Abstract. — Little is known 

 about cetaceans in the oceanic Gulf 

 of Mexico (depths >200 m). From 

 July 1989 to June 1990, we con- 

 ducted aerial surveys in the oceanic 

 north-central Gulf (long. 87.5°W- 

 90.5°W) with the following objec- 

 tives: 1) to determine which ceta- 

 cean species were present; 2) to 

 document temporal and spatial dis- 

 tribution for each species; and 3) 

 to estimate relative abundance for 

 each species. We surveyed a total 

 of 20,593 transect km and sighted 

 at least 18 species. Of 278 identi- 

 fied herds (6,084 animals), 94% of 

 the herds and 98% of the animals 

 represented seven species or spe- 

 cies groups: Risso's dolphin. Gram- 

 pus griseus (22% of the herds, 13% 

 of the animals); sperm whale, Phy- 

 seter macrocephalus (16%, 1%); bot- 

 tlenose dolphin, Tursiops truncatus 

 14%, 7%); Atlantic spotted dolphin, 

 Stenella frontalis (13%, 15%); 

 pygmy sperm whale, Kogia bre- 

 viceps, and dwarf sperm whale, Ko- 

 gia simus (12%, 1%); striped dol- 

 phin, Stenella coeruleoalba, spin- 

 ner dolphin, S. longirostris , and 

 clymene dolphin, S. clymene (9%, 

 34%); and pantropical spotted dol- 

 phin, S. attenuata (8%, 27%). Each 

 of these species or species groups 

 was sighted throughout the area 

 surveyed in at least three seasons. 

 Mean water depths of bottlenose 

 dolphin and Atlantic spotted dol- 

 phin sightings were less than 400 

 m; mean water depths of Risso's 

 dolphins and pygmy and dwarf 

 sperm whales were between 400- 

 600 m; and mean water depths of 

 striped, spinner, and clymene dol- 

 phins, sperm whales, and 

 pantropical spotted dolphins were 

 greater than 700 m. Mean herd 

 sizes varied by species and species 

 groups and ranged from 1.9 ani- 

 mals for pygmy and dwarf sperm 

 whales to 87.8 animals for striped, 

 spinner, and clymene dolphins. 



Cetaceans on the upper 

 continental slope in the 

 north-central Gulf of Mexico 



Keith D. Mullin 

 Wayne Hoggard 

 Carol L. Roden 

 Ren R. Lohoefener* 

 Carolyn M. Rogers 



Southeast Fisheries Science Center 



National Marine Fisheries Service, NOAA 



PO Drawer 1207. Pascagoula. Mississippi 39568 



Brian Taggart 



NOAA Aircraft Operations Center 



PO Box 6829. MacDill Air Force Base, Florida 33608 



Manuscript accepted 9 May 1994. 

 Fishery Bulletin 92:773-786. 



The Gulf of Mexico encompasses an 

 area of over 1,500,000 km 2 and has 

 an average depth of 1,700 m (Gore, 

 1992). The continental shelf (depths 

 <200 m) is wide (up to 260 km) in 

 most parts of the northern Gulf (Fig. 

 1). Directed studies (Fritts et al., 

 1983; Scott et al. 1 ) and opportunistic 

 sightings (Schmidly, 1981; Rade- 

 macher 2 ) have suggested that only 

 the bottlenose dolphin, Thrsiops trun- 

 catus, and the Atlantic spotted dol- 

 phin, Stenella frontalis, are common 

 in most continental shelf waters of 

 the U.S. Gulf. However, there are 

 records (primarily from strandings) 

 of 29 cetacean species from the Gulf 

 (Schmidly, 1981; Perrin et al., 1981; 

 Hersh and Odell, 1986; Perrin et al., 

 1987; Bonde and O'Shea, 1989; Bar- 

 ron and Jefferson, 1993). Therefore, 

 if species other than the bottlenose 

 dolphin and the Atlantic spotted dol- 

 phin are represented in substantial 

 numbers, their distributions must be 

 primarily oceanic (depths >200 m). 



Mineral deposits have been 

 mined widely in U.S. Gulf shelf 

 waters west of Mobile, Alabama, 

 and as of 1988, over 4,500 drilling 

 structures have been in use for oil 



and gas production. Mineral devel- 

 opment on the continental slope 

 (depths 200-2,000 m) in the central 

 and western Gulf has begun and 

 additional exploratory drilling is 

 being planned. Before large-scale 

 exploration, development, and pro- 

 duction can take place, an assess- 

 ment of cetacean diversity, distribu- 

 tion, and abundance is required to 

 satisfy the intent of the U.S. Ma- 

 rine Mammal Protection Act and 

 the U.S. Endangered Species Act. 

 Both acts mandate that federal 

 agencies take appropriate actions to 

 ensure that their activities do not 



Present address: U.S. Fish and Wildlife 

 Service, 3100 University Boulevard, South 

 Suite 120, Jacksonville, Florida 32216 



1 Scott, G. P., D. M. Burn, L. J. Hansen, and 

 R. E. Owen. 1989. Estimates of bottlenose 

 dolphin abundance in the Gulf of Mexico 

 from regional aerial surveys. U.S. Dep. 

 Commer., NOAA, Nat. Mar. Fish. Serv., 

 Southeast Fish. Sci. Cent., Miami Labora- 

 tory, 75 Virginia Beach Drive, Miami, FL 

 33149. Admin. Rep. CRD-88/89-07, 24 p. 



2 Rademacher, K. R. 1991. Opportunistic 

 sightings of cetaceans in the Gulf of Mexico 

 from NOAA Ship Chapman, 1989-90. U.S. 

 Dep. Commer., NOAA, Nat. Mar. Fish. 

 Serv., Southeast Fish. Sci. Cent., 

 Pascagoula Facility, P.O. Drawer 1207, 

 Pascagoula, MS 39568. Unpubl. data. 



773 



