The insular shelf south of St. Thomas and St. John, in the U.S. Virgin Islands (USVI) is an expansive geomorphologic feature with extensive mesophotic coral reefs occurring at 30 to 150 meter depths. These mesophotic reefs are part of a broader coral reef community in the U.S. Caribbean that provide a wide range of ecosystem goods and services estimated to provide $187 million per year (in 2007 US dollars) to local communities. While these reefs provide great economic value to local communities, they are increasingly under threat from multiple human-caused stressors, making it critical for the USVI and Puerto Rico jurisdictions to find ways to preserve and sustainably manage them. Ideally, the first step in any marine management process is to comprehensively map and inventory the location of coral reef resources. The habitat map products provided in this report represent the first complete habitat map for the insular shelf south of St. Thomas and St. John. These products also represent the culmination of an extensive seafloor mapping campaign conducted by NOAA’s National Centers for Coastal Ocean Science (NCCOS) in collaboration with regional partners.
In total, 652 km2 of seafloor were characterized on the insular shelf south of St. Thomas and St. John. These habitat map products are ‘pixel-based,’ developed from 11×11 meter resolution raster images and mathematical modeling techniques called boosted regression and classification trees. These modeling techniques generate continuous spatial predictions by finding relationships between the geographic distribution of habitats (extracted from 1,005 underwater videos) and environmental conditions that may be influencing or correlated with these distributions (extracted from 20 raster images describing the oceanography, geography, and seafloor topography of the area). The final habitat map products describe the probability of occurrence and associated precision for four individual substrata (i.e., ‘Coral Reef’, ‘Pavement’, ‘Rhodoliths’, and ‘Sand’) and two biological cover types (i.e., ‘Live Hard Coral’ and ‘Live Soft Coral’). These six layers were also combined to create a single, composite benthic map with five habitat classes including: ‘Coral reef colonized with live coral’, ‘Rhodoliths with macroalgae’, ‘Bare sand’, ‘Rhodoliths with macroalgae and bare sand’, and ‘Pavement colonized with live coral.’ The performance and thematic accuracy of these products were evaluated using an independent set of underwater videos (n=348). The performance of the habitat models was considered to be good to outstanding based on four quantitative metrics, including percent deviance explained (x̄ =37.4% ±16.2 SE), area under the receiver operating characteristic curve (x̄ =0.86 ±0.05 SE), mean error (x̄ =-0.04 ±0.03 SE) and root mean square error (x̄ =0.33 ±0.04 SE). The overall accuracy and tau value for the composite habitat map was high at 85.6% and 0.82 ±0.05, respectively.
‘Coral Reef colonized with live coral’ was the most abundant habitat type on the insular shelf, comprising 34.3% (223.6 km2) of the area. It was dominant throughout the Virgin Passage, Hind Bank, along the upper shelf of French Cap Bank, and along ridge features south of St. Thomas. ‘Bare sand’ was often found around the edges of these coral reef structures, particularly on the eastern portion of the insular shelf. Linear patches of bare sand were also common in the middle of the insular shelf between Hind Bank and Frenchcap Bank. Interestingly, these patches were fairly regularly spaced, and commonly oriented in the north-south direction, suggesting they were formed by consistent bottom currents in the area. ‘Rhodoliths with macroalgae’ was the second most abundant habitat, comprising 30.2% (196.8 km2) of the insular shelf. This habitat was more common in deeper depths (greater than approximately 45 m), especially near Frenchcap Bank, Tampo Bank, and west of El Seco. ‘Rhodoliths with macrolagae’ often transitioned into ‘Rhodoliths with macroalgae and bare sand’ at this depth threshold. This transition was particular sharp south of Frenchcap Cay and the Midshelf Reef. Lastly, ‘Pavement colonized with Live Coral’ was the least abundant habitat type, comprising 4.8% (31.3 km2) of the area. It occurred primarily in the southeast region on the insular shelf. As these complex spatial patterns suggest, seafloor habitats vary widely across the insular shelf south of St. Thomas and St. John.
The products described here capture these complex patterns and translate them into maps that can be used to plan for potential activities both onshore nearby islands and offshore on the insular shelf. They can also be integrated with other spatially coincident datasets collected by NCCOS and regional partners, including underwater videos collected using remotely operated vehicles and acoustic information describing the size, abundance and distribution of fish. The best way to access and use these products is through geographic information systems (GIS) software that allows users to zoom in and create custom maps. These GIS-ready layers are archived at NOAA’s National Centers for Environmental Information (NCEI) and are freely available for viewing and download from the following project page: https://coastalscience.noaa.gov/project/habitat-map-insular-shelf-south-of-st-thomas-st-john/. For users that do not have GIS software or expertise, these map products are accessible as print-ready maps at the end of this report, and through an online map viewer linked to the above project page.