Douglas et al. Seasonal distribution and abundance of cetaceans off Southern California 
215 
Long-beaked common dolphins were the fourthmost 
commonly encountered and secondmost abundant small 
cetacean in the study area. Distribution of long-beaked 
common dolphins was limited to waters near the Cali- 
fornia coast or Channel Islands — a result that is consis- 
tent with findings that this species is commonly found 
within ~93 km of the coast and ranges from Baja Cali- 
fornia to central California, with the highest densities 
observed during warm-water events throughout their 
range (Heyning and Perrin, 1994). The uncorrected 
abundance estimate from summer-fall surveys at shal- 
low depths for long-beaked common dolphins (49,118 
individuals [CV=0.52]) was about 3 times higher than 
Barlow’s (2010) abundance estimate determined from 
pooled data from line-transect surveys conducted during 
1991-2008, but our mean group size (217.2 individuals 
[CV=0.52]) and abundance estimate were much lower 
than his mean group size and abundance estimates from 
the 2009 line-transect survey, where corrected mean 
group size was 481.0 individuals and abundance was 
111,738 individuals (CV=0.44) (Carretta et al., 2011a). 
Our estimates are likely negatively biased, given the 
relatively large number of common dolphin sightings 
that were not identified to species. However, the 2009 es- 
timates were much greater than the results from earlier 
surveys, and there was an indication that the moderate 
El Nino event in 2009 may have caused an influx of 
dolphins from the south. Our surveys, conducted during 
2004-08, show that this species is present year-round 
but increases 22-fold in abundance during the summer- 
fall period, indicating that dolphins are shifting south 
for the winter and spring. 
Although the number of sightings was insufficient 
from the winter-spring period to quantify year-round 
seasonality of long-beaked common dolphins, this study 
is the first to provide evidence of seasonal habitat use 
for the 2 common dolphin species found along Southern 
California. For previous publications that have docu- 
mented seasonality, aerial surveys were used for cold- 
water seasonal surveys; however, at the time of those 
studies, there was not an effective method for distin- 
guishing the 2 species from an aerial platform (Dohl 
et al., 1986; Forney et al., 1995; Forney and Barlow, 
1998). In marked contrast to the ratio of encounters of 
short- and long-beaked common dolphins reported here 
(6:1), Carretta et al. (2011a) encountered the 2 species 
at a 1:1 ratio in 2009; their observation supports the 
hypothesis of a dramatic shift or pronounced interan- 
nual variability from the preceding years off Southern 
California. 
Pacific white-sided dolphins were encountered in all 
seasons, with the greatest abundance estimate (14,898 
individuals [CV=0.21]) from both depth categories com- 
bined in the winter-spring seasonal period. Although 
density was markedly different between the shallow 
and deep categories during the winter-spring season, 
abundance was fairly constant throughout the entire 
study area. During the summer-fall period, we found 
that density and abundance (9.24 individuals/1000 km' 2 ; 
2204 individuals [CV=0.35]) decreased by almost 15% 
from the previous winter-spring period, with greater 
abundance in shallow waters than in deep waters. Bar- 
low and Forney (2007) published a similar pooled abun- 
dance estimate of 2196 individuals (CV=0.39) for sur- 
veys conducted in all depths during the summer-fall 
period during 1991-2005. From the data on encounters 
by season, we found that a significant shift into deep 
water occurred during the winter-spring period (Table 
7, Fig. 7). Along the coast of California, the 2 forms of 
Pacific white-sided dolphins are primarily found in wa- 
ters over the continental shelf and slope (Forney, 1994). 
The northern form is thought to enter coastal South- 
ern California waters during the winter months and to 
congregate with the southern form (Walker et al., 1986; 
Lux et al., 1997; Soldevilla et ah, 2011). Because we 
were unable to differentiate between the 2 forms, it is 
possible that the increase in observed abundance dur- 
ing the winter-spring season was a result of capturing 
both forms that use the study area rather than captur- 
ing only the southern form. 
Risso’s dolphins were encountered year-round in 
shallow water, with abundance estimates of 2546 indi- 
viduals (CV=0.36) for the winter-spring period and of 
279 individuals (CV=0.55) for the summer— fall period. 
Our findings agreed with those from visual surveys 
that found high seasonal variability in occurrence and 
distribution of this species off California (Shane, 1994; 
Forney and Barlow, 1998; Kruse et ah, 1999; Benson 
et ah, 2002; Barlow and Forney, 2007) and that their 
abundance along the California coast could be an or- 
der of magnitude higher during the winter than dur- 
ing the summer (Forney and Barlow, 1998). However, 
further research is needed to understand our results 
in relation to the findings of Soldevilla et al. (2010), 
who found peak Risso’s dolphin echolocation activity off 
Southern California in the fall. 
On the basis of genetics and morphology, bottlenose 
dolphins along the coast of California and elsewhere 
worldwide are split into offshore and coastal popula- 
tions (Hansen, 1990; Carretta et ah, 1998; Defran and 
Weller, 1999; Bearzi et ah, 2009; Perrin et ah, 2011). 
The Southern California coastal population typically is 
encountered within 500 m of shore (this species was 
sighted within that boundary 99% of the time during 
a previous study; Hanson and Defran [1993]), and the 
offshore population is found outside of a few kilome- 
ters from the mainland. The mean distance from a 
land mass that bottlenose dolphins were recorded in 
this study was 34 km; the minimum distance was just 
over 2 km. The study area did not include nearshore 
waters sufficiently to encounter coastal bottlenose; 
therefore, we assume that our abundance estimate is 
for the offshore bottlenose dolphin population. For our 
stratum of the summer-fall period and shallow depth, 
the abundance estimate (2879 individuals [CV=0.69]) 
is greater than Barlow and Forney’s (2007) abundance 
estimate (1831 individuals [CV=0.47]) for this popula- 
tion off Southern California during the same period. 
