FISHERY BULLETIN VOL 77. NO 4 



dolphins in the portion of the eastern Pacific from 

 the Equator and long. 145° W north and east re- 

 spectively, from strandings, collections, and sight- 

 ing records, and examines the data for patterns in 

 distribution, movement, and seasonal occurrence. 



METHODS 



Inherent in the approach to this paper is the 

 opinion that for this as for other areas, there are 

 numerous scientists and organizations which 

 have small amounts of information of little sig- 

 nificance alone, but when the data are combined, 

 they can yield a better understanding of what is 

 known about a given cetacean species (e.g., see 

 Leatherwood and Walker ( 1979) on Lissodelphis 

 borealis). 



We reviewed previously published records of 

 at-sea sightings of Risso's dolphins in the study 

 area (Table 1). We then examined over 250 previ- 

 ously unpublished reports of sightings of the 

 species in the area from 1958 to 1975 for reliabil- 

 ity of identification. Interviews with observers and 



*A summary of verified records of observations of Grampus 

 griseus in the northeast Pacific is available from Leatherwood or 

 Perrin. 



photos assured us of the accuracy of most records. 

 Descriptions of animals with slate gray to nearly 

 all-white coloration, extensive scarring, a bifur- 

 cated melon, and a prominent dark dorsal fin, all 

 distinctive characteristics of Risso's dolphins 

 (Figure 1), aided in verification of the remainder. 

 We discarded questionable records. 



Many of the reports included estimates of herd 

 size. Since many of these were stated as ranges 

 (e.g., 30-40 animals), we used the midpoint of each 

 estimate. If the estimate was such that the mid- 

 point was ahalfnumber (e.g.,an estimate of 10-15 

 animals) we took the lower of the numbers (e.g., 

 12). 



Some records also included measurements of 

 sea surface temperature at or near the location of 

 the observation. The few of those most important 

 to interpreting apparent trends in the more 

 northern portions of the study area were used, 

 along with annual summaries of temperature 

 trends. 



Incidental sighting records alone cannot be used 

 to reliably determine trends in distribution, 

 movements, or abundance. Data on sighting effort 

 are essential. Although a few major marine sur- 

 veys have been conducted in the study area, effort 

 is difficult to quantify for most other sources of 



Table 1. — Previously published at-sea sightings of Risso's dolphins in the eastern North Pacific. In the few cases 

 where collections were reported, as in Orr (1966), herds from which animals were collected are not included as 

 sight records. 



Figure l. — Risso's dolphins off south- 

 em California, 1973. The animal's dis- 

 tinctive whitish head (in adults), scar- 

 ring, and high subtriangular dorsal fin 

 enhance the reliability of "incidental" 

 observational records. ( Photo by G. E. 

 Lingle, courtesy Naval Oceans Systems 

 Center, San Diego, Calif.). 



