The "Spillover Effect:"
One of the most interesting aspects of Marine Reserves is the so-called "Spillover Effect," by which improved biomass and diversity within the reserves enhances nearby fisheries and ecosystems. Beginning with the work of Roberts, et. al. (1997), the existence of such an effect has been noted and studies for nearly a decade. In particular, he noted that the dispersal of pelagic larvae from protected areas enhanced the productivity of "downstream" regions and made them more resilient to change. There are numerous documented examples of spillover benefits, from all parts of the world, ranging from the closed area off Cape Canaveral in Florida (Johnson, Funicelli, & Bohnsack 1999) to the examples in the Philippines (Russ, Alcala, & Maypa 2003) and off the coasts of Kenya and Tanzania (McClanahan, Verheij, & Maina 2006).
Although there is only limited long-term evidence of spillover, such evidence has increasing, especially as longer-term data has become available (R.A. Abesamis et al., 2006). Abesamis, et. al. (2006) proposes that a shallow "decreasing gradient of abundance of targeted fish across a no-take reserve boundary" is evidence for spillover. For instance, if population increases inside a reserve, some of the fish might move away from the more population-dense areas because of resource competition. However, local environmental characteristics can also influence an abundance gradient, since population is usually higher in areas with more resources and in complex habitats (R.A. Abesamis et al., 2006). One study on abundance gradients of fish across the boundaries of protected areas near two small Philippine Islands (which used a control section without a reserve) found that three of four reserve boundaries had shallow gradients of decreasing abundance. Although habitat factors could not explain the cases where there was sharp decline in abundance across the boundary, the evidence, in general suggests the existence of spillover in many cases (R. A. Abesamis et al., 2006).
Another example of tangible evidence for dispersal around marine protected areas was an experiment in which 90 blue cod were tagged and released at four sites, two of which were in and two of which were next to Long Island (R.G. Cole et al., 2000). Fewer resightings of tagged fish occurred in the reserve than outside, but the resights in the reserve were larger on average than those outside (R.G. Cole et al., 2000). The smaller number of resights in marine reserves than in fished areas, despite the absence of fishing in the protected areas, suggests that blue cod migrate longer distances in reserve sites than in fished areas; this evidence supports the claim that blue cod will grow to larger sizes in marine reserves and, through spillover, help supply nearby fisheries through spillover (R.G. Cole et al., 2000).
As part of increasing evidence of spillover in general, it is strongly predicted that protected areas will allow the spillover for certain important species. For example, several studies in New Zealand suggest that a small number of spiny lobsters in a population will migrate large distances (R. J. Davidson et al., 2002). Based on this evidence, it is likely that marine protected areas can protect a signification percentage of the population of spiny lobsters and also provide spillover from the reserve (R. J. Davidson et al., 2002). Another study in the Western Mediterranean using catch and effort data about lobster, as well as data from tagged lobsters released in the reserve, suggests that the declining density gradient of lobster is caused by lobsters migrating away from the reserve (Goñi et al. 2006). While it is possible that other factors could have caused the density gradient, because of lobsters' general inability to move long distances compared to the size of the reserve and its overfished status, the establishment of the protected area there probably caused increased export and spillover (Goñi et al. 2006).