AU and FERRYMAN: DOLPHIN HABITATS 



MO' 

 30' 



150' 



— T" 



70' 

 30' 



Hawaiian lalandt 



SPOTTED AND SPINNER DOLPHIN 



SCHOOLS PER 100 MILES SEARCHED 



RESEARCH CRUISES JAN.-MAR. 



1976. 1977, 1979, 1980 



30" 



I 

 O.S 



Chrittmaa Island * 



i 



Figure 6— Distribution of relative abundance of spotted and spinner dolphins, inferred from data of the January-March research cruises. 



Hawaiian and Marquesan sightings are not considered. 



spinner dolphins (Fig. 7). Conspicuous lobes extend- 

 ed from off Baja California and also broadly from 

 the coasts of Central America and northern South 

 America out to and along the Equator. Within the 

 latter lobe, centers of higher relative abundance oc- 

 curred west of Nicaragua and Costa Rica and west 

 of the Galapagos Islands, all areas where reduced 

 abundance of spotted and spinner dolphins occurred. 

 The Galapagos area and the lobe off Baja Califor- 

 nia were dominated by the common dolphin (cf. Fig. 

 5). 



Dolphins of Tropical Water and 

 Upwelling-Modified Water Habitats 



The areas of greater frequency of spotted and spin- 

 ner dolphins during January-March are the typical 

 tropical waters of the eastern tropical Pacific. These 

 waters are underlain by a sharp thermocline, 

 generally >2°C/10 m, at depths usually much <50 

 m. The surface temperatures are >25°C, and the 

 salinities <34''/oo. Such tropical waters are defined 

 by Wyrtki (1966, 1967) as Tropical Surface Water 

 (see Fig. 1). In particular the warm "Inner Tropical" 



Waters (Wyrtki 1964a) lying primarily north of the 

 Equator comprise the major habitat of spotted and 

 spinner dolphins (Fig. 6). The waters south of the 

 Equator, where the relative abundance of these 

 dolphins is also higher, are seasonally tropical, and 

 are therefore called southern Subtropical Surface 

 Water (Wyrtki 1966, 1967). These waters, occurring 

 approximately south of lat. 2.5° S, have surface 

 salinities >357oo, and during January-March 

 (southern summer), warm to more than 26°C over 

 a shallow, sharp thermocline Since spotted and spin- 

 ner dolphins occur there frequently, at least during 

 January-March, it appears that spotted and spinner 

 dolphins prefer all waters whose characteristics are, 

 or become, tropical in the eastern Pacific However 

 the primary habitat appears to be the "Inner" 

 Tropical Waters north of the Equator. 



In contrast striped and common dolphins appear 

 to prefer waters with more variable conditions dur- 

 ing January-March. Their most important habitat ap- 

 pears to be broadly centered about equatorial waters 

 (Fig. 7). This band of distribution extends into the 

 central Pacific along the Equator. In the east it 

 broadens widely to include Tropical Water off Cen- 



629 



