Early indicators of MPA effects are detected by stereo-video

Abstract

Resource managers often outline discrete goals when implementing marine protected areas (MPAs), such as increases in organism abundance, biomass, or body size, to evaluate MPA success and thus inform future management decisions. Understanding which biological indicators are most sensitive to protection is essential for adaptive management, and while density is a commonly used metric to monitor MPAs, previous work has pointed to a lack of reliability of this metric to effectively evaluate MPAs, particularly during initial stages of protection. To determine which biological indicators of MPA success were evident 5 yr after protection began, we examined the differences in populations inside vs. outside MPAs for 3 fishes targeted by anglers—kelp bass Paralabrax clathratus, barred sand bass P. nebulifer, and California sheephead Semicossyphus pulcher—and 4 non-targeted species. We used stereo-video to determine the average body size, biomass, and density of each species in 7 MPAs in Southern California that were paired with nearby non-MPA sites. Responses of targeted species were detectable after 5 yr of protection in some MPAs. In most but not all MPAs, individuals were larger compared to outside MPAs for kelp bass and California sheephead. Biomass and density of targeted species appeared to be less sensitive to MPA effects. As expected, non-targeted species did not show evidence of MPA effects. Similar to past studies, our findings indicate that the choice of response variable and species studied may influence the perceived efficacy of MPAs, and we emphasize that increases in length may be a particularly sensitive indicator of effective protection.