9781888569605-ch48

Benthic Habitats and the Effects of Fishing

Pockmarks on the Outer Shelf in the Northern Gulf of Mexico: Gas-Release Features or Habitat Modifications by Fish?

Kathryn M. Scanlon, Felicia C. Coleman, and Christopher C. Koenig

doi: https://doi.org/10.47886/9781888569605.ch48

Abstract. Side-scan sonar and multibeam data and video observations from six outer continental shelf sites in the northern Gulf of Mexico reveal evidence of three distinct types of seafloor pockmarks. Two sites in the northwestern Gulf (Flower Garden Banks and the Pinnacles area off Mississippi and Alabama) have large-diameter (up to 65 m) pockmarks that are probably generated by the seepage of gas through the sediments. Three sites (Madison–Swanson, Twin Ridges, and an area north of Steamboat Lumps) have small-diameter (less than 2 m) pockmarks that occur in fine-grained sediments at water depths greater than about 100 m. These may be attributed to excavation by fish and other animals. The sixth site, Steamboat Lumps, has medium-diameter (5–25 m) pockmarks in sand with loose cobblesized to boulder-sized rocks. Red grouper Epinephelus morio are associated with these pockmarks and appear to be maintaining and excavating them. Because this area may have experienced freshwater seeps in the past, the pockmarks may be the result of a combination of water seepage followed by fish excavation.