FISHERY BULLETIN: VOL. 84, NO. 2 



Based upon the distribution of sightings on our 

 bimonthly aerial surveys, movement of common 

 dolphins into the SCB appeared to follow the net- 

 work of escarpments and seamounts noted by Evans 

 (1971). The major corridor was along the Coronado 

 Escarpment to Thirty-Mile Bank, up to the Cata- 

 lina Escarpment, around both sides of Santa Cata- 

 lina Island, along the western margins of the San 

 Pedro and Santa Monica basins to Santa Cruz and 

 Santa Rosa Islands (Fig. 1). The population front 

 then advanced westward along the southern margin 

 of these islands until reaching the Santa Rosa-Cortes 

 Ridge where it shifted south, spreading out along 

 the western slope of this prominant underwater 

 feature. Some elements of this influx stopped and 

 along the way, increasing summer-autumn popula- 

 tions significantly in the San Pedro Channel, Gulf 

 of Santa Catalina, and, to a lesser extent, in near- 

 shore waters from Dana Point to La Jolla. A sec- 

 ondary pathway was from Forty-Mile Bank in the 

 south, up the San Clemente Escarpment west of San 

 Clemente Island to reach the Santa Rosa-Cortes 

 Ridge area. 



During periods of peak occupancy common 

 dolphin sightings west of long. 119°W were dis- 

 tributed along the western slope of the Santa Rose- 

 Cortes Ridge centered at lat. 33°00'N, long. 

 120°00'W. As waters cooled, the distributional 

 center shifted eastward to locate over the eastern 

 slope of the Santa Rosa-Cortes Ridge at 33°00'N, 

 119°20'W, while a smaller element moved north- 

 westerly to a new location around 33°30'N, 

 120°30'W. With continued cooling of the western 

 waters, the majority of the animals along the east- 

 ern edge of the Ridge appeared to move southeast- 

 erly to merge with existent populations south and 

 east of San Clemente Island. The remaining small 

 number of animals wintering-over moved westward, 

 centering near 33°00'N, 119°30'W, south of San 

 Nicolas Island. 



The destination of common dolphins that moved 

 northwesterly from the summering grounds over 

 the western edge of the Santa Rosa-Cortes Ridge 

 is unknown. However, several pieces of incomplete 

 evidence lead us to believe that they are part of a 

 "pelagic" population that returns in late autumn or 

 early winter to offshore waters over the Rodriguez 

 Seamount or Patton Escarpment. During several 

 midsummer ship surveys and three aerial surveys 

 of offshore waters over the Patton Escarpment and 

 San Juan Seamount, we recorded sightings of large 

 schools of robust-bodied, brilliantly marked, 

 "pelagic" common dolphins. On two occasions, our 

 crew on the catch boat head-netted, brought on 



board, photographed, measured, tagged, freeze- 

 branded, and released, examples of these "pelagic" 

 animals from within schools containing predom- 

 inantly the paler, smaller, nearshore variety of 

 Delphinus. Ships' logs indicate that the presence of 

 these "pelagic" animals increased with distance 

 from shore, and percentages as high as 50% were 

 found in mixed schools of common dolphins at the 

 western boundary of catch trips, usually south of 

 lat. 33°45'N and west of long. 120°00'W. West of 

 the Patton Escarpment, mixed schools were not 

 noted, and the few schools encountered contained 

 only "pelagic" animals (Dohl unpubl. data). 



In summary, this study establishes an extended 

 distributional range of the common dolphin within 

 the SCB, identifies areas of significantly greater 

 seasonal use, and provides seasonal mean popula- 

 tion estimates. Our results confirm the findings of 

 earlier studies that common dolphins move into the 

 SCB following major features of underwater topog- 

 raphy in response to increasing seasonal water 

 temperatures. Observations on surveys also seem 

 to indicate that most of the population moves 

 through the SCB in a generalized counterclockwise 

 direction, and that the western summer-autumn 

 population is augmented by an influx of "pelagic" 

 animals from far offshore. 



ACKNOWLEDGMENTS 



The original data for this paper were collected 

 under contract to the University of California, Santa 

 Cruz, from the Minerals Management Service 

 (formerly a part of Bureau of Land Management), 

 U.S. Department of the Interior. 



The analysis of these data and the development 

 of the distributional model described here were sup- 

 ported by Woodward-Clyde Consultants (WCC), 

 Walnut Creek, CA, by a contract from the Minerals 

 Management Service, Department of the Interior. 



We are grateful to the many individuals involved 

 in the collection of these data: J. D. Bryant, R. C. 

 Guess, J. D. Hall, L. J. Hobbs, M. W. Honig, K. S. 

 Norris, and P. N. Sund. We also thank T. P. Win- 

 field and R. K. Christiansen at WCC for technical 

 assistance, and particularly K. P. Burnham, T. D. 

 Smith, and R. S. Holt for valuable assistance and 

 comments on the manuscript. 



LITERATURE CITED 



Burnham, K. P., D. R. Anderson, and J. L. Laake. 



1980. Estimation of density from line transect sampling of 

 biological populations. J. Wildl. Manage. Manage. Monogr. 



342 



