nSHERY BULLETIN: VOL 77. NO 4 



TIME OF YEAR 



Figure 6. — Frequency of sightings of Risso's dolphins by month 

 in Zone II in the eastern North Pacific (defined in Figure 5). 



most northern records, the 4 from February to 

 April were associated with water temperatures of 

 12° and 13° C, unusually high temperatures for the 

 season. 



Of the records, 12 published and 191 unpub- 

 lished provided usable estimates of herd sizes. 

 Numbers of animals sighted ranged from 1 to 220, 

 about a geometric mean of 10.65. About 75% of the 

 groups contained fewer than 20 animals (Figure 

 7). No statistically significant differences could be 

 demonstrated among herd sizes from different 

 zones (I, II, III) or different seasons (Mann- 

 Whitney U Test, a = 0.05). 



SUMMARY AND DISCUSSION 



Risso's dolphins are clearly abundant and 

 widely distributed year-round in tropical and 

 warm temperate waters of the northeastern 

 Pacific. 



-I 1 1 r- 



O 



z 



60 



50. 



40 



DETAIL 



Geometric Mean 10. 6S 



rm 



N= LSI 



 '  '"V''" ' " i;i' ''ti , — y-'-i E i - i v : 



2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 

 Herd Size. No of Animals 



N = 1R4 



jpn. 



10(1 120 140 



Herd Size, No of Animals 



160 



ISO 



200 



220 



240 



Figure 7.— Distribution of estimated "herd" sizes of Risso's dolphins, in the eastern North Pacific (1958-75). Inset gives detail for herd 



sizes with  30 animals. 



960 



