When you’re close to hitting bottom, there’s a whole lot more room for success. This appears true with regard to a long-held belief about the benefits of marine reserves, protected areas where fishing of many species is illegal: the commonly cited idea that the reserves provide spillover benefit to neighboring fisheries may only be true when that fishery is poorly managed. In other words, if you’ve already screwed up the situation pretty badly, the well-managed situation next door will help you out. If the fishery is well-managed, though, the benefit is likely minimal.
A number of previous studies have banged the Marine Protected Area (MPA) spillover drum: this one, this one, and this one, for example. The general idea is that those protected fish in one area will wander over the invisible lines into the non-protected areas, helping to replenish fishery stocks. The new study used modeling of a theoretical fishery and neighboring reserve to see in what circumstances the reserve provides any benefit to the fishery.
The modeling shows that it is really only when a fishery is being badly mismanaged that there is any benefit from the MPA. Consider two fisheries, one where the fishing “effort” is perfectly matched to produce the maximum sustainable yield, and one where the “effort” is allowed to badly surpass that line and is thus close to extinction. The well managed example will get no benefit at all from the MPA, since the system is already fished sustainably and to equilibrium. In fact, the authors write that “reserves will generally negatively impact yield for well managed fisheries.”
For poorly managed fisheries, where excess effort beyond that sustainable threshold is allowed, there is indeed some benefit. This makes sense intuitively, in that it seems more likely for an abundance of fish in an MPA to drift into an area less densely populated by that same fish. The authors do point out that even in well managed fisheries, a next-door MPA won’t guarantee a negative effect because there are often catch limits below the maximum sustainable yield as a sort of buffer.
So why have earlier studies claimed to find benefit from MPAs? In several cases in the Mediterranean, the total fishery had been “severely depleted,” which fits with the modeling results here. The same was true in other studies from Africa and Asia, “where the fisheries in question were over-exploited and where there was limited application and/or enforcement of standard fisheries management controls.”
The take-home here is that there isn’t any substitute for solid management of a fishery from the start. MPAs are great, but the ancillary benefits that fall outside the MPA’s boundaries aren’t going to manifest unless we’re really bad at everything else. For now, the authors say that means the messaging has to change: “We contend that it is misleading for governments to promote reserves on the basis of net spillover benefit in the context of well-managed fisheries.” - Dave Levitan | September 9 2014
Source: Buxton CD, Hartmann K, Kearney R, et al (2014). When is spillover from marine reserves likely to benefit fisheries? PLoS One, 9 (9) e107032. DOI: 10.1371/journal.pone.0107032