Baumgartner et al.: Cetacean habitats in the northern Gulf of Mexico 
233 
whales was less than a quarter of the average and was 
one-sixth the sighting rate in waters where the depth of 
the 15°C isotherm was shallower than 200 m (Fig. 1 IB ) 
Discussion 
Previous studies have indicated that cetacean habitat 
within several hundred kilometers of the coast is most 
effectively partitioned by depth (Davis et al., 1998; 
CETAP 5 ; Dohl et al 6 .; Dohl et al. 7 ; Green et al. 8 ; Davis 
et al. 11 ). In the northern Gulf of Mexico, each of the five 
species examined in our study could be distinguished 
from at least three of the others by its distribution with 
depth alone. Although the distributions of Risso’s dolphin 
and the Kogia spp. with respect to depth overlapped on 
the upper continental slope, their distributions over the 
upper slope could be distinguished by using depth gradi- 
ent and zooplankton biomass. The distributions of pan- 
tropical spotted dolphins and sperm whales with respect 
to depth were very similar over the continental slope and 
deep Gulf, but their distributions differed with respect to 
the depth of the 15°C isotherm. These results suggest that 
cetaceans partition the north- 
ern Gulf of Mexico according 
to each species’ habitat pref- 
erences which are presumably 
based on different prey distri- 
butions. It is important to note 
that this partitioning does not 
necessarily imply spatial sep- 
aration. Given the right condi- 
tions (e.g. waters 500-750 m 
deep over a steep section of 
the continental slope with high 
zooplankton biomass in the 
upper 200 m), many of these 
cetaceans could be encoun- 
tered in the same area. Some 
of the environmental variables 
that are important descrip- 
tors of cetacean habitat (e.g. 
zooplankton biomass, depth of 
the 15°C isotherm, and surface 
temperature variability) vary 
over time, and therefore the 
locations and spatial extent of 
each species’ habitat may vary 
over time as well. 
Two regions of the northern Gulf of Mexico seem to be 
particularly important habitats for some of the more fre- 
11 Davis, R. W., G. S. Fargion, W. E. Evans, L. N. May and T. 
D. Leming. 1996. Cetacean habitat. In Distribution and 
abundance of cetaceans in the north-central and western Gulf 
of Mexico: final report, vol. II: technical report (R. W. Davis 
and G. S. Fargion, eds.), p. 329-349. U.S. Department of 
the Interior, Minerals Management Service, OCS Study MMS 
96-007. [Available from Public Information Office, MS 5034, 
Gulf of Mexico Region, Minerals Management Service, 1201 
Elmwood Park Blvcl., New Orleans, LA 70123-2394.] 
quently encountered species during the spring (Fig. 4): the 
vicinity of the Mississippi River plume and just seaward 
of the southwestern Florida continental shelf. The Mis- 
sissippi River injects nutrients into an otherwise oligotro- 
phic oceanic Gulf in a region where the continental shelf 
is very narrow and the upper continental slope is quite 
steep. The rate of primary productivity and the standing 
stocks of chlorophyll and plankton associated with the nu- 
trient-rich, fresh-water plume are high in relation to other 
regions in the oceanic Gulf (El-Sayed, 1972; Dagg et al., 
1988; Ortner et al., 1989; Miiller-Karger et al., 1991). Con- 
sequently, the plume region may provide feeding oppor- 
1.0 
0 8 
0 4 
Cl 
3 0 . 2 - 
o 
0 
0.0 J 
1000 2000 3000 4000 
Depth (m) 
B 
1 0 
-0 6 

100 150 200 250 450 500 
Depth of 15°C isotherm (m) 
Figure 11 
Sighting rate distributions of sperm whales with respect to (A) depth and (B) the depth of 
the 15°C isotherm. 
