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sperm whales during the GulfCet II focal cruises were in 
regions characterized by cyclonic mesoscale features. 
Jaquet ( 1996) reviewed a variety of sperm whale habitat 
studies that seemed to have contradictory conclusions re- 
garding the primary oceanographic processes influencing 
sperm whale distribution (namely upwelling and down- 
welling). Jaquet attributed these discrepancies to a prob- 
lem of defining the appropriate spatial and temporal scales, 
and she and others illustrated this point by demonstrating 
a varying but positive correlation between historical sperm 
whale catches and surface chlorophyll over increasing tem- 
poral and spatial scales in the equatorial Pacific (Jaquet et 
al., 1996). These results seem to indicate that upwelling, 
which contributes to increased surface phytoplankton bio- 
mass, is a predominant factor in influencing sperm whale 
distribution in the equatorial Pacific. Historical catches in 
temperate waters, however, are not at all correlated with 
surface chlorophyll (see Fig. 1 of Jaquet, 1996 and Fig. 1 
of Jaquet et ah, 1996) which suggests that other oceano- 
graphic processes or physiographic influences may be im- 
portant (e.g. downwelling or biological-physical processes 
associated with continental slopes). At comparatively short 
time scales and small spatial scales, we found no evidence 
to suggest a relationship between the distribution of sperm 
whales and surface chlorophyll in the northern Gulf of 
Mexico. Even at longer temporal and larger spatial scales, 
we would expect this same result because the oceanic Gulf 
is persistently oligotrophic both in time and space (Miillei- 
Karger et ah, 1991; Longhurst, 1998). 
Berzin ( 1971) examined harvest records from the world- 
wide sperm whale fishery and suggested that sperm whale 
distribution was closely linked to processes that support- 
ed the meso- and bathypelagic food webs. Because sperm 
whales feed almost exclusively on mesopelagic or demer- 
sal cephalopods (Clarke, 1986, 1996), they probably aggre- 
gate in areas where these prey are abundant. These deep- 
water prey species are entirely dependent on the rain of 
organic matter from the surface for their sustenance and 
so these species will be found in regions where the export 
of detritus from the surface is enhanced. This process oc- 
curs in convergence zones where downwelling forces sur- 
face biomass and oxygen into the deep ocean, such as in 
the middle of anticyclonic eddies, at the peripheries of cy- 
clonic eddies, to the right (left) of surface ocean currents 
in the northern (southern) hemisphere, in the middle of 
the large-scale anticyclonic ocean gyres (e.g. the Sargasso 
Sea), or along fronts where surface water masses abut. 
The global sperm whale distribution maps provided by 
Townsend ( 1935 ) and Berzin ( 197 1 ) do indeed suggest that 
this species was frequently harvested in or near large- 
scale oceanic convergence zones, especially along the sub- 
tropical convergence zones and the Antarctic polar front. 
The distribution of sperm whales in the northern Gulf 
of Mexico and northwestern Atlantic Ocean (Waring et al., 
1993; Griffin, 1999) seems contradictory to Berzin’s hy- 
pothesis, however. Features such as the Loop Current or 
warm-core eddies rotate anticyclonically and have conver- 
gent centers in which downwelling occurs. According to 
Berzin’s hypothesis, the interior of these features would be 
favorable to sperm whales because of the enhanced export 
of surface biomass to the deep ocean and the resultant in- 
crease in prey species. The interior of anticyclonic eddies 
in the northern Gulf of Mexico are, however, low in surface 
zooplankton biomass (Biggs, 1992). Although the rate of 
detrital export to the deep is enhanced by increased verti- 
cal velocities within these features, the amount of biomass 
actually exported may be too small to support large popu- 
lations of deep-water prey. 
Another possible explanation for the distribution of 
sperm whales with respect to the depth of the 15°C iso- 
therm is related to the availability of prey. Berzin (1971) 
characterized cephalopods as thermophilic and thus in- 
dicated that they are distributed within a narrow range 
of ocean temperatures according to their species-specific 
thermal requirements or to the thermal requirements of 
their prey. These requirements not only govern the hori- 
zontal distribution of cephalopods, but their vertical dis- 
tribution as well. Because warm-core features are charac- 
terized by depressed isotherms (e.g. Fig. 3), cephalopods 
within these features may be hundreds of meters deeper 
in the water column than in the waters outside these fea- 
tures. Despite their well-known ability to dive to great 
depths, foraging continuously at greater depths under 
warm-core features would be much more energetically ex- 
pensive than foraging outside these features. Thus, when 
prey abundance inside and outside of warm-core eddies 
are equivalent, sperm whales may feed on prey distribut- 
ed at shallower depths outside of these features to reduce 
their energy expenditure. 
Caveats 
It is important to remember that this study was limited 
to surveys conducted during the spring season. The spa- 
tial distribution of cetaceans may be different in other sea- 
sons because the oceanographic conditions of the northern 
Gulf of Mexico change over the course of the year. The 
northward penetration of the Loop Current into the Gulf 
varies on a quasi-annual basis (Vukovich et al., 1979; Stur- 
ges and Evans, 1983; Vukovich, 1995) and the variability 
in the position of the Loop Current affects the generation 
and positions of both anticyclonic and cyclonic eddies. This 
variability may, in turn, greatly influence the productiv- 
ity and availability of prey species in the eastern Gulf of 
Mexico. In the northwestern Gulf, the slow march of warm- 
core eddies from east to west toward the “eddy graveyard” 
over the continental slope also varies with time and may 
affect the seasonal distribution of cetaceans. Hansen et 
al. 3 observed seasonal differences in cetacean abundance 
in the western and central regions of the northern Gulf 
of Mexico that may have been influenced by temporal 
changes in the local oceanography. 
Another potential limitation of our study was the rather 
coarse environmental sampling. Although the CTD/XBT 
sampling strategy was sufficient to identify the large-scale 
oceanographic features, some of the most biologically sig- 
nificant processes in the oceanic Gulf of Mexico occur on 
smaller spatial scales. In particular, the outer edge of the 
Loop Current is frequently a site of upwelling and these 
divergent features often develop into cyclonic meanders 
