Spillover of adult fish from marine protected areas to bordering fisheries is one desired goal of such areas; through net export, they can strengthen nearby fisheries. However, there is only limited long-term evidence of spillover; recent evidence is increasing, but not as convincing as long-term data (R.A. Abesamis et al., 2006). A "decreasing gradient of abundance of targeted fish across a no-take reserve boundary" may be a way to measure spillover, since, 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 (R.A. Abesamis et al., 2006). However, local environmental characteristics can also influence such 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 only one significant gradient of decreasing abundance across a reserve boundary out of four, but claimed that habitat factors could not explain the sharp decline in abundance across the boundary; the evidence, therefore, suggests the existence of spillover (R. A. Abesamis et al., 2006).
One example of tangible evidence for limited dispersal around a marine protected areas involves 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 (R.G. Cole et al., 2000).
In addition to increasing evidence of spillover, evidence exists that strongly predicts protected areas will create spillover for some species. For instance, while empirical proof of spillover of spiny lobsters is difficult to obtain, since they have nonuniform distribution and varying behavior, and live in complex habitats (R. J. Davidson et al., 2002). However, 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 recapturing lobsters tagged and released in the reserve, suggests that the declining density gradient of lobster is caused by lobsters migrating away from the reserve, or spillover (Goñi et al. 2006). While it is possible other factors could cause 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 therefore spillover (Goñi et al. 2006).
When setting up marine reserves, it is important to consider potential ecological barriers that will prevent the occurrence of spillover, and therefore reduce the ability of the protected area to enhance adjacent fisheries. For instance, one study in the Turks and Caicos Islands found that numerous shallow sand habitats enclosed a population of conch, reducing its spillover into nearby areas (A. Tewfik and C. Bene, 2003).
Tewfik A, Benen C. 2003. Effects of natural barriers on the spillover of a marine mollusc: implications for fisheries reserves. Aquatic Conservation: Marine and Freshwater Ecosystems 13: 473-488.
Davidson RJ, Villouta E, Cole RG, Barrier RGF. 2002. Effects of marine reserve protection on spiny lobster (Jasus edwardsii) abundance and size at Tonga Island Marine Reserve, New Zealand. Aquatic Conservation: Marine and Freshwater Ecosystems 12: 213-227.
Abesamis RA, Russ GR, Alcala AC. 2006. Gradients of abundance of fish across no-take marine reserve boundaries: evidence from Philipine coral reefs. Aquatic Conservation: Marine and Freshwater Ecosystems 16: 349-371.
Cole RG, Villouta E, Davidson RJ. 2000. Direct evidence of limited dispersal of the reef fish Parapercis colias (Pinguipedidae) within a marine reserve and adjacent fish areas. Aquatic conservation: Marine and Freshwater Ecosystems 10: 421-436.
Goñi R, Quetglas A, Reñones O. 2006. Spillover of spiny lobsters Palinurus elephas from a marine reserve to an adjoining fishery. Marine Ecology Progress Series 308: 207-219.