Habitat use of shelf-edge topographic highs in the northwestern Gulf of Mexico by sharks and rays (subclass Elasmobranchii)

Abstract

The continental shelf of the northwestern Gulf of Mexico includes an array of submerged hard-banks and reefs in addition to a great concentration of offshore petroleum platforms that provide significant vertical and structural relief in an otherwise level landscape, and are defined as “topographic highs”. This study investigates the habitat use of shelf-edge topographic highs by wide-ranging sharks and rays occurring in the northwestern Gulf of Mexico. Three topographic highs (East and West Flower Garden Banks and the High Island A-389A offshore production platform) located near the shelf-edge (200 m isobath) were visited from July 1992 through April 1998. Underwater surveys on SCUBA were conducted to gather information on the species composition, abundance, life history stages, sexes, and social groups of elasmobranchs occurring at the study sites. Thirteen species were identified that include Ginglymostoma cirratum (nurse shark), Rhincodon typus (whale shark), Galeocerdo cuvier (tiger shark), Carcharhinus falciformis (silky shark), Carcharhinus obscurus (dusky shark), Carcharhinus perezi (Caribbean reef shark), Carcharhinus plumbeus (sandbar shark), Sphyrna lewini (scalloped hammerhead), Dasyatis americana (southern stingray), Aetobatis narinari (spotted eagle ray), Mobula hypostoma (lesser devil ray), Mobula tarapacana (sicklefin devil ray), and Manta birostris (manta ray). These species form three assemblages (winter, summer, and resident) at the Flower Garden Banks. Carcharhinus falciformis was the only species found utilizing the High Island platform throughout the year. Data gathered in this study show the Flower Garden Banks function as nursery areas, adult feeding areas, and possibly as a mating area to these elasmobranch species, while the High Island platform functions as a secondary nursery area to C. falciformis. Seasonal movements relative to other habitat areas in the region are also modeled for the sharks and rays observed at shelf-edge topographic highs.