We conducted a baseline characterization of Gray’s Reef National Marine Sanctuary, a protected natural reef on the continental shelf off the Georgia coast. We examined reef fish, bottom features, and impacts from fishermen (fishing gear and other debris) throughout the area, creating the most complete assessment available of the area’s range of marine life and habitats.
Why We Care
Gray’s Reef National Marine Sanctuary (GRNMS) protects one of the largest near-shore areas of live-bottom reefs and ledges along the U.S. southeast coast. These ledges are often densely populated with fish and encrusting organisms sought by recreational fishers and divers. Despite its rarity and importance, a comprehensive inventory of GRNMS’s reef fauna, bottom features, and patterns of human use had never been conducted; information about the fish and seafloor communities was limited to just a few reefs. To support responsible stewardship, including management, education, research, and recreational activities, the sanctuary staff needed a comprehensive scientific assessment conducted over the entire area.
What We Did
First, we mapped benthic habitats of the sanctuary using sonar imagery. Divers used the maps to guide extensive surveys of fish communities, marine debris (including fishing gear), sponges, corals, and other seafloor organisms. Given the importance of the sanctuary’s ledge-type seafloor, we paid particular attention to characterizing the different ledge types, their fish communities, and the marine debris associated with them.
What We Found
Of the four seafloor types in the sanctuary, ledges were found to contain the greatest number of fish and fish species and compose 42 percent of the benthic cover. Based on measurements taken at each site (total height, undercut height, and undercut width), we classified ledges as short, medium, and tall.
Fish communities at GRNMS are closely linked by specific benthic habitat. Species richness, diversity, composition, abundance, and biomass varied strikingly depending on bottom type, with ledge areas showing the highest values of most metrics. Species richness and total abundance of fish are positively related to total percent cover of sessile invertebrates and ledge height. Our data indicate that there are five distinct combinations of ledge type and species assemblage, suggesting that species assemblage, richness, and abundance might be predicted by knowing the basic characteristics of a ledge. Comparisons with previous studies reveal some major changes in the fish community at GRNMS since the late 1980s, although the causes of the changes are unknown.
Because GRNMS is a popular site for recreational fishing, there was increasing concern about the accumulation of trash (marine debris) in the reef. Approximately two-thirds of all observed debris items we found was fishing gear, and about half of that was monofilament fishing line. Debris is concentrated in the center of the sanctuary and is most frequently associated with ledges where sports fishing species are known to congregate. On ledges, the presence and abundance of debris is associated with boating density and physiographic features such as ledge height, ledge area, and percent cover.
Our project (see 2007 report below) established quantitative baseline characteristics of many of the key resources at GRNMS and was the a basis for new management and use policies, including a ban on spear fishing and the establishment of a research area. The results can be applied to other hard seafloor areas to prioritize conservation efforts.