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National Centers for Coastal Ocean Science

Characterization of the Fish, Benthos, and Marine Debris at the Grays Reef National Marine Sanctuary

Office of National Marine Sanctuaries

Gray’s Reef National Marine Sanctuary


Rockin' the Bottom: Fish and Habitat of Gray's Reef

The nooks and crannies, bumps and ledges of the reefs within Gray’s Reef National Marine Sanctuary (GRNMS) provide plenty of places for critters to latch on to and for fish to hide. Scientists from the National Centers for Coastal Ocean Science and GRNMS have now identified connections between these fish communities and the sea-floor features of the sanctuary.

Surveying organisms along a ledge in Gray's Reef NMS

Scientists surveyed fish communities and bottom characteristics of limestone ledges within Gray’s
Reef National Marine Sanctuary.

Located off the coast of Georgia, Gray’s Reef is known as a “live bottom” reef because, in some places, animals form a dense carpet of living creatures that completely covers the underlying rock.

Within the 17-square-nautical-mile sanctuary, there are both rocky ledges and sandy flat places. The reefs' rock ledges, submerged beneath 60 to 70 feet of water, can be as tall as six to eight feet. Made of limestone, ledges such as those in the sanctuary cover only about one to five percent of the southeastern U.S. continental shelf. However, these ledges support some of the greatest diversity of life along the coast, making it important for us to understand these areas for conservation and management purposes.

A black sea bass

Black sea bass were found to occur at 98 percent of the ledges surveyed and were evenly distributed throughout the sanctuary in much higher numbers than either gags or scamps, which were concentrated at only 11 percent of the ledges.

NOAA scientists have found that the amount of hard bottom, the height of the ledges above the ocean floor, and the density of attached invertebrates influences the kinds and numbers of fish found at a particular location. At Gray's Reef, the number of species of fish, their abundance, and the weight and size (or biomass) of fish were higher at rocky ledges with a lot of attached sponges and corals than in areas with a flat bottom.

One group of fish species was associated with ledges that were tall, heavily colonized with sessile (permanently attached) invertebrates, large in area, and did or did not have undercuts. Another distinct group of species was associated with ledges that were short, not undercut, smaller in area, and were or were not heavily colonized by invertebrates.

Management plans currently exist for both Gray’s Reef National Marine Sanctuary and reef fish species associated with the sanctuary. However, as scientists continue to learn more about the connections between species and habitat within Gray’s Reef, they will be able to more effectively manage the sanctuary and surrounding marine habitats.


A CORS station

Scientists found that the abundance of fish at ledges in Gray's Reef increased with the amount of cover provided by the ledges and also as the height of the ledges increased.