I ATHERWOOD ET AL ; DISTRIBUTION AND MOVEMENTS OF GRAMPUS GRISEUS 



The rather dramatic decrease in the number of 

 sightings north of about lat. 13° N and the very 

 limited number of offshore sightings in the broad 

 belt from about the latitudes of Cedros and 

 Guadalupe Islands south to approximately 

 Acapulco, appear to reflect an £u-ea of apparent 

 very low density in the species' distribution, since 

 survey effort in the area was heavy even where no 

 sightings were reported. Pronounced distribu- 

 tional gaps in portions of the same ocean area have 

 been documented for Delphinus delphis (Evans 

 1975) and Stenella spp. (Perrin 1975). 



Risso's dolphins appear to occur year-round in 

 offshore waters from about central Baja California 

 northward to about San Francisco. Movements 

 onto the continental shelf of southern California 

 are seasonal and appear to be related to surface 

 temperatures. For example, records of Risso's dol- 

 phins over the continental shelf were more 

 numerous in 1974 than in previous years since 

 1968, despite an equal effort, and more numerous 

 than in 1975, despite increased survey effort in 

 that year. In 1974 and 1975, surface temperatures 

 were unusually high (California Cooperative 



19 



Oceanic Fisheries Investigations ). 



A poorly defined area of apparent low density in 

 distribution, centering at about lat. 43° N, proba- 

 bly reflects generally poor sampling in the area 

 from about San Francisco north to the latitude of 

 Seattle and not any real change in the species' 

 density there. 



Records from lat. 45° to 51° N are most abundant 

 during summer and are primarily ofT the conti- 

 nental shelf. Like the movements onto the south- 

 ern California continental borderland and those 

 into more northern latitudes, this change appears 

 to relate to warming of surface waters. 



The reports of abundance near Monterey in the 

 late 19th century seem inconsistent with modern 

 records of low abundance in the area. It may well 

 be that this indication of the common occurrence of 

 the species in Monterey Bay in the 1870's and 

 1880's represents a holdover of the occurrence of 

 tropical animals in central California in the 1850's 

 (Hubbs 1948). This being the case, the movement 

 of Risso's dolphins north and inshore in some 

 abundance during that period is consistent with 

 behavior in 1974 and 1975 off southern California. 

 Southward movements of the Ball's porpoise. 



"California Cooperative Oceanic Fisheries Investigations. 

 Unpublished data in files of CalCOFl at the Southwest Fisheries 

 Center La Jolla Laboratory, National Marine Fisheries Service, 

 NOAA, La Jolla, CA 92038. 



Phocoenoides dalli. into southern California (Nor- 

 ris and Prescott 1961) and seasonal movements of 

 the right whale dolphin, Lissodelphis borealis 

 (Leatherwood and Walker 1979), and the Pacific 

 whitesided dolphin, Lagenorhynchus obliquidens 

 (Leatherwood and Reeves 1978), in the eastern 

 North Pacific have been similarly linked to sea- 

 sonal changes in water temperature. 



Despite extensive survey effort in the northern 

 temperate and Arctic eastern Pacific, Risso's dol- 

 phins have not been reported north of lat. 51° N. 

 Therefore, since it provided no new data, the 

 summary report of the species' occurrence in the 

 Bering Sea (Clark 1945) is of doubtful accuracy. 



Considered together, these records tend to sup- 

 port Davies' (1963) summary of the species' dis- 

 tribution, at least in the northeast Pacific. It ap- 

 pears, as he contended, to be primarily tropical, 

 extending its range poleward to overlap with 

 temperate forms, though not penetrating as far 

 into high latitudes. Perhaps the most important 

 point supported by these records is the dynamic 

 nature of distribution of this (and probably other) 

 marine mammal species. In addition to well- 

 documented short-term and seasonal movements, 

 there appears to have been a long-term fluctuation 

 in the boundries of species' ranges, apparently in 

 responses to long-term environmental changes. 



ACKNOWLEDGMENTS 



The authors thank all the following institutions 

 and individuals for contributing unpublished 

 data: Fisheries Research Board of Canada 

 (MaCaskie from Pike's records); University of 

 Southern California (Capt. F. Zeischenhene); Mo- 

 clips Cetological Society (K. C. Balcomb); North- 

 west and Alaska Fisheries Center, NMFS, NOAA 

 (C. H. Fiscus, R. L. DeLong, and D. W. Rice); Uni- 

 versity of California, Santa Cruz (T. P. Dohl, J. D. 

 Hall, and L. J. Hobbs); U.S. Fish and Wildlife 

 Service (R. L. Brownell, Jr.); Museum of Verte- 

 brate Zoology, Berkeley (R. E. Jones); Humboldt 

 State College (W. J. Houck; San Diego Natural 

 History Museum (R. M. Gilmore); Los Angeles 

 County Museum (W. A. Walker, D. R. Pattern, and 

 W. A. Samaras); Hopkins Marine Station (J. E. 

 Vandevere); University of Oregon (T. Wahl); 

 USAAJSSR Marine Mammal Cooperative Pro- 

 gram (D. W. Rice and W. E. Evans); and Hubbs-Sea 

 World Research Institute (W. Evans and J. Jehl, 

 Jr.). J. D. Hall, F. G. Wood, M. Schaeffer, and 

 D. W. Rice critically reviewed the manuscript. 



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