Whilst there is no doubt about the effectiveness of closing an area for the purpose of conserving a marine ecosystem or rehabilitating sedentary resources within that area, there is some doubt about the value of closing an area for the purpose of sustaining an entire fishery. Although proponents point out that restricting fishing in a percentage of the available fishing grounds will maintain an irreducible minimum population of fishable organisms, whose offspring can contribute to replenishing the fishable stock via the spillover effect, critics maintain that this benefit is outweighed by several factors:
- the fishing effort pushed out of the closed area has to go somewhere. Usually it has to be accommodated within the surrounding fishery, which is often already stretched to the limit, and a general collapse may result, unless some sort of general effort reduction mechanism, buy-back scheme, or alternative livelihood is provided (and all of these mechanisms in turn have their drawbacks, if they work at all);
- whilst closed areas undoubtedly act as refugia (and in many cases seem to actually attract fish from surrounding areas, thus decreasing the sustainability of surrounding fisheries), the "spillover" of young, recently-spawned fish, from the refuge to surrounding areas is likely to be of variable extent for different species, and for some species with non-mobile larval stages may not occur to any significant level;
- Creating a few MPAs makes politicians feel happy and secure that they have done all that needs to be done for the marine environment, and that further money does not need to be spent on other, possibly more effective fishery maintenance measures;
- Enforcing the perimeter of a protected area can require considerable resources, and the consequences of even a limited amount of poaching can be relatively severe, wiping our years of conservation effort;
- A permanent MPA may require permanently taking a fishing ground away from a community, and substituting government regulatory mechanisms.
Critics thus suggest that if you are creating an MPA for purely fishery management purposes, the social resources spent on maintaining an MPA would be better spent on an appropriately tailored mixture of fishery management methods across the entire fishery.
The modern reality is that MPAs are set up for a variety of reasons anyway - for the conservation of pristine marine environments, to attract visitors to eco-tourism ventures (one of the possibly more succesful forms of alternative livelihoods to artisanal fishing), and for heritage reasons, and these MPAs all have consequences for fisheries, some of which may be negative and some positive. All of these effects need to be taken into account in fishery management.
However, recognising that there are both pros and cons, a middle view is reasonable. Whilst it would be unwise to invest all of society's fishery management resources in MPAs, it is prudent to maintain a proportion of no-fishing areas within fisheries for site-associated organisms as a form of natural "insurance" - as a buffer against the uncertainties of natural resource monitoring science, and the natural cycles to which the recruitment of marine organisms are particularly prone.