Preventing overexploitation of migratory fish stocks: the efficacy of marine protected areas in a stochastic environment

Abstract

Abstract West, C. D., Dytham, C., Righton, D., and Pitchford, J.W. 2009. Preventing overexploitation of migratory fish stocks: the efficacy of marine protected areas in a stochastic environment. – ICES Journal of Marine Science, 66: 1919–1930. Marine protected areas (MPAs) have been widely proposed for protecting overexploited fish populations. It has been suggested that fisheries may be enhanced by spillover of individuals from MPAs into fishing grounds. However, traditional spillover studies fail to account for the seasonal migrations of many populations. Most fisheries models also fail to include the stochasticity inherent in marine environments explicitly. Here we assess MPA efficacy using a simple population model simulating the migration of fish populations between a spawning ground MPA and a fishery. Including realistic environmental stochasticity in our model allows the population to deviate from, and shift between, positive stable equilibria, something that is impossible in a deterministic analysis. This deviation may result in population collapse in cases where deterministic analysis predicts population persistence. We show that, although effective at low migration levels, the ability of MPAs to protect stocks from collapse generally decreases as migration increases. However, an MPA provides greater protection and greater expected fisheries yield than a system without an MPA, irrespective of migration level. Combining MPAs with a harvest control rule may further increase protection and yield. We therefore argue that MPAs can play a role in the protection of migratory species.