AU and FERRYMAN: DOLPHIN HABITATS 



the spring and summer months (Calkins 1975). 

 However there is little information from that area 

 during the winter months (when there is little 

 fishing), except for results from the January-March 

 research cruises, which indicated that population 

 densities there were not high (Fig. 6). This suggests 

 a summer buildup in the concentration of these 

 dolphins. This is likely because the offshore habitat 

 is centered close to or along the divergence zone at 

 the northern boundary of the North Equatorial 

 Countercurrent, where the thermocline ridges and 

 biological production is increased (Cromwell 1958). 

 During the northern summer, the trade winds over 

 the offshore habitat abate, ridging intensifies (Wyrt- 

 ki 1964a, 1974), and porpoise-tuna fishing expands 

 west of Clipperton Island (at ca. lat. 10°N, long. 

 109°W). The increase in fishing may be due to bet- 

 ter weather, but possibly also to an increased abun- 

 dance of dolphins and tuna. 



The seasonal change in environment of the eastern 

 tropical Pacific most likely to affect the distribution 

 of dolphins is that due to the movement of the 

 southern border of the tropical waters. During the 

 northern winter, tropical conditions usually extend 

 to about 15° south of the Galapagos Islands, when 

 a shallow surface layer of warm water develops over 

 what is actually Subtropical Water. During the north- 

 ern summer, the cool Peru and South Equatorial 

 Current strengthen, and a conspicuous thermal 

 front, marking the southern boundary of Tropical 

 Surface Water, usually develops. This Equatorial 

 Front (Wyrtki 1966) is located a few degrees north 



of the Equator except for a short section east of the 

 Galapagos Islands. 



We studied the relationship of the Equatorial 

 Front to dolphin populations during an October- 

 November 1977 cruise of the David Starr Jordan. 

 The Equatorial Front was conspicuous, as were the 

 effects of the cool Peru and South Equatorial Cur- 

 rents (Fig. 9). Only 4 of 27 sightings of spotted and 

 spinner dolphins schools in the equatorial region oc- 

 curred in the cool, Equatorial and Subtropical 

 Waters south of the front. The majority of the re- 

 maining 23 sightings were along the Equatorial 

 Front, at the southern border to the warm Tropical 

 Surface Water. These same species had occurred 

 throughout these southern waters during January 

 to March (Figs. 3, 4), when sea surface temperatures 

 of 25°C or more prevailed over this entire area. The 

 apparent redistribution of dolphin schools along the 

 warm edge of the Peru Current and Equatorial Front 

 appeared to be restricted to the spotted and spin- 

 ner species, suggesting their seasonal movements 

 away from cool southern waters. By implication these 

 same dolphin species may migrate into southern 

 waters during the warm season. Seasonal move- 

 ments of dolphins (unidentified) in the southern 

 waters is also suggested by results from the 1967 

 and 1968 EASTROPAC cruises (Love 1971, 1972). 

 On the other hand, we did see during the October- 

 November cruise four schools of spotted and spin- 

 ner dolphins in cool Subtropical Water, and observers 

 aboard tuna seiners have also reported these same 

 species there during the cool season. 



I30» 



IQo — 



c - 



I0« - 



Figure 9.— Distribution of dolphin schools in the equatorial region during October-November 1977, 

 relative to surface isotherms (°C). Notice the Equatorial Front or zone of rapid temperature change 

 just north of the Equator. Clumping of sightings along the track is due to nighttime travel by ship. 

 "Other" dolphins are primarily pilot whales. 



633 



