Cetacean habitats in the northern Gulf of Mexico

Surveys were conducted in the northern Gulf of Mexico during the spring sea sons of 1992, 1993, and 1994 to determine the distribution, abundance, and habitat preferences of oceanic cetaceans. The distributions of bottlenose d olphins (Tsursiops truncatus), Risso's dolphins (Grampus griseus) Kogia spp . (pygmy [kogia breviceps] and dwarf sperm whales [Kogia sima]) pantropical spotted dolphins (Stenella attenuata), and sperm whales (Physeter macrocep halus) were examined with respect to depth. depth gradient, surface tempera ture, surface temperature variability the depth of the 15 degreesC isotherm , surface chlorophyll concentration, and epipelagic zooplankton biomass. Bo ttlenose dolphins were encountered in two distinct legions: the shallow con tinental shelf (0-150 mi and just seaward of the shelf break (200-760 mi. W ithin both of these depth strata, bottlenose dolphins were sighted more fre quently than expected in regions of high surface temperature variability wh ich suggests an association with ocean fronts. Risso's dolphins were encoun tered over the steeper sections of the upper continental slope (200-1000 m) , whereas the Kogia spp. were sighted more frequently in waters of the uppe r continental slope that had high zooplankton biomass. The pantropical spot ted dolphin and sperm whales were similarly distributed over the lower cont inental slope and deep Gulf (>1000 m), but sperm whales were generally abse nt from anticyclonic oceanographic features (e.g. the Loop Current. warm-co re eddies) characterized by deep occurrences of the 15 degreesC isotherm. H abitat partitioning, high-use areas, species accounts, environmental sampli ng limitations, and directions for future habitat work in the Gulf of Mexic o are discussed.