In the process of implementing marine reserves, policy makers typically are occupied with (1) choosing the spatial configuration of areas to protect, and (2) addressing the concerns of fishermen regarding the effects of proposed reserves on fishery yield. The spatial configuration is typically set by choosing the habitat, species, and ecosystems to protect, assuming that the associated species will be sustained in that configuration. The concerns of fishermen are typically addressed by describing various spillover mechanisms and suggesting that yields will increase. There is a growing scientific understanding of the effects of reserves on the sustainability of populations and fishery yield, and the practical implications of those results should be incorporated into policy decisions. While there are exceptions, analytical and simulation results from models with sedentary adults indicate that yield will increase only if a population has been fished hard enough to cause a substantial decline in recruitment. This is consistent with the rough equivalence between yields possible with marine reserves and conventional management. This equivalence is a useful benchmark in the absence of information on larval advection and spatial variability in productivity. With reserves, lower fishery yields will be obtained from species dispersing shorter distances. Both yield and preservation goals depend on species in reserves being sustainable. Sustainability of species in reserves will depend on their dispersal distances and the spatial configuration of reserves. Species will be sustained in marine reserves if the alongshore dimension of the reserve is greater than its mean dispersal distance (assuming little alongshore displacement of the dispersal pattern), butspecies dispersing all distances will be sustained in networks of reserves, if a specific fraction of the coast is covered. Yield will be greater as the size of individual reserves in that network becomes smaller. Shorter-distance dispersers are always more likely to persist. Sustainability and yield in marine reserves depend on three categories of uncertainty: (1) uncertainty in population response to management is less when employing marine reserves than in conventional management, (2) uncertainty inthe slope of the stock-recruitment relationship at low abundance affects both reserves and conventional fishery management, and (3) uncertainty in the pattern of larval dispersal affects management by reserves much more than conventional fishery management. Most of the available results are modeling results, and there is a need for better empirical information on both sustainability and yield. We need to know more than just whether marine reserves sustain populations and increase yield; rather, we needto know which kinds of reserves (i.e., size and spacing) sustained populations and which kinds of species (i.e., dispersal distance) were sustained and showed increased yields.
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