A recent paper in PLOS ONE (Public Library of Science) follows the production of several commercially important fish species in a Marine Protected Area (MPA) in the Mediterranean. I admit, I am not well versed in MPAs, but this paper offers some interesting results regarding fishery recovery within MPAs.
First, recovery can take a while. In the studied MPA the authors show that several species required two decades to reach top densities. One of the species that took this long to recover included a species of grouper related to the Nassua grouper. Other, smaller species took shorter durations to reach maximum abundance in the MPAs, but in all cases at least 10 years was required.
Second, the export of fish to surrounding fisheries was not detected until the populations within the MPA were near maximum abundance. This is not terribly surprising given that the reason fishes might leave a habitat is partially due to the density of fishes within the MPA. Therefore, only by protecting the MPA from harvest will spillover to surrounding harvest areas be detectible.
Third, not all species increased or reached maximum abundance after 25 years. Fisheries are complex. The different species that comprise a community interact and respond differently to harvest pressure and environmental variation. So non-synchronous responses of individual species are hardly surprising. However, what is remarkable here is that the communities have not stabilized even after such a long time of protection. One species is still declining (Sparus aurata – Gilt-headed Bream)and another is still increasing (Dentex dentex – European Sea Bass). While this could be due to predator prey dynamics inside the MPA (D. dentex is a predator and S. aurata could be prey), the authors hypothesize a different reason for declining S aurata abundance. Rather they cite – ‘density independent biomass export‘ due to intensive harvests at spawning aggregations outside the MPA. What this means is that when S. aurata form spawning aggregations outside the reserve they are targeted for harvests which causes a continuous decline inside the reserve.
Fourth, the increased densities of some fishes inside the reserve (Dusky Grouper) are likely due to recruitment to the MPA from outside the reserve. Basically, the production of fishes inside the reserve is dependent on spawning populations elsewhere. In this respect, the MPA only works because fishes elsewhere are spawning and the larval fish settle and survive better in the MPA…producing higher abundances of fish.
These last two points are important because they underscore one of the main issues with MPAs. Basically, fish have diverse life histories. Therefore, protecting fish may require a scale of protection that is difficult to achieve. Protecting spawning aggregations is one component of fisheries management that seems rather obvious, but may not be covered under traditional MPA design because even if fish were produced within a MPA, intensive harvests at spawning areas can decimate populations rapidly.
Fisheries management requires management that covers a diversity of species. That means a diversity of life histories. Therefore MPAs need to integrate the various needs of the focal species…not an easy task. MPAs may also take decades to reach their maximum production…a point at which this paper suggests harvests in surrounding areas begin to increase. Given the long duration to response, and the diversity of responses to protection shown in this paper (not all studies show the same responses) it is not surprising that there is a lot of questions about how well MPAs work, how they should be designed, and how to judge success. More research is obviously needed, and long term studies such as this provide some context for which to understand future and past research on the subject.
Go ahead and give the paper a read….it’s not too technical and there is a bit more detail and context that i’ve glossed over. The paper is an free online and ready for download here.
Antoni García-Rubies, Bernat Hereu, Mikel Zabala. Long-Term Recovery Patterns and Limited Spillover of Large Predatory Fish in a Mediterranean MPA. PLOS ONE, 2013; 8 (9): e73922 DOI: 10.1371/journal.pone.0073922