220 
Fishery Bulletin 99(2) 
nose dolphin (Tursiops truncatus ) and Atlantic spotted 
dolphin (Stenella frontalis), occur regularly over the conti- 
nental shelf (Fritts et al., 1983; Mullin et ah, 1994; Davis 
et ah, 1998). In contrast, the oceanic Gulf supports a wide 
diversity of cetacean species by potentially supplying a 
large number of ecological niches. Although predator avoid- 
ance, interspecific competition, and reproductive strategies 
all affect cetacean distribution to some extent, energetic 
budget studies indicate that most cetaceans must feed 
every day (Smith and Gaskin, 1974; Lockyer, 1981; Kenney 
et ah, 1985; CETAP 5 ) and thus habitat is assumed to be pri- 
marily determined by the availability of food (Kenney and 
Winn, 1986). The distribution of the oceanic species, then, 
is presumably linked to the rather dynamic oceanography 
of the Gulf of Mexico through physical-biological interac- 
tions and trophic relationships between phytoplankton, 
zooplankton, micronekton, and cetacean prey species. For 
most cetaceans in the Gulf of Mexico, specific prey species 
are not known but likely include epi- and mesopelagic fish 
and cephalopods (Fitch and Brownell, 1968; Perrin et ah, 
1973; Wurtz et ah, 1992; Clarke, 1996). 
The physical and biological oceanography of the north- 
ern Gulf of Mexico is highly variable in both space and 
time. The eastern Gulf contains the Loop Current, an ex- 
tension of the Gulf Stream system that enters the Yucat- 
an Channel, turns anticyclonically, and exits through the 
Straits of Florida. The northward penetration of the Loop 
Current into the Gulf of Mexico normally varies between 
24° and 28°N on a quasi-annual basis (Sturges and Evans, 
1983). Cold, potentially biologically rich, upwelling fea- 
tures are frequently found at the edge of the Loop Current 
and often develop into cyclonic, cold-core eddies (Vukovich 
et ah, 1979; Maul et ah, 1984; Vukovich and Maul, 1985; 
Richards et ah, 1989). Large, anticyclonic, warm-core ed- 
dies can shed from the Loop Current during its maximum 
northerly penetration into the Gulf (Cochrane 1972; Hurl- 
burt and Thompson, 1982) after which they move slowly 
westward at an average speed of 5 km/day. More than 
one of these warm-core eddies can be found in the west- 
ern Gulf of Mexico because their translation (net) speed 
and decay are slow (Elliot, 1982). During their transit 
from the eastern to western Gulf of Mexico, these warm- 
core features can also have associated cyclonic features at 
their peripheries which are biologically productive (Biggs, 
1992). Another major source of nutrients that can drive 
primary productivity in the oceanic Gulf is the Mississippi 
River. The Mississippi River Delta protrudes into the Gulf 
4 ( continued from previous page) Geological Survey, Biological 
Resources Division, USGS/BRD/CR-1999-0006 and Minerals 
Management Service, OCS Study MMS 2000-003. (Available 
from Public Information Office, MS 5034, Gulf of Mexico Region, 
Minerals Management Service, 1201 Elmwood Park Blvd., New 
Orleans, LA 70123-2394.] 
’ CETAP (Cetacean and Turtle Assessment Program). 1982. A 
characterization of marine mammals and turtles in the mid- and 
north Atlantic areas of the U.S. outer continental shelf. LIS. 
Department of the Interior, Bureau of Land Management, con- 
tract AA551-CT8-48. 584 p. [Available from National Techni- 
cal Information Service, LIS. Department of Commerce, 5285 
Port Royal Road, Springfield, VA 22161.] 
in a region where the continental shelf is narrow and the 
continental slope is steep. The river’s nutrient-rich fresh 
water plume extends over the deep Gulf and supports high 
rates of primary productivity and large standing stocks 
of chlorophyll and zooplankton biomass (El-Sayed, 1972; 
Dagg et al., 1988; Ortner et al., 1989). 
Our study examines the distribution of five commonly 
encountered cetacean species or species groups in the 
northern Gulf of Mexico with respect to several physical, 
biological, and physiographic variables. These species are 
the bottlenose dolphin, Risso’s dolphin (Grampus griseus), 
Kogia spp. (pygmy [ Kogia breviceps | and dwarf sperm 
whale [Kogia sima]), pantropical spotted dolphin ( Stenel- 
la attenuata ) and sperm whale (Physeter macrocephalus) . 
The environmental and cetacean survey data for our 
study were collected by the U.S. National Marine Fish- 
eries Service. Subsets of these data have been analyzed 
by Baumgartner ( 1997 ) to characterize the distribution of 
Risso’s dolphins with respect to the physiography of the 
northern Gulf of Mexico and by Davis et al. (1998) to de- 
scribe cetacean habitats over the continental slope in the 
northwestern Gulf. One of the major objectives of these 
surveys was to help assess the impact of large-scale oil 
and gas exploration and development in the northern Gulf 
of Mexico on cetaceans. An understanding of the habitat 
preferences of each of these species will greatly improve 
management and conservation efforts by providing a con- 
text for interpreting future anthropogenic influences on 
cetacean distribution. 
Materials and methods 
Data collection and treatment 
We examined the distribution of each cetacean species with 
respect to seven environmental variables (Table 1) to char- 
acterize habitat. These variables were selected because they 
represent specific oceanographic or physiographic features 
or conditions. Depth and depth gradient (sea floor slope) 
were included to represent the physiography of the Gulf 
of Mexico because the distribution of some cetaceans has 
been associated with specific topographic features in the 
Gulf (Baumgartner, 1997; Davis et al., 1998) and elsewhere 
(Evans, 1975; Hui, 1979, 1985; Selzer and Payne, 1988; 
CETAP 5 ; Dohl et al. 6 ; Dohl et al. 7 ; Green et al. 8 ). A com- 
6 Dohl, T. P., K. S. Norris, R. C. Guess, J. D. Bryant, and M. W. 
Honig. 1978. Summary of marine mammal and seabird sur- 
veys of the Southern California Bight area 1975-78, vol. Ill: 
Investigators’ Reports, part II: Cetacea of the Southern Cali- 
fornia Bight. U.S. Department of the Interior, Bureau of Land 
Management, Contract AA550-CT7-36, 414 p. [Available from 
National Technical Information Service, U.S. Department of 
Commerce, 5285 Port Royal Road, Spring-field, VA 22161.] 
7 Dohl, T. P, R. C. Guess, M. L. Duman, and R. C. Helm. 1983. 
Cetaceans of central and northern California, 1980-1983. Status, 
abundance and distribution. U.S. Department of the Interior, 
Minerals Management Service, contract 14-12-0001-29090, 284 p. 
[Available from National Technical Information Service, U.S. 
Department of Commerce, 5285 Port Royal Road, Springfield, 
VA 22161.1 
