RjSSO's DOLPHIN 

 Crampus griesus (G. Cuvier, 1812) 



The cosmopolitan Risso's dolphin, also known as grampus, has been 

 little studied anywhere. In the eastern Pacific it is known to occur from 

 as far north as latitude 5 5 49 N m the Gulf ot Alaska and as far south as 

 central Chile. It appears to be primarily a tropical to mid-temperate 

 pelagic species, occurring consistently relatively far offshore (beyond 

 the 100 fathom curve). Risso's dolphins appear at present to occur 

 yearround m offshore waters from about central Baja California 

 northward to about San Francisco. However, movements farther north 

 and onto the continental shelf, mcludmg the SCB, appear to be 

 seasonal and related to changes in sea surface temperatures. The species 

 was virtually absent from the SCB during a protracted cold period but 

 is now present, at least seasonally, m significant numbers. 



In the SCB, these large dolphins are generally seen along the 

 continental rise west ot San Miguel and San Nicolas islands. However, 

 we and other workers have seen them occasionally closer to shore, near 

 Dana Point, in the Santa Catalina Channel, oft Santa Barbara Island, in 

 the Santa Barbara Channel and more frequently between Santa Cata- 

 lina and San Clemente islands, primarily in late winter and early spring. 

 Risso's dolphins are almost exclusively scjuid eaters and their habitua- 

 tion of the pelagic zone wherever they occur evidently reflects that food 

 preference. Their appearance m inshore waters has occurred when 

 pelagic squid have been available in larger than usual quantities over the 

 continental shelt. 



Risso's dolphins seldom occur in large herds 1 more than 200 have 

 been seen on occassion but the vast majorit\' ot herds are 50 or fewer 

 and groups ot about ten are most common i. Wherever they occur, 

 however, Risso's dolphins are gregarious and often assemble with other 

 species, particularly northern right whale dolphins and pilot whales. 



Figure 2^. A ^jroup of Risso's dolphins seen from aircraft oft San Nicolas 

 Island in .August 19-9. Photo courcesv Hubhs Marine Research Center. ,1 



Figure 24. Risso's dolphins, absent or scarce in the SCB during cold water periods, are relatively abundant when warm water currents dominate. Their 

 movements, both local and long distance, are thought to be related to movements of pelagic squid. Photo by P. C. Howorth. i 



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