Accurate assessment of marine protected area success depends on metric and spatiotemporal scale of monitoring

Abstract

Marine protected areas are being monitored to determine whether they increase abundance of fished populations, with responses often expected within a few years. Evaluations typically compare abundance inside versus outside or after versus before implementation, but the temporal and spatial scales over which these measures can reflect marine protected area success are untested. We modeled the response of fished populations for a range of marine protected area sizes, fishing intensities, larval dispersal distances, and adult movement ranges. Our results, which can inform experimental design and interpretation of monitoring, show that the spatial and temporal scale of population responses to marine protected areas will be determined by simple relationships between marine protected area size, larval and adult movement distances, and generation time, in addition to the effects of exploitation rate. The largest effects of marine pro- tected areas should be expected with 'outside' samples located at least 2 dispersal units from the edge, and after 2 generations have passed since establishment. In general, monitoring studies over time (after versus before) should provide better assessment of marine protected area success than monitoring over space (inside versus outside), but understanding of the limitations of each type of measurement is key. Because it may take many years for marine protected area effects to be fully realized, we strongly caution against judgment of marine protected area effectiveness at inappropriately short time frames.