Baumgartner et al.: Cetacean habitats in the northern Gulf of Mexico 
225 
Table 2 
Number of 1-km transect sections (units of effort) with valid data for each environmental variable. 
Variable 
1992 
1993 
1994 
Total 
Depth 
3454 
2373 
3274 
9101 
(100%) 
Depth gradient 
3454 
2373 
3274 
9101 
(100%) 
Surface temperature 
1414 
2245 
2915 
6574 
(72%) 
Surface temperature standard deviation 
688 
1084 
498 
2270 
(25%) 
Depth of 15°C isotherm 
2357 
1939 
2669 
6965 
(77%) 
Surface chlorophyll concentration 
2844 
2277 
2859 
7980 
(88%) 
Zooplankton biomass 
2127 
1103 
1419 
4649 
(51%) 
Table 3 
Correlation matrix of environmental variables. Correlation coefficients for surface chlorophyll and zooplankton biomass were com- 
puted from the station samples (not from the interpolated fields). SD = standard deviation. 
Variable 
Depth 
Depth 
gradient 
Surface 
temperature 
Surface 
temperature 
SD 
Depth of 
15°C 
isotherm 
Surface 
chlorophyll 
Depth gradient 
-0.003 
Surface temperature 
0.104 
0.098 
Surface temperature SD 
-0.067 
0.032 
0.019 
Depth of 15°C isotherm 
0.297 
-0.139 
0.199 
0.365 
Surface chlorophyll 
-0.341 
0.013 
-0.250 
-0.165 
-0.166 
Zooplankton biomass 
-0.224 
-0.064 
-0.192 
0.141 
-0.380 
0.710 
indicates P < 0.05. 
indicates P < 0.01. 
Table 4 
Number of group sightings in ), sighting rate (group sightings per 100 km) and mean, 
standard deviation (SD), minimum (Min) and maximum (Max) group size of the five 
most frequently encountered species or species groups. 
Group size 
Species 
n 
Sighting rate 
Mean 
SD 
Min 
Max 
Bottlenose dolphin 
89 
0.98 
14.7 
22.7 
1 
150 
Risso’s dolphin 
67 
0.74 
10.8 
7.3 
2 
40 
Kogia spp. 
56 
0.62 
2.1 
1.6 
1 
8 
Pantropical spotted dolphin 
107 
1.18 
51.8 
42.4 
3 
180 
Sperm whale 
43 
0.47 
2.5 
1.9 
1 
11 
their surface temperature signa- 
tures were strong. 
Many of the environmental vari- 
ables in the effort data set were 
significantly correlated with one 
another (P<0.05), but correlation 
coefficients were less than 0.3 in 
most cases (Table 3). The high 
correlation between surface chlo- 
rophyll and zooplankton biomass 
was strongly influenced by stations 
on the continental shelf where 
both surface chlorophyll and zoo- 
plankton biomass were typically 
quite high. The surface chloro- 
phyll measurements collected on 
the shelf were significantly high- 
er than those from oceanic waters 
(Mann-Whitney, £7=4.44, P<0.0001), whereas the median 
zooplankton biomass measured on the shelf ( 10.1 cc/100 nr 3 ) 
was almost twice as large as the median of the oceanic obser- 
vations (5.4 cc/100 m 3 ). The correlation coefficient between 
the surface chlorophyll and zooplankton biomass measured 
in oceanic waters was not significantly different from zero 
(P>0.05). This contrast between the continental shelf and 
more oceanic waters was also manifested in the inverse re- 
lationships detected between depth and surface chlorophyll 
and between depth and zooplankton biomass. 
Of the 614 cetacean groups sighted between 1992 and 
1994, the most frequently encountered species were the 
bottlenose dolphin, Risso’s dolphin, Kogia spp., pantropi- 
cal spotted dolphin, and sperm whale (Table 4). The Ko- 
