FISHERY BULLETIN: VOL. 83, NO. 4 



The purpose of this paper is to describe the 

 January-March distributions of schools of spotted, 

 spinner, striped, and common dolphins as determin- 

 ed from research ship surveys. Being independent 

 and distinct from data collected by observers aboard 

 tuna seiners, these survey data enable a separate 

 evaluation of habitats and ecological relationships. 

 The distributions derived will be discussed in terms 

 of habitat features and interspecific associations in- 

 cluding those of other cetaceans, yellovvrfin tima, and 

 certain seabirds. We propose that tv^o major ceta- 

 cean communities can be recognized, centered 

 broadly about the tropical and about the equatorial- 

 subtropical surface water provinces (Fig. 1). 



METHODS 



The Southwest Fisheries Center conducted or par- 

 ticipated in 17 research cruises studying cetaceans 

 between 1976 and 1981. Eight major cruises were 

 carried out between January and March of 1976, 

 1977, 1979, and 1980, with the NOAA ships David 



Starr Jordan and Tovmsend Cromwell. During these 

 surveys, schools of all cetacean species encountered 

 (at least 23 species in the areas of interest) were ap- 

 proached to allow close observation. The combined 

 cruise tracks of these January-March surveys form- 

 ed an extensive coverage of the eastern tropical 

 Pacific and included areas not frequently searched 

 by fishing vessels (Fig. 2). The latter are the 

 equatorial waters and areas both south of lat. 8°N 

 and west of long. 110°W. The cruise tracks during 

 any particular year were chosen to investigate cer- 

 tain aspects of dolphin distribution, ag., the inten- 

 sive surveys during 1979 off southern Mexico and 

 central America constituted a joint aircraft-ship 

 survey of the nearshore habitat. The remaining other 

 nine cruises were either for special studies or of ships 

 of opportunity. In the latter case, the ships did not 

 usually divert course to inspect cetacean schools that 

 were sighted. 



Cetacean observations from a research ship were 

 conducted by scanning the waters ahead and to the 

 sides of the ship through 20 x 120 mm USN MK-3 



Figure L— Surface water masses and boundaries of the tropical ocean in the eastern Pacific, based on Wyrtki's (1964a, 1967) analysis 

 to long. 140°W. Boundaries are a function of the 24°C surface isotherm and the TC/IO m thermocline gradient. Notice that in October 

 the southern boundary to the eastern tropical ocean lies mostly north of the Equator, while in March this boundary lies far to the south 

 so that southern subtropical waters become tropical. 



624 



