210 
Fishery Bulletin 112(2-3) 
Table S (continued) 
Density 
Species 
Season, depth 
Number of 
groups 
Mean group 
size 
(individuals/ 
1000 km 2 ' 
Uncorrected 
abundance 
CV 
Bottlenose dolphin 
Winter-spring, shallow 
4 
11.2 
22.12 
1580 
0.54 
Winter-spring, deep 
1 
2 
0.97 
161 
0.79 
Summer-fall, shallow 
5 
13.6 
40.32 
2879 
0.69 
Summer-fall, deep 
0 
0.0 
0.00 
0 
NA 
Northern right whale dolphin 
Winter-spring, shallow 
5 
39 
107.31 
7662 
0.50 
Winter-spring, deep 
4 
13.5 
20.30 
3392 
0.49 
Summer-fall, shallow 
1 
6 
6.72 
480 
0.78 
Summer-fall, deep 
2 
25 
11.10 
1855 
0.68 
Dali’s porpoise 
Winter-spring, shallow 
13 
4.8 
45.50 
3249 
0.32 
Winter-spring, deep 
32 
5.3 
48.65 
8128 
0.28 
Winter-spring, all depths 
45 
5.1 
47.71* 
11,378 
0.26 
Summer-fall, shallow 
2 
3 
2.11 
151 
0.58 
Summer-fall, deep 
1 
17 
2.73 
456 
0.78 
encountered marine mammal species in the Southern 
California region. Although seasonal variation was 
seen in cetacean encounters, large numbers of whales 
and dolphins were observed year-round off Southern 
California, both on and between transect lines (Table 
4; Figs. 3 and 4). 
Abundance and density of cetaceans: overall comparisons 
with previous surveys 
Although our analyses of relative density by seasonal 
period and depth are robust for the most commonly en- 
countered species, absolute densities and uncorrected 
abundances reported here may differ from values re- 
ported by the NOAA Southwest Fisheries Science Cen- 
ter (SWFSC) for its previous studies in Southern Cali- 
fornia; those differences primarily are due to 5 factors. 
First, our study relied on data collected by the naked 
eye or with 7x binoculars, hence ESWs by species group 
in our study were calculated as half (or less) of the 
ESWs used for the same species groups from 5 years of 
pooled SWFSC sighting data, which were collected with 
25x binoculars as the primary search method (Barlow 
and Forney, 2007). Second, we assumed that detection 
on the transect line was certain, org(0)=l; this decision 
likely had the greatest negative impact on density of 
cryptic or long-diving species, like sperm whales. Third, 
we had a relatively high proportion of sightings that 
were not identified to species, and we did not prorate 
unidentified cetaceans, thinking that it would be bet- 
ter to compute a best estimate of cetaceans positively 
identified to species than to make assumptions about 
the detectability of unidentified and identified species. 
Fourth, we used uncalibrated group-size estimates — an 
approach different from the one for SWFSC cruises 
in which observers make group-size estimates inde- 
pendently and each observer is “calibrated” with the 
use of photogrammetry of select sightings (Gerrodette 
and Perrin 4 ). Carretta et al. (2011b) stated that uncali- 
brated group-size estimates could result in estimated 
counts that were 50% lower than actual group sizes. 
Fifth, we did not correct for reactions to vessel ap- 
proach by small cetaceans — an issue that is primarily 
a concern with the Dali’s porpoise and vessel-attracted 
dolphin species, like the short-beaked common dolphin. 
Lastly, the SWFSC southern stratum, with an area of 
318,500 km 2 , is larger than the study area of the Cal- 
COFI surveys by 25%. We compared density and abun- 
dance of species from these 2 studies because the 2 ar- 
eas overlap by 75% and the CalCOFI study area occurs 
completely inside the SWFSC southern stratum. That 
said, our study provides the most recent and best repli- 
cated shipboard assessment of seasonal densities for 11 
species of cetaceans off Southern California, including 
3 species of baleen whales, the sperm whale, 6 species 
of delphinids, and the Dali’s porpoise. 
Baleen whales The most commonly occurring large 
whales that used this area for feeding were fin, hump- 
back, and blue whales; because of their presence along 
the coast in greater numbers during the summer and 
fall, compared with other seasons, these species have 
been well represented in previous line-transect and 
4 Gerrodette, T., and C. Perrin. 1991. Calibration of ship- 
board estimates of dolphin school size from aerial photo- 
graphs. Southwest Fish. Sci. Cent. Admin. Rep. W-91-36, 73 
P- 
