Mechanisms matter for evaluating the economic impacts of marine reserves

Large areas of marine and coastal environments have been protected to satisfy diverse policy goals, but there has been limited work understanding the economic impacts of such closures. While methods for establishing causal impacts are prevalent, less attention has been paid to explaining the mechanisms through which the causal relationship came to be. Understanding mechanisms is crucial for designing policies that foster the mechanisms that achieve the intended objectives of marine reserves and mitigate the mechanisms that do not. We estimate the treatment effect of a large marine reserve on the net earnings of a commercial fishery using difference-in-differences and synthetic-control designs, and decompose the treatment effect into its constituent mechanisms through structural equation modeling. We find minimal evidence that closing the marine reserve to fishing had a significant economic cost for the industry; however, several counteracting mechanisms are critical for explaining the effect and for generalizing to other settings.