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Risso’s dolphins (Grampus griseus) occur throughout the world in tropical and temperate waters. The best estimates of abundance for Risso’s dolphins are 1,589 (CV = 0.27) for the northern Gulf of Mexico and 20,479 (CV = 0.59) for the U.S. Atlantic Ocean (Waring et al. 2006). G. griseus is the fourth most abundant cetacean species in the oceanic waters of the northern Gulf and is found in all seasons (Mullin et al. 1994, 2004; Maze-Foley and Mullin 2006).
Risso’s dolphins typically can be found over the continental shelf edge and the upper continental slope, where sea surface temperatures exceed 10◦C (Baumgartner 1997, Kruse et al. 1999). They frequent areas of high seafloor relief, where they may feed on mesopelagic and vertically migrating cephalopods (Norris and Dohl 1980, Clarke and Pascoe 1985, Clarke 1986), but no previous studies of diving behavior of Rissos’s dolphins have been reported (Baird 2002). Their oceanic distribution limits opportunities for systematic study of movements and dive patterns.