Coral Reef Benthic Video Surveys Facilitate Long-Term Monitoring in the Commonwealth of the Northern Mariana Islands: Toward an Optimal Sampling Strategy.

Abstract

This study describes a step-by-step process used to design an effective benthic video survey component of the Commonwealth of the Northern Mariana Islands long-term monitoring program. Documenting abundance of major benthic groups at relatively large spatial scales, at the appropriate localities, can empower monitoring programs with the capacity to detect changes over time and assess whether management practices are working. Most pertinent to any long-term monitoring program is the overriding question: do we have enough information, or statistical power, to detect changes if changes occur? To assess the power of our benthic video surveys to detect change in coral cover and diversity we varied (1) transect lengths, (2) number of transects, (3) number of frames per transect, and (4) number of data points per frame. Five replicated 50-m transects yielded the most consistent estimates with the highest statistical power, compared with more numerous replicates of shorter (35-m and 15-m) transects. Increasing the number of frames analyzed per 50-m transect yielded greater power than increasing the number of data points per frame, but increasing the number of data points was more effective at estimating species richness. The greatest power of detecting a change in the benthos at each site, within a feasible sampling period, was evident using 5 by 50 m random transects, extracting 60 frames per transect, and analyzing five data points on each frame. This optimal sampling strategy was tested at 23 other long-term monitoring sites and yielded 90% power to detect a 20–30% relative change in dominant benthos abundance estimates (benthos >20% coverage). Our study addresses the sampling unit, accuracy, and ways to improve estimates, but this does not remove the onus of concisely stated questions for monitoring programs pertaining to management.