Effects, benefits and costs of MPAs (as a fisheries management tool)
The effect of MPAs on fish populations, ecosystems and people depends on where they are located, how many there are, how big they are, the nature of protection within the MPA (e.g. is all fishing prohibited or only fishing with some gears?), effectiveness of management and enforcement, and the movement of the fish species (of all life stages) across MPA boundaries. It is also important to take into account activities that are occurring outside of the MPA itself.
Sustaining specific fish populations
With regard to sustaining specific fish populations, the MPA has to be large enough so that some of the eggs and larvae they produce are retained within it. In theory, MPAs can be designed as a connected network to sustain fish populations, but there is little evidence so far to support the theory. For mobile fish species, the amount of area that will need to be contained in MPAs to sustain a population will be large if fishing is intense outside of MPAs.
What happens to fish populations and habitats within MPAs?
When designed appropriately, there is substantial scientific evidence showing that there are more fish and bigger fish, with a higher biomass, inside MPAs than outside. MPAs can protect habitats within their boundaries and there is evidence that disturbed habitat can recover. MPAs can reduce bycatch, discards and harmful effects on non-target species. There is some evidence that biodiversity is higher within MPAs than outside, but it is ambiguous.
Some of the fish inside MPAs will probably migrate across MPA boundaries, and particularly once biomass and density increase, and can then be caught by fishers. This so called spillover effect is a potential benefit generated by MPAs, but the fishing mortality rate outside the MPA still needs to be regulated to ensure sustainability. Fish eggs and larvae may also disperse across MPA boundaries, which may improve recruitment, but there is little direct evidence of this occurring.
MPAs serve as a resource reallocative mechanism, within and among groups of resource users, and at varying spatial and temporal scales. As such, MPAs can either help or hurt local people and communities around them depending on the local context and how they are designed and implemented. An important distributional issue with MPAs is that the benefits tend to be diffuse while costs are concentrated. A potential cost to the fisher is that catch, and revenues, may be decreased, at least in the short-term, as a result of the implementation of a closure. The coastal community adjacent to the MPA, especially those with a high economic dependency upon the fishery, may face a disproportionate impact, particularly in the short-term, as a result of aggregate reduction in fishing revenue. The potential benefits in the form of reduced variations in aggregate catch levels, increased total catches or more valuable larger size fish catches thanks to spillover effects from increased fish populations inside the MPA might only be generated in the longer-term.