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Fishery Bulletin 112(2-3) 
(e.g., Dohl et al., 1986; Barlow, 1995; Forney and Barlow, 
1998; Forney, 2000; Barlow and Forney, 2007; Becker, 
2007). Risso’s and common dolphins preferred waters of 
intermediate and warmer temperature (14-20°C) (For- 
ney, 2000; Reeves et al., 2002; Becker, 2007). In con- 
trast, sightings of Dali’s porpoises, Pacific white-sided 
dolphins, and northern right whale dolphins peaked in 
the cool spring season or with cool SSTs. In addition, 
long-beaked common, bottlenose, and Risso’s dolphins 
and Dali’s porpoises showed a preference for inshore 
or island-associated waters. Short-beaked common and 
Pacific white-sided dolphins were observed both inshore 
and slightly offshore. Northern right whale dolphins 
were associated with the slope region, and striped dol- 
phins were observed only in deep offshore waters. The 
relationship between SST and depth is complex and dif- 
ficult to separate, and these models likely oversimplify 
the observed trends. However, these results do seem to 
indicate some habitat or resource partitioning is occur- 
ring because these small cetacean species presumably 
follow preferred water conditions and prey. 
Although the seasonal distribution patterns here 
are generally consistent with those found by Forney 
and Barlow (1998) for temperate species, an increase 
in common dolphin sightings was observed in that 
study in winter rather than in summer for 1991-92. 
In contrast, a summer peak in sightings for common 
dolphins was found by Dohl et al. (1986). Our results, 
however, support the findings of both of these studies. 
ENSO was included as a predictor in the model for 
the long-beaked common dolphin. The strong El Nino 
that occurred in 1991-92 may explain the increase in 
winter sightings for common dolphins in the surveys 
conducted by Forney and Barlow (1998) over that time 
period. If the winter of 1991-92 was uncharacteristi- 
cally warm, then there may have been more common 
dolphins present than usual at that time of year. In 
contrast, the 1975-78 surveys conducted by Dohl et al. 
(1986) overlapped with the 1976-77 PDO regime shift 
from cool to warm; this shift could account for the in- 
crease in common dolphins during the warmer summer 
months of 1975-78. 
Patterns of temperature oscillation 
Temperature fluctuation patterns like ENSO, PDO, and 
the North Atlantic Oscillation have been documented 
to affect the prey of marine animals. An example of 
this effect is the strong relationship between the 
North Atlantic Oscillation, the life cycle of the copepod 
Calanus finmarchicus, and the recruitment of larval 
Atlantic Cod (Gadus morhua) that prey on copepods 
(Stenseth et al., 2002). Atlantic Cod in turn are a ma- 
jor food source for the gray seal (Halichoerus grypus), 
and Calanus spp. are an important prey for the North 
Atlantic right whale (Eubalaena glacialis) (Wishner et 
al., 1995; Mohn and Bowen, 1996). Calanoid copepods in 
the California Current system also have exhibited pop- 
ulation-level step changes in abundance in response to 
strong ENSO events and PDO shifts (Rebstock, 2002). 
For example, during the PDO phase switch in the late 
1970s, 28% of the copepod species sampled increased in 
abundance. In contrast, around 1990 a biological step 
change occurred in copepod populations, when 28% of 
the species declined in abundance. 
Population fluctuations of small pelagic fishes, such 
as anchovies ( Engraulis spp.) and Pacific Sardine 
( Sardinops sagax ), are also correlated strongly with 
both ENSO and PDO indices in the California Current 
system and in the Peru-Chile Current (Tibby, 1937; 
Hubbs, 1948; Niquen and Bouchon, 2004; Lehodey et 
al., 2006). These fish species are prey for many species 
of cetaceans in the California Current, including the 
short- and long-beaked common dolphins, bottlenose 
dolphin, Pacific white-sided dolphin, and Dali’s por- 
poise (e.g., Stroud et ah, 1981; Walker and Jones, 1993; 
Heise, 1997; Amano et al., 1998; Osnes-Erie, 1999). 
Isolated instances of cetaceans changing their dis- 
tribution patterns have been noted during and after 
strong climatic events. One example is the permanent 
expansion of the northern extent of the range of coastal 
bottlenose dolphins along the California coast during 
the 1982-83 El Nino (Defran et al., 1999). Another ex- 
ample is the increased abundance of humpback whales 
(Megaptera novaeatigliae) in Monterey Bay during the 
1997-98 El Nino (Benson et al., 2002). SST fluctuations 
have been shown to affect the distribution and com- 
munity composition of seabirds in the California Cur- 
rent system as well (Hyrenbach and Veit, 2003; Yen et 
al., 2006). A decline of 2% per year in overall seabird 
density was recorded for the last 25 years — a drop that 
was attributed to declines in nearshore abundance of 
forage fishes (Sydeman et ah, in press). 
The models for most species included the PDO and 
ENSO indices as significant variables, although they 
were not strong predictors in most cases. During posi- 
tive PDO and ENSO phases, upwelling and productiv- 
ity decrease while water temperature increases, partic- 
ularly as warm equatorial waters are pushed poleward 
and the California Current system is found closer in- 
shore (Sette and Isaacs, 1960; McGowan, 1985). These 
conditions may explain the apparent association of the 
Dali’s porpoise and Pacific white-sided dolphin with 
positive PDO indices. These species may be pushed 
closer to shore by the contraction of the California 
Current, or they could be concentrating in the remain- 
ing areas of productivity, as has been hypothesized for 
the increase in rorquals in Monterey Bay during the 
1997-98 El Nino (Benson et ah, 2002). 
Alternately, the patterns observed here may re- 
flect changes that occur in other parts of these spe- 
cies’ ranges. For example, during negative, cool PDO 
phases, the overall range of warm-temperate species 
may contract southward; therefore, a slight increase in 
the number of common dolphins and even bottlenose 
dolphins may occur during this phase. Likewise, if the 
cold-temperate species range as far south as Baja dur- 
ing negative PDO and ENSO periods, then their ranges 
