Hawaiian Marine Protected Areas Produce Spillover

by Katie Huang

Marine protected areas (MPAs) can be beneficial to fisheries through spillover effects, which occur when protected fish stocks recover and migrate into open areas. As a result, fishers tend to react by increasing fishing pressure near MPA boundaries to capitalize on these biomass gradients. To supplement previous research on spillover, Stamoulis and Friedlander (2013) studied a Hawaiian MPA with a new seascape approach that incorporated habitat variables, multiple scales of study, and information on fishing pressure. They took visual surveys of fish populations both targeted and not targeted for conservation along random transects and determined their biomass, species abundance, and density in protected and open areas. The authors found that all fish wellbeing measures were observed to be significantly higher inside the reserve. Also, as distance increased from the MPA boundaries, biomass decreased for resource fish but remained constant for non-resource fish, indicating the existence of a spillover gradient. Although fishing was more concentrated near MPA borders, current harvest rates are sustainable for the time being. The authors suggest that similar comprehensive studies be made throughout Hawaii but that further research should also include analysis on larval and egg export, a second benefit to fisheries besides spillover.

Stamoulis and Friedlander focused their study on the Pupukea Marine Life Conservation District, a Hawaiian MPA that forbids virtually all fishing. Using randomly located stations in both open and protected areas, divers took visual surveys of fish lengths and abundances along multiple transects. For analytical purposes, they separated the fish into the categories of resource (targeted for conservation) and non-resource. They also conducted studies at broad and fine scales, which examined small (125 m2) and large (1000 m2) transects respectively. From the fish lengths, the authors calculated area biomasses using mathematical parameters from existing data of the same or similar species. They also conducted 22 visual surveys that recorded the number of fishers observed during randomized daylight hours as well as the type and number of gear that they used. They combined those results with a dataset of fishing violations from a local community organization to calculate an approximate weekly amount of fishers. With the gear data, they mapped a spatial analysis of relative fishing effort that included spearfishing, a fishing method with high catch rates. Stamoulis and Friedlander also measured habitat variables such as percentage benthic cover that could have influenced observed differences across areas. In their data analyses, they removed what they found to be significant habitat variables in order to minimize the influence of the environment on results. To determine biomass gradients, the authors used linear regressions to test the relationships between biomasses from resource and non-resource fish and the distance from the MPA boundary.

The authors found that there were significantly more fish of all types within the MPA, indicating a strong reserve effect. Also, the difference in biomass, species richness, and density between the protected and open areas was much larger for resource than non-resource fish, suggesting that targeting specific populations for conservation is working. On broad and fine scales, there was a significant decrease of both resource and non-resource fish biomass the further the distance from the MPA boundary, signifying the existence of a spillover gradient. Although there were more fishers in the open area than in the MPA, the relative proportions of type of gear used were similar. Spearfishing occurred significantly more along the MPA boundary than further from the bounds, at times even crossing into the reserve. This suggests that fishers have noticed and are responding to the biomass gradient that developed as a result of the MPA. According to calculations, the current level of fishing effort is likely to be sustainable due to the influx of spillover fish biomass, although it should be monitored to ensure this remains true. The results of this study support previous spillover research but this approach added controls for habitat variables, investigations at multiple scales, and logs recording fishing effort to provide a more nuanced understanding of MPA-induced gradients. The authors suggest that similar studies be conducted in Hawaii that also include analysis on the effects of larval and egg export, another potential benefit to fisheries.

Stamoulis K.A., Friedlander A.M., 2013. A seascape approach to investigating fish spillover across a marine protected area boundary in Hawai‘i. Fisheries Research 144, 2–14. http://www.kaikuleana.net/spillover.pdf

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