By: Dana Sackett

When marine protected areas (MPAs) are established as a strategy for fisheries management, spillover is one of the primary goals.  Conceptually, spillover is relatively simple: protect an area from fishing to allow fish populations to grow inside the MPA and eventually the benefits caused by the protected status (larger and more fish) spill over into the surrounding areas outside the MPA. Thus, over time, not fishing in the MPA benefits the fisheries outside the MPA.  Despite the seemingly intuitive simplicity of this idea, spillover has been a controversial topic in conservation ecology for several decades.

Much of this controversy stems from the difficulties many have in providing evidence for the occurrence of spillover.  To be fair, studies on MPAs have demonstrated both positive and negative consequences of protecting an area from fishing.  For example, studies have described how the size of the reserve, the target species mobility (how much and how far fish move), and whether adjacent areas were managed, strongly influenced whether spillover would and could occur.  Some other studies even suggested that MPAs displaced fishing effort further depleting already depleted resources, changed local community structure to facilitate an invasion by an exotic species, or caused density dependent declines in population size inside the reserve.

However, many of the studies that demonstrated negative consequences helped scientists identify the circumstances in which MPAs are most useful and under which they are not.  Further, now that the formula for a successful MPA is better understood, more studies are demonstrating how protecting an area from fishing can lead to spillover and financially benefit a fishery.  However, negative consequences are still a concern and MPAs may have unexpected consequences for those species that are not targeted.  For instance, increasing the number of large predators (which are often the exploited species we are trying to protect) may cause prey species to decline.

In a recent study, species richness and community complexity spilled over the boundary of an area protected for 25 years in the Philippines.  These results were a direct consequence of the spillover of multiple species from this MPA.  Our own work in Hawaii has demonstrated that deepwater MPAs can benefit deepwater snapper species similar to shallow MPAs, resulting in increases in fish size and abundance over time.  Even more recently, we are finding that targeted species are spilling-over from these deepwater protected areas and even benefiting the local fishery (e.g. larger and more fish in catch data).

MPAs are not a panacea, but in the right circumstances, the export of larger fish, abundance, species richness and community complexity resulting from a MPA can profit local fisheries and provide hope that MPAs may help reverse the decline of fish sizes, abundance and biodiversity seen all-too frequently across the world.

References and related material

Guidetti P. 2007. Potential of marine reserves to cause community-wide changes beyond their boundaries.  Conservation Biology 21:540-545.

Kerwath, SE, Winker H, Gotz A, Attwood CG.  2013. Marine protected area improves yield without disadvantaging fishers.  Nature Communications 4:2347.

Russ G, Alcala A. 2010. Enhanced biodiversity beyond marine reserve boundaries: the cup spilleth-over. Ecological Applications DOI:10.1890/09-1197