Sonar Mapping of Deep Reef Sites off South Carolina
Biologically Engineered and Other Complex Habitats at the Shelf Edge and Upper Slope of the South Atlantic Bight
August 19 - 30, 2006
George R. Sedberry
South Carolina Department of Natural Resources
The sea floor on the continental shelf and slope off the southeastern United States includes vast areas of smooth sandy bottom. Interspersed on the smooth bottom are rugged rocky outcrops. These rocky reefs range in height from a few inches to several hundred feet, and in area from small patches to hundreds of square miles. This rugged hard bottom provides substrate (a surface on which an organism grows or is attached) for sponges, corals, and other invertebrates, and is a favored habitat of many species of fish, including important fishery species. Snappers, groupers, porgies, and many other fisheries species are attracted to the complex bottom habitats provided by rocky relief. In addition to its protruding rocks, the habitat is often further altered by living organisms (including the fishes themselves) in ways that make it more useful and attractive to other fishes and organisms. We refer to these habitats as being "biologically engineered."
Tropical coral reefs are spectacular examples of biologically engineered complex habitats. The reef structure, built up by billions of living coral polyps, provides habitat for sponges, corals, crabs, worms, fishes, and all kinds of marine plants and animals. Off the southeastern U.S., hard corals and worms that produce reefs of hard calcareous (containing calcium carbonate) tubes also build up reefs and construct complex habitats on the outer continental shelf and upper slope. Fishes often excavate soft bottom and the sandy areas around reefs, creating complex habitats used by other organisms. Rock, coral, and worm reefs have been observed visually from submersibles on our previous Ocean Exploration (OE) expeditions, and they contain many more fishes than nearby (but less complex) habitats.
This image shows a portion of the shelf-edge reef, at about 180 ft depth, included within the proposed Edisto Marine Protected Area off South Carolina. This is an important spawning site for several important reef fishes, including scamp grouper and vermillion snapper. Click image for larger view and image credit.
During our South Atlantic Bight 2006 expedition, we wanted to use state-of-the-art sonar to rapidly map rock, coral, and other physically and biogenically formed bottom features of the outer continental shelf and upper slope off South Carolina. These habitats consist of low- to high-relief hard grounds and rocky reefs, smooth sand or mud, and mud that has been excavated by tilefish. In addition to tilefish burrows, other biologically engineered habitats include grouper and triggerfish excavations, worm reefs, and coral reefs. We are particularly interested in mapping habitats that are spawning grounds of reef fishes, especially those species that form aggregations by migrating to specific locations or habitat features.
This mapping will complement previous and current NOAA-funded projects aimed at determining the factors that constitute spawning grounds for reef fishes, especially deep-reef species. We need sonar mapping and the groundtruthing of sonar signatures (using video cameras on remotely operated vehicles) around known spawning locations in order to determine the characteristics that make them attractive to spawning reef fishes. By characterizing the sites with sonar (groundtruthed with visual observations), we can then use rapid sonar surveys to map other lesser-known or previously unknown spawning locations, and to discover additional essential fish habitat. In addition, many fishes build nests during spawning, or otherwise alter the bottom in ways that show up on sonar records; those records can be used to map spawning and residence areas for fishes that “bioengineer” the bottom.