Expansion of an established fishery-independent survey into the US Virgin Islandsʼ upper mesophotic zone: feasibility and management implications

A three-year pilot study, the Deep Coral Reef Monitoring Program (DCRMP), expanded the National Coral Reef Monitoring Program’s (NCRMP) established fishery-independent, diver-based reef fish visual survey to upper mesophotic reefs (>30 to 50 m) in the United States (US) Caribbean for the first time. The new DCRMP sample domain (>30 to 50 m) encompassed 2.4 times more survey area than NCRMP (0 to 30 m) and collected high quality data (CV <20%) on coral reef fishes [three survey years, 29 (5) species; mean (standard deviation)]. For the four representative, fishery-targeted, analysis species selected (i.e., a grouper, snapper, triggerfish, and parrotfish), domain-wide density and length comparisons between surveys showed similar or statistically higher abundances and larger lengths for fishes at deeper depths (>30 to 50 m). These results highlight the importance of surveying the entire insular shelf in St. Thomas and St. John, US Virgin Islands for fisheries management applications. Furthermore, the DCRMP survey leveraged NCRMP’s methods and resources resulting in a seamless extension to deeper waters. However, if these programs were fully integrated and optimized within a single survey design, approximately half the sites would be needed to achieve the same level of precision, offering substantial time and cost savings. The principles of probabilistic sampling successfully used in the present fishery-independent survey design (0 to 50 m) can be applied more broadly to develop an “ideal” large-scale, multi-gear survey from 0 to about 500 m to encompass the entire depth ranges of managed species in the US Caribbean.