LANDSCAPE MODIFICATION AND SEDIMENT MIXING BY RED GROUPER AND OTHER FISH ON THE WEST FLORIDA SHELF

Open circular pits, narrow burrows, and patches of sediment-free hardbottom having dimensions of 1 - 25 m have been observed at numerous sites on the west Florida shelf between the Florida Panhandle and Florida Bay. We attribute these features to excavation by fish. The distribution of pits was mapped using sidescan-sonar imagery and/or multibeam bathymetry at all sites except the shallow Florida Bay site. Ground truth observations were made using submersibles, SCUBA divers, and ROVs.

Small (<2 m diameter and 1 m deep) pits occur in fine-grained sediments at water depths >90 m at three sites in the northeastern Gulf of Mexico. Commonly a narrow tunnel extends downward or laterally from the bottom of the pit. The density of these features is ~600 per km2 and the volume of excavated sediment is ~2500 m3/km2. We observed a blueline tilefish (Caulolatilus microps) occupying a pit in the Madison Swanson area (100 km south of Panama City) and sand tilefish (Malacanthus plumieri) near pits on Pulley Ridge (250 km west of Cape Sable). These burrows are similar to tilefish burrows described on the Atlantic margin slope and probably have the same origin.

Red grouper (E. moria) create excavations of different styles depending on the geologic substrate. In shallow Florida Bay juvenile red grouper remove sandy carbonate sediment from small (< 1 m diam.) solution holes in the Miami Limestone and clear the thin layer of sediment away from the area surrounding the hole. The exposed rock provides substrate for sessile organisms and habitat for numerous other fish, crabs, and lobsters. Adult red grouper create areas of similar exposed rock habitat at ~65 m water depth in the Madison Swanson area. In the Steamboat Lumps area (~200 km west of Tampa) adult red grouper construct large open circular pits, typically 6 m across and 2 m deep, in the thick sand lens that covers the seafloor at 75 m water depth. There are about 250 such pits per km2, representing 8100 m3/km2 of sediment moved. At Pulley Ridge, red grouper occupy smaller (1 - 4m diam.) pits excavated in gravelly carbonate substrate, having a density of ~200 per km2.

Fish can be a significant geologic agent, mixing large volumes of sediment over wide areas. The process has important implications for seafloor engineering projects and for interpretation of the stratigraphic record.