fishery, seasonal closures have long been used to manage the size composition of the commercial catch. Although primarily aimed at short-term economic benefits of improved yield per recruit, closures were also thought to reduce the risk of recruitment overfishing. Until recent years, seasonal closures took the form of a single closed season in late summer to protect juvenile banana prawns Penaeus merguiensis during months of peak recruitment. However, with the growing commercial importance of tiger prawns P. esculentus and P. semisulcatus and associated concern about recruitment overfishing, a second midyear closure was introduced to further protect subadult tiger prawns. In this paper, we use a multispecies simulation model of the northern prawn fishery to assess the biological and economic effectiveness of seasonal closures in relation to yield, income, net operating income, and spawning stock indices per recruit. The model incorporates factors such as seasonal trends in export prices, seasonal changes in catchability, recent reductions in the fleet size, and the effect of the additional midyear closure. Our single most important finding was that little would be gained by any further finetuning of seasonal closures in this fishery. For example, any opening date for the season within a 2.5-month range (17 March to 2 June) would still produce net operating income per recruit within 5% of the maximum possible. Appropriate changes to the current closure regime (1993) could increase net operating revenue per recruit by 3.7%; however, these changes would also result in a 5.6% drop in tiger prawn spawning stock index per recruit.