Henderson et al.: Effects of sea-surface temperature on the occurrence of small cetaceans off Southern California 
171 
may contract northward during positive PDO phases, 
leading to an increase in sightings of Dali’s porpoises 
and Pacific white-sided dolphins in Southern California 
waters. 
Implications in regard to climate change 
We have demonstrated changes in distributions of 
small cetaceans on scales of months to decades. De- 
spite a limited understanding of the mechanisms be- 
hind those changes, the model results may help cre- 
ate a basis for understanding the potential effect of 
climate change upon these species. Studies of climate 
change in the California Current system indicate that, 
in addition to increasing temperatures, a rise in atmo- 
spheric carbon dioxide levels is predicted to lead to 
more intense upwelling (Bakun, 1990; Snyder et al., 
2003), stronger thermal stratification, and a deepening 
of the thermocline (Roemmich and McGowan, 1995). 
These changes may alter large-scale circulation pat- 
terns (Harley et al., 2006). Fluctuations in these physi- 
cal mechanisms will lead to changes in ecosystem dy- 
namics and biodiversity from primary producers to top 
predators (Sydeman et al., 2001; Harley et al., 2006; 
Hooff and Peterson, 2006). 
Globally, species associated with sea ice or with 
highly limited ranges are the most obvious species to 
be affected by changing ocean temperatures and sea 
levels (Moore and Huntington, 2008). However, even 
pelagic species, such as the ones discussed here, are 
likely to be affected (Learmonth et al., 2006; Simmonds 
and Eliott, 2009). For example, as water temperatures 
off Scotland increased, the abundance of common dol- 
phins increased, whereas the number of white-beaked 
dolphins ( Lagenorhynchus albirostris), which are asso- 
ciated with cold water, decreased. Such trends could 
indicate a poleward shift in range for both species (Ma- 
cLeod et al., 2005; Simmonds and Isaac, 2007). In addi- 
tion, an influx of cold freshwater in the northern Gulf 
of Mexico in 2011 may have contributed to an unusu- 
ally high mortality rate in bottlenose dolphins (Char- 
michael, et al. 2012). 
We predicted that the ranges of the common dol- 
phins, Risso’s dolphin, and bottlenose dolphin would 
expand northward as ocean temperatures warmed, es- 
pecially as seasonal, ENSO, and PDO events were com- 
pounded (e.g., a positive PDO with a positive ENSO). 
Conversely, we predicted that the ranges of the Pacific 
white-sided dolphin, northern right whale dolphin, and 
Dali’s porpoise would contract poleward and inshore. 
These patterns have held true for observations made 
during previous shorter-term studies. For example, 
Dali’s porpoises and Pacific white-sided dolphins domi- 
nated the odontocete species assemblage off central 
California in the decade before the strong El Nino of 
1997-98 (Benson et al., 2002; Keiper et al., 2005). 
Keiper et al. (2005) noted that during the strong El 
Nino of 1997-98 there was a deepened thermocline, a 
narrow, inshore distribution of Pacific Sardine eggs, and 
an overall decrease in abundance of macrozooplankton. 
During that El Nino, sightings of Dali’s porpoises were 
greatly reduced, whereas common and Risso’s dolphin 
sightings increased. Furthermore, Pacific white-sided 
dolphin sightings decreased after this period, while 
sightings of common (particularly the long-beaked spe- 
cies) and bottlenose dolphins increased (Keiper et al., 
2005). 
However, over the longer-term, our study showed 
an association of the Pacific white-sided dolphin and 
Dali’s porpoise with positive PDO indices, of common 
and bottlenose dolphins with negative PDO indices, 
and of the northern right whale dolphin with positive 
ENSO indices. These results indicate a more complicat- 
ed relationship between distribution patterns and SST 
than we allowed for in our initial predictions or that 
has been observed on shorter temporal scales. Contin- 
ued monitoring efforts should be made to ensure that 
future changes in distribution or reproductive success 
are documented. 
Model considerations 
The results presented here provide insight into long- 
term distribution trends of small cetaceans over sev- 
eral decades. The results are both supported by and 
build upon the current knowledge base for these spe- 
cies. Nonetheless, we recognize some caveats to this 
study that warrant discussion. 
The PDO and ENSO indices were developed with the 
use of broad regions of the Pacific. Therefore, they may 
not reflect precisely the specific dynamics of the South- 
ern California study area. The seasonal SSTs, although 
averaged for each grid section and quarter, were also 
still quite broad, as was the selected size of grid cells. 
However, this scope was used intentionally to capture 
the large temporal- and spatial-scale dynamics of these 
changing SST patterns, rather than to examine meso- 
scale dynamics on shorter temporal scales. In addition, 
the SST, ENSO, and PDO variables have the potential 
to be correlated, as the indices are similar over time. 
A correlation analysis was conducted, and correlations 
between ENSO and PDO and between seasonal SSTs 
and PDO were detected for some species. In those cas- 
es, they were not included together, and only the most 
significant predictors were included. 
Only one cruise occurred per year before 1987. To 
account for potential differences between Survey Types, 
we repeatedly reran each model while randomly drop- 
ping out data from different years. The results indicat- 
ed that the models were robust against missing years 
of data, and the variation in the number of surveys 
per year did not affect the results. The survey methods 
from each Survey Type were quite different, making 
it a challenge to combine these data sets. However, by 
using only the group SPUE and by limiting our sight- 
ings to the ones made in sea states of 3 or less on the 
Beaufort scale, we tried to make the data as compa- 
rable as possible. 
