Baumgartner et al.: Cetacean habitats in the northern Gulf of Mexico 
235 
ties because of enhanced local productivity or because it 
forms a bolder between two separate, exploitable ecosys- 
tems. The observations of higher than expected surface 
temperature variability at Risso’s dolphin sightings seem 
to support this hypothesis, but the evidence is rather tenu- 
ous given the small sample size (n- 14). 
Kogia spp. 
Kogia spp. were predominantly encountered along the 
upper continental slope in regions with high epipelagic 
zooplankton biomass. Their distribution with respect to 
depth in the northern Gulf of Mexico is in agreement 
with inferences drawn from a stomach content study of 
stranded Kogia spp. in South Africa where cephalopods 
typical of the continental slope were identified as the 
largest constituent of the stranded whales’ stomach con- 
tents. 13 Further stomach content and stable isotope analy- 
ses suggest that pygmy and dwarf sperm whales consume 
different prey species and therefore may occupy different 
habitats. 14 ' 15 No such separation was detectable in our 
study because of the low sample size for the individual 
species and the difficulty of positively identifying each 
at sea. Some diet overlap was observed between the two 
species off South Africa 14 and therefore the association 
between Kogia spp. and high epipelagic zooplankton bio- 
mass in the northern Gulf of Mexico may be due to the uti- 
lization of zooplankton in the diet of one or more of their 
common prey species. 
Of all the cetaceans, Kogia spp. had the highest median 
value for surface temperature variability which suggests a 
similar association with ocean fronts as that observed for 
Risso’s dolphins. Sample size for this variable was unfortu- 
nately low (n-18) however; therefore it is difficult to accu- 
rately describe this potential association. Because the up- 
per continental slope can be a region of persistent frontal 
activity, it is conceivable that the distribution of both Ris- 
so’s dolphins and Kogia spp. with respect to surface tem- 
perature variability may have been a consequence of their 
distribution with depth. The low sample sizes for each of 
these species precludes any analysis that may have been 
able to further support or refute these hypotheses. 
Pantropicai spotted dolphin 
The distribution of the pantropicai spotted dolphin was 
not significantly different from a uniform distribution 
with respect to any of the environmental variables, except 
depth. Pantropicai spotted dolphins are rarely encoun- 
tered on the continental shelf in the northern Gulf of 
Mexico (Jefferson and Schiro, 1997) and from the results 
1! Klages, N. 2000. Persona! commun. Port Elizabeth Museum 
at Bayworld, P.O. Box 13147, Humewood 6013. South Africa. 
14 Plon, S. 2000. Personal commun. School of Biological Sci- 
ences, Thomas Building, Level 2, Univ. Auckland, Private Bag 
92019, Auckland, New Zealand. 
1:1 Barros, N. 2000. Personal commun. Center for Marine Mam- 
mal and Sea Turtle Research, Mote Marine Laboratory, 1600 
Ken Thompson Parkway, Sarasota, FL 34236-1096. 
obtained in our study, are probably evenly distributed 
with depth over the continental slope and deep Gulf. These 
results were surprising in light of this species’ spatial dis- 
tribution in the northern Gulf of Mexico (Fig. 4). Local 
maxima in group encounter rates occurred southwest of 
Panama City and along the Florida Escarpment northwest 
of Key West. The coherent pattern in Figure 4 strongly 
suggests the existence of high-use areas for this species, 
but the characteristics that make these regions attractive 
to pantropicai spotted dolphins were not observed in 
the chosen set of environmental variables used in our 
study. Davis et al. 16 reported that oceanic stenellids (pan- 
tropical spotted dolphins, striped dolphins [ Stenella coe- 
ruleoalba ], spinner dolphins [ Stenella longirostris] , and 
Clymene dolphins [ Stenella clymene ]) were more fre- 
quently encountered in cyclonic, cold-core eddies and less 
frequently encountered in anticyclonic, warm-core eddies 
than expected based on the distribution of the GulfCet 
program sighting effort in the northern Gulf of Mexico. 
Although no such relationship was detected in our study, 
species grouping, confounding by other environmental or 
behavioral factors or temporal variability in habitat associ- 
ations (or both) or prey availability could easily account 
for the apparent discrepancy between these two studies. 
Sperm whale 
Like the pantropicai spotted dolphin, the sperm whale was 
never encountered on the continental shelf and appears 
to have a roughly even distribution with respect to depth 
over the continental slope and deep Gulf. The distribu- 
tions of these two species with respect to the depth of 
the 15°C isotherm were significantly different, however, 
and the canonical LDF analysis suggested that this vari- 
able contributed to the separation between sperm whales 
and the other oceanic cetaceans (Fig. 6B). Sperm whales 
were encountered much less frequently in regions where 
the depth of the 15°C isotherm was quite deep (Fig. 11B), 
which suggests that this species avoids the interior of anti- 
cyclonic, warm-core features such as the Loop Current or 
warm-core mesoseale eddies. Waring et al. ( 1993) and Grif- 
fin (1999) described similar results from studies of sperm 
whale distributions in and around the periphery of warm- 
core eddies associated with the Gulf Stream in the north- 
west Atlantic Ocean. Davis et al. 16 and Biggs et al. (2000) 
reported that sperm whales were not only encountered 
outside of anticylonic features in the northeastern Gulf of 
Mexico, but most of the visual and acoustic contacts with 
16 Davis, R. W„ J. G. Ortega-Ortiz, C. A. Ribic, W. E. Evans, I). C. 
Biggs, P. H Ressler, J. H. Wormuth. R. R. Leben, K. D. Mullin, and 
B. Wursig. 2000. Cetacean habitat in the northern Gulf of 
Mexico. In Cetaceans, sea turtles and seabirds in the north- 
ern Gulf of Mexico: distribution, abundance and habitat asso- 
ciations, vol. II: technical report (R. W. Davis, W. E. Evans, and 
B. Wursig, eds.), p. 217-253. U.S. Department of the Interior, 
Geological Survey, Biological Resources Division, LTSGS/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 Ser- 
vice, 1201 Elmwood Park Blvd., New Orleans, LA 70123-2394.] 
