FISHERY BULLETIN: VOL. 83, NO. 4 



of the eastern Pacific The patterns of species 

 distribution we describe will not always be obvious. 

 Nevertheless there is evidence from other distribu- 

 tion studies, using data collected from the fishery, 

 that these patterns are real (Perrin et al. 1983). 



The results described in this paper are based upon 

 our personal experiences at sea where the procedure 

 of investigating all cetacean schools and continuously 

 monitoring the physical and biotic environment 

 enabled the development of an ecological under- 

 standing of these dolphins. Continuity in these 

 studies was provided by the fact that on each ship 

 there was at least one experienced observer who par- 

 ticipated during all cruise years. We present our 

 results as an independent, research-ship based, 

 assessment of species distributions and habitat 

 areas. 



RESULTS 



Distribution and Relative Abundance 

 of Dolphin Schools 



Plots of the sighting localities of schools of spot- 



ted, spinner, striped, and common dolphins, obtain- 

 ed during the January-March research cruises, show 

 the geographical distributions of these species dur- 

 ing the northern winter season. Table 1 summarizes 

 the numbers and sizes of the schools which were 

 identified and studied. Geographic locations referred 

 to in the text can be found in Figure 1. 



Spotted and spinner dolphins are often in mixed 

 schools (33.5% of spotted dolphins schools also con- 

 tained spinner dolphins. Table 1), and so the sight- 

 ings of spotted or of mixed spotted plus spinner 

 dolphin schools are shown together (Fig. 3). Most of 

 the mixed schools were encountered off southern 

 Mexico, where the eastern form of spinner dolphin 

 (Perrin 1975a, b; Perrin et al. 1977) usually accom- 

 panied spotted dolphins (cf. Figs. 3, 4). Mixed schools 

 were uncommon along the Equator, as was the spot- 

 ted dolphin itself, especially west of long. 110°W. 

 Large schools (>300 individuals) of spotted dolphin 

 were widely scattered, but tended to be more com- 

 mon off southern Mexico. Westward extensions of 

 distribution appeared to occur as three main lobes: 

 about lat. 10°n, between lat. 0° and 5°N, and be- 

 tween lat. 2°S and 5°S. 



170° 

 30° 



140° 



— r 



130° 



— r- 



120° 



— r 



70° 

 30° 



Hawaiian Islands 



\ 



O o° 



Christmas Island * 



o o 



>4 . i^ 

 A. 



Marquasas Islands 



O SPOTTED DOLPHIN 



• SPOTTED a SPINNER DOLPHIN 



o • 



o °0° 



Figure 3.— Distribution of spotted and spotted plus spinner dolphin schools seen during the January-March research cruises. Larger circles 



indicate schools of >300 animals. 



626 



