PHYSICAL HABITAT OF CETACEANS ALONG THE CONTINENTAL-SLOPE IN THE NORTH-CENTRAL AND WESTERN GULF-OF-MEXICO

The physical habitat of cetaceans found along the continental slope in the north-central and western Gulf of Mexico was characterized from s hipboard sighting data, simultaneous hydrographic measurements, and sa tellite remote sensing. The study area was encompassed by the longitud e of the Florida-Alabama border (87.5 degrees W), the southernmost lat itude of the Texas-Mexico border (26.0 degrees N), and the 100-m and 2 ,000-m isobaths. Shipboard surveys were conducted seasonally for two y ears from April 1992 to May 1994. A total of 21,350 km of transect was visually sampled in an area of 154,621 km(2). Sighting localities of species in the study area were differentiated most clearly with bottom depth. Atlantic spotted dolphins (Stenella frontalis) were consistent ly found in the shallowest water on the continental shelf and along th e shelf break. In addition, the bottom depth gradient (sea floor slope ) was less for Atlantic spotted dolphins than for any other species. B ottlenose dolphins (Tursiops truncatus) were found most commonly along the upper slope in water significantly deeper than that for Atlantic sported dolphins. All the other species and species categories were fo und over deeper bottom depths; these were Risso's dolphins (Grampus gr iseus), short-finned pilot whales (Globicephala macrorhynchus), pygmy/ dwarf sperm whales (Kogia spp.) rough-toothed dolphins (Steno bredanen sis), spinner dolphins (Stenella longivostris), sperm whales (Physeter macrocephalus), striped dolphins (Stenella coevuleoalba), Mesoplodon spp., pantropical spotted dolphins (Stenella attenuata), Clymene dolph ins (Stenella clymene) and unidentified beaked whales (Ziphiidae). Ris so's dolphins and short-finned pilot whales occurred along the upper s lope and, as a subgroup, were significantly different from striped dol phins, Mesoplodon spp., pantropical spotted dolphins, Clymene dolphins , and unidentified beaked whales, which occurred in the deepest water. Pygmy/dwarf sperm whales, rough-toothed dolphins, spinner dolphins, a nd sperm whales occurred at intermediate depths between these two subg roups and overlapped them.