Henderson et al.: Effects of sea-surface temperature on the occurrence of small cetaceans off Southern California 
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Table 1 
The final best-fit generalized additive models are presented here for each of the 8 species of small cetaceans investigated 
for this study in Southern California waters in 1979-2009. Also included are the restricted maximum likelihood (REML) 
score, explained deviance (Expl. dev.), and residual degrees of freedom (df) for each model. See Appendix 2 for the P-values 
of each variable in these models. The 8 species were the short-beaked common dolphin ( Delphinus delphis), long-beaked 
common dolphin ( D . capensis ), Risso’s dolphin ( Grampus griseus), northern right whale dolphin ( Lissodelphis borealis), 
Pacific white-sided dolphin (Lagenorhynchus obliquidens ), Dali’s porpoise ( Phocoenoides dalli), striped dolphin ( Stenella 
coeruleoalba), and bottlenose dolphin (Tursiops truncatus); a third model for common dolphins incorporated data for both 
the short- and long-beaked common dolphins. Variable abbreviations: DepthMin=minimum depth (m), DepthMean=mean 
depth (m), MaxDepth=maximum depth (m), SeasAv=seasonal averaged sea-surface temperature, ENSO=El Nino-Southern 
Oscillation, and PDO=Pacific Decadal Oscillation. 
Species 
Final model 
REML 
Expl. dev. 
Residual df 
Short-beaked common dolphin 
Quarter + Slope + DepthMean + SeasAv 
754.0 
23.9% 
642 
Long-beaked common dolphin 
Quarter + DepthMax + ENSO + SeasAv 
211.7 
57.4% 
652 
Both common dolphins 
Quarter + SurveyType + Slope + DepthMax 
+ PDO + SeasAv 
2751.8 
32.5% 
2415 
Risso’s dolphin 
Quarter + SurveyType + Slope + DepthMean 
+ ENSO + SeasAv 
644.7 
36.6% 
2421 
Northern right whale dolphin 
SurveyType + DepthMax + ENSO + SeasAv 
270.3 
26.1% 
2428 
Pacific white-sided dolphin 
Quarter + SurveyType + Slope + DepthMean 
+ PDO 
706.2 
21.2% 
2419 
Dali’s porpoise 
Quarter + SurveyType + Slope + DepthMean 
+ PDO + SeasAv 
726.8 
27.5% 
2423 
Striped dolphin 
SurveyType + DepthMin 
88.3 
41.8% 
2437 
Bottlenose dolphin 
SurveyType + Slope + DepthMean + PDO 
376.0 
46.4% 
2429 
encountered in a year, the group size would not neces- 
sarily also increase or decrease. 
The best-fit models are shown in Table 1. Values for 
explained deviance ranged between 21.2% and 57.4% 
across species. A summary of group sighting rates 
is given in Table 2. Six of the 9 models included the 
Quarter variable, indicating intra-annual variation in 
the SPUE for each species. Of the 9 models, 7 models 
included the SurveyType variable; the 1987-2004 Cal- 
COFI cruises ranked lowest and the SWFSC cruises 
ranked highest in sighting numbers for most species. 
Of the 9 models, 6 models included the seasonal SST 
average variable, and 7 models also included either the 
PDO or ENSO index. The latter results indicate the 
importance of those temperature fluctuations on small 
cetacean distribution. All models also included at least 
one depth metric, previously shown to be an important 
predictor variable for Southern California cetaceans 
(e.g., Becker, 2007). Finally, 5 of the 9 models included 
slope as a predictor. 
Common dolphins 
Three different models were used for common dol- 
phins: both species of common dolphin in a single 
combined category, short-beaked common dolphin, 
and long-beaked common dolphin. The similarities 
in the model results for both common dolphins and 
the short-beaked common dolphin indicate that the 
data for the combined category likely are dominated 
by sightings of short-beaked common dolphins. Com- 
mon dolphins were associated with seasonal SSTs of 
about 14-18°C in all 3 models, indicating possible 
avoidance of extremely warm or cold temperatures 
(Fig. 4). For all common dolphin groups, most sight- 
ings occurred in the summer and fall, and generally 
the fewest sightings occurred in the spring. Depth 
was an important predictor of common dolphin dis- 
tribution in all 3 models, and slope was included in 
the models for the combined category and the short- 
beaked common dolphin. Long-beaked common dol- 
phins were found almost exclusively inshore, and 
sightings of short-beaked common dolphins and dol- 
phins in the combined group were recorded both in- 
shore and offshore in areas with shallow slopes. The 
model for both common dolphins combined showed a 
very slight increase in sightings with negative PDO 
anomalies, although the overall response was fairly 
flat (Fig. 4). 
