AU and FERRYMAN: DOLPHIN HABITATS 



the spotted and spinner dolphins of the Tropical 

 Water and the striped and common dolphins of the 

 Upwelling-Modified Water may be quite apparent. 

 This was the case during the intensive winter surveys 

 of 1979 in the Central American Bight, shown in 

 Figure 8. Superimposed in the figure are contours 

 of the 20°C isotherm depth (essentially the ther- 

 mocline depth) which were obtained from expendable 

 bathythermograph probes dropped at 30-60 mi in- 

 tervals (55.6-111.1 km). Notice that spotted and spin- 

 ner dolphins were encountered mainly off southern 

 Mexico, where the deeper 20°C isotherms indicated 

 the occurrence of a large surface lens of warm water. 

 The warmest surface waters in the eastern tropical 

 Pacific normally occur in this area; the thermocline 

 gradient is weaker, and the annual variation in sur- 

 face temperature is relatively small (Wyrtki 1964a). 

 In the more variable tropical waters of the Central 

 American Bight, where the thermocline had shoal- 

 ed or ridged to <60 m, both these species were seen 

 too. However, striped and common dolphin schools 

 predominated, especially near the shallower iso- 

 therms that mark the location of the Costa Rica 

 Dome Finally the equatorial distribution of primar- 

 ily striped and common dolphins was evident. The 



sampling suggested that Subtropical Waters south 

 of the Galapagos Islands were probably also impor- 

 tant to these latter two species. A 1977 aerial survey 

 of cetaceans in these waters off Central America 

 (SWFC 1977'^) obtained results similar to those just 

 described. 



Though there appear to be large-scale geographic 

 differences in the habitats of spotted and spinner 

 dolphins and of striped and common dolphins, there 

 was no evidence of negative association among these 

 species. The frequency of days in the Central 

 American Bight, both in 1979 and 1980, with dif- 

 ferent combinations of these species encountered, 

 are summarized in Tkble 2. There was no evidence, 

 using chi-square contingency tests for association 

 among spotted and/or spinner dolphins and striped 

 or common dolphins, that the species were not oc- 

 curring independently on any particular day. 



Our contention that spotted and spinner dolphins 

 and striped and common dolphins differentially in- 

 habit waters of different oceanographic characteris- 



^Southwest Fisheries Center (SWFC). 1977. Aerial survey trip 

 report January - June 1977. Admin. Rep. No. LJ-78-01, 73 p. 

 Soutliwest Fisheries Center La Jolla Laboratory, National Marine 

 Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 92038. 



120 



30° 



110" 



100° 



90° 



80° 



20< 



10° 



o SPOTTED OB SPINNER DOLPHIN 

 • STRIPED OR COMMON DOLPHIN 

 — DEPTH OF 20° C ISOTHERM (M) 



10° 

 120° 



70° 

 -130° 



20' 



- 10° 



SOUTH 

 AMERICA 



10' 



110° 



100° 



90° 



80° 



70° 



Figure 8.— Distribution of dolphin schools by species type in the Central American Bight, with 

 reference to the depth of the 20°C isotherm. Data are from the January-March cruises of 1979. 



631 



