Marine protected areas for spatially structured exploited stocks

Many harvested marine and terrestrial populations have segments of their ra nge protected in areas free from exploitation. Reasons for areas being prot ected from harvesting include conservation, tourism, research, protection o f breeding grounds, stock recovery, harvest regulation, or habitat that is uneconomical to exploit. In this paper we consider the problem of optimally exploiting a single species local population that is connected by dispersi ng larvae to an unharvested local population. We define a spatially-explici t population dynamics model and apply dynamic optimization techniques to de termine policies for harvesting the exploited patch. We then consider how r eservation affects yield and spawning stock abundance when compared to poli cies that have not recognised the spatial structure of the metapopulation. Comparisons of harvest strategies between an exploited metapopulation with and without a harvest refuge are also made. Results show that in a 2 local population metapopulation with unidirectional larval transfer, the optimal exploitation of the harvested population should be conducted as if it were independent of the reserved population. Numerical examples suggest that rel ative source populations should be exploited if the objective is to maximis e spawning stock abundance within a harvested metapopulation that includes a protected local population. However, this strategy can markedly reduce yi eld over a sink harvested reserve system and may require strict regulation for conservation goals to be realised. If exchange rates are high, results indicate that spawning stock abundance can be less in a reserve system than in a fully exploited metapopulation. In order to maximise economic gain in the reserve system, results indicate that relative sink populations should be harvested. Depending on transfer levels, loss in harvest through reserv ation can be minimal, and is likely to be compensated by the potential envi ronmental and economic benefits of the reserve.