Efficacy of probability surveys for ESA-listed coral species in southern Florida

As coral populations have declined on coral reefs in the United States, one federal management tool to conserve species is the US Endangered Species Act (ESA). Evaluation of the population status and trends for each species at the ecosystem scale requires periodic surveys, but surveys require a sampling effort sufficient to detect change at high levels of precision over time. Therefore, we assessed the feasibility of diver-based survey data to quantitatively evaluate the seven ESA-listed coral species on Florida’s coral reefs. Data are from two complementary monitoring programs constructed from the same probability-based survey design that incorporates stratification created from density mean-variance spatial patterns of abundant target species. The survey design also included along-shore subregional gradients and marine spatial planning to tailor the sample frame specifically to the spatial distributions of each threatened species. We evaluated a multispecies design by comparing precision-sample size relationships from two stratification schemes. Results showed that constructing a composite scheme from the habitat use of four ESA-listed species resulted in a multispecies design with predicted precision estimates (CV) from 7.5% to 20% for six threatened species. These precision levels enable detection of change in mean density from 15% to 40% (approximately 2 SEs or twice the CV). This level of precision ranked as high as those for many abundant species for four threatened species and somewhat lower for the two rarest species. However, controlling survey precision at the regional scale would likely require sample sizes at least three times larger than a typical survey effort for most ESA-listed species.