Biological and ecological considerations in the design, implementation and success of MPA

BOTSFORD, L. W.; MICHELI, F.; PARMA, A.M.

FAO Fisheries Report

FAO

Lugar: Roma; Año: 2007 p. 109 - 148

0429-9337

1) While Marine Protected Areas (MPAs) have a long history, there has recently been increasing interest in implementing them as a tool for fishery management. This interest is driven in large measure by the observation that some 25 percent of world fisheries are over-exploited or recovering from over-exploitation. To aid decision makers in deciding whether implementing MPAs will improve their fisheries and marine ecosystems, and in selecting the best MPA designs, we review what is known about their ecological performance from empirical observations and modelling studies. 2) The answer to the question of whether species will benefit inside reserves is reasonably well known. Empirical evidence from 70 existing marine reserves indicates that 63 percent of them have higher densities inside them than outside, 90 percent have higher biomass, 80 percent have a larger mean size of individual fish, and 59 percent have greater taxonomic diversity. The higher biomasses are primarily in fished species, and the effect is stronger for species at higher trophic levels and for species with greater body size. 3) The answer to the question of how reserves will contribute to the fisheries outside reserve is less well understood. The species that attain higher biomass or abundance inside reserves could contribute to fisheries outside through larvae produced in reserves being transported out of them. However, there is little empirical evidence regarding the magnitude and extent of the contribution of larvae produced within reserves to recruitment outside reserves. 4) Fish whose juvenile and adult stages are highly mobile could also contribute to fisheries outside reserves by juvenile and adult movement to outside the MPAs. However, individuals spending a large amount of time outside reserves will be exposed to fishing outside the reserves, hence more mobile species will be less well protected by marine reserves. Although empirical comparisons among species with varying adult mobility show that less mobile species increase more in abundance inside reserves, in other cases such comparisons are confounded by the stronger influences of fishing intensity. 5) Empirical evidence indicates that catch and CPUE of mobile species can be higher near marine reserves than far away from them, but few studies have assessed the net gain in catch and CPUE due to marine reserves when the loss of fishing area to reserves is accounted for. These few studies yielded conflicting results on whether increased CPUE around MPAs compensates losses associated with closure of fishing grounds. 6) There are no empirical comparisons of increases in catch with reserves to the increases possible through a reduction in fishing effort. Modelling studies indicate that the yield possible through management by marine reserves is approximately equal to the yield possible through conventional management. This implies that reserve implementation will increase yield only for heavily fished species (empirical evidence in Bullet 2). Exceptions to this approximate equivalence include species with pre-dispersal density-dependence, species with ontogenetic migration (e.g. specific spawning areas) and species with a distinct source/sink structure. For species with pre-dispersal density dependence, models suggest that yield may be less with reserves than with conventional management, whereas the opposite is predicted for species with distinct source/sink structure, if MPAs can be designed to protect sources. 7) The question of how to choose the size and spacing of MPAs currently depends on modelling results because there is no empirical information available regarding the dependence of yield and sustainability on size and spacing of marine reserves. Modelling studies of the level of sustainability brought about by different size and spacing of reserves indicate that, when there is intense fishing: (1) single reserves will sustain species with larval dispersal distances less or equal to the linear dimension of the reserve, and (2) systems of many marine reserves will sustain species with larvae dispersing any distance, when the fraction of area covered in reserves is greater than the minimum fraction of lifetime egg production needed for replacement. The latter is a network effect. The reserve area required for sustainability is less when there is less fishingoutside reserves. 8) Management by MPAs depends on different uncertainties than conventional fishery management. The significant uncertainties relevant to the design of marine reserves are the limited knowledge of larval dispersal patterns, a poor understanding of the minimal individual replacement (i.e. minimum Lifetime Egg Production or Spawning Potential Ratio) required for population sustainability, and indirect effects of protection in reserves through species interactions. Conventional fishery management shares a critical dependence on the last two elements, and has, in addition, considerable implementation uncertainty. 9) The decision as to whether to employ marine reserves for fishery management may be approached differently depending on the amount of ecological and socio-economic data available. For datarich management environments, the ecological aspects of design and implementation of reserves can be approached through a modelling framework. Uncertainties can be identified and explored and performance of different reserve designs combined with other conventional management tactics can be compared under different scenarios that represent the existing uncertainty. Even in data-rich situations, major uncertainties remain (see bullets 3 and 8) and it is important that learning through monitoring is included in the implementation plans. 10) For data-poor management environments, rules of thumb from the modelling studies may be useful. The influences of management costs, practicality of implementation and enforcement, and uncertainties on the decision of whether to implement reserves or conventional quota or effort management differ from the data-rich case. Reserves, probably in combination with some form of effort limitation, tend to be more advisable than catch quotas when many species are taken by the same gear and when the resource has a persistent spatial structure due to low mobility of the individuals.