fiuL !-a 



Whii. .iJ.u ,;,., 



1-'' 



t init;KiUung ^!.u whales. Photo ott And^:.ii\i LslanJ. Icbruarv w^^s b\ S. Leachcrwood. 



within several miles of Anacapa and Santa Cruz islands on both coasts, 

 west and south of San Miguel Island, around Santa Barbara, San 

 Nicolas and San Clemente islands, in the San Pedro Channel and near 

 the Coronado Islands. During the past several \'ears we have also found 

 stranded white-sided dolphins at San Miguel Island. 



Within their broad range. Pacific white-sided dolphins appear to 

 fluctuate in abundance seasonally. Earlier researchers speculated that 

 such fluctuations were evidence of shifts m population centers off 

 Northern Calitornia, in the SCB and off Baja California's west coast. 

 We have been unable to unequivocallv demonstrate migration m any 

 area, although observed peaks m abundance suggest regular seasonal 

 influxes into waters north ot 40 N in spring and summer and between 

 25 N and 10 N in autumn. 



Apparently the only two areas in the Northeast Pacific in which 

 observed distribution patterns are clearK' not an artifact of sampling 

 effort are the waters off Baja California and those in the SCB. Evidence 

 for seasonal use is most convincing off Southern California, where 

 peak numbers occur inshore from November through April and lesser 

 numbers for the remainder of the year. It has been suggested that such 

 movements are related to changes in prey distribution and water 

 temperature. Northern anchovy. Pacific whiting ; hake i, and market 

 squid appear to be the most frequently consumed prey of white-sided 

 dolphins. These dolphins apparently do most of their feeding at night 

 in the epipelagic and, to a lesser extent, mesopelagic zones. White- 

 sided dolphins are, however, also known to eat sanddab, eulachon, 

 night smelt, )uvenile rockfish and plamfin midshipman. These observa- 

 tions suggest that white-sided dolphins may be generalized predators 

 feeding on both inshore schooling and bottom duelling prev. 



Small numbers of white-sided dolphins have been killed inciden- 

 tally during commercial fishing operations for tuna, anchovy, and 

 salmon and squid in subtropical, mid-temperate and northern tempe- 

 rate waters, respectiveh'. 



Among the most gregarious of eastern Pacific delphinids, the white- 

 sided dolphin occurs in herds of up to several thousand, though groups 

 of less than tv\'o hundred are more usual. They often intermix with 

 herds of northern right whale dolphins, Dalls porpoises, pilot whales, 

 common dolphins, Rissos dolphins and bottlenose dolphins and have 

 been seen with gray and humpback whales. Female white-sided dol- 

 phins apparently calve in summer after a gestation period of about ten 

 months; little else is known about their reproductive biology. 



No population estimate is possible with currently available data. On 

 aerial surieys in the well studied SCB, white-sided dolphins were 

 observed to occur at a peak frequency of 1.42 individuals per nautical 

 mile flown. In a region off Baja California they have been estimated to 

 occur in densities of up to 0.06 individuals per square nautical mile. 

 Based on these surveys, white-sided dolphins appear to be the second 

 or third most abundant delphinid in Southern California waters in 

 winter, behind common and perhaps northern right whale dolphins. 

 The ijoo herds sighted during surveys off the Pacific coast of North 

 America contained an average of 88 individuals; herds were signifi- 

 cantly larger, however, in southern and northern portions than in the 

 central portion of their range. 



Figure jy. Three right whale dolphins 1 upper left 1 ride the bow wave with 

 some Pacific white-sided dolphins. The two species are frequently found 

 travelling together. Photo by S. Stanshurw from Leathenvood et al.. 1982. 

 Figure 26s top. 



iiMm 



27 



