Evaluating the potential impacts of the 'larval subsidy' effect for management of fish populations on the Great Barrier Reef, Australia

Closing areas of marine habitat to fishing ("no take" marine reserves) has been suggested as a strategy for enhancing harvested fish stocks possibly by providing a source of either emigrating adults (the spillover effect) or dispersive larvae (the larval subsidy effect). We have developed a spatially-structured simulation model of the population dynamics and harvest by line fishing of common coral trout (Plectropomus leopardis) on the Great Barrier Reef, in order to evaluate potential management strategies, including marine reserves. The simulation model incorporates harvest by multiple sectors (commercial, charter and recreational) and a spatially structured sub-model of the full life-history of common coral trout, including larval dispersal and hence the possibility of a "larval subsidy effect". In this paper, we use this tool, the Effects of Line Fishing Simulator (ELFSim), to evaluate the conditions, extent and potential effects on the fishery that a "larval subsidy effect" might impart. Simulations were performed for different levels of larval settlement on the same reef to which they were spawned, for a range of spatial closures. Results showed that under a constant effort scenario, as the amount of area closed to fishing increased, catch tended to decrease as the effort concentrated on a smaller proportion of the population. The spawning biomass increased however, as a larger proportion of the population was protected. The catch supported per unit of biomass was higher when there were area closures and when larvae were advected to reefs after being spawned. This occurred because the areas closed to fishing had biomass at near pre-exploitation levels, and thus subsidised the areas open to fishing with larval input. When larvae were advected among reefs, closing small portions of habitat to fishing also enhanced catches over what would have been obtained had no areas been closed to fishing.