FISHERY BULLETIN: VOL. 83, NO. 4 



Dolphin Communities 



The habitat differences discussed also apply to 

 other delphinid species, so that there seem to be dif- 

 ferent communities of dolphins in the eastern Pacific 

 To show how the cetacean communities differ be- 

 tween the habitats dominated by spotted and spin- 

 ner and by striped and common dolphins, the eastern 

 Pacific was divided into Areas I and II (Fig. 10, in- 

 set) that separate the main habitat areas of these 

 two species pairs. Area I is primarily Tropical Sur- 

 face Water and includes most areas where thermo- 

 cline ridging is a dominant physical feature Area 

 II is primarily Equatorial-southern Subtropical Sur- 

 face Water, but also includes the wedge-shaped area 

 of variable Tropical Water in the Central American 

 Bight. Area II comprises most of the waters we have 

 called Upwelling-Modified. 



In each of these two areas, only schools sighted 

 at <1.0 nmi perpendicular to the ship's tracks were 

 listed. This requirement was imposed so that per- 

 cent species composition could be based on species 

 schools, that to the largest practical degree, could 

 all be sighted with equal probability, if present. The 

 change in "sightability" with distance is different for 

 each species because of differences in behavior, 

 coloration, size, etc 



Our data indicate that the species composition of 

 delphinids is different in these two areas. Percent- 

 age composition was determined for spotted; spin- 



ner; striped; common; bottlenose, Tursiops trun- 

 catus; rough-toothed, Steno bredanensis; and Risso's, 

 Grampus griseus, dolphins, and for "blackfish", 

 Peponocephala electra/Feresa attenuata; pilot whales, 

 Globicephala macrorhynchus; and others (Tkble 4). 

 Among 8 of 10 species-groups specifically identified 

 in Tkble 4, there were significantly higher percent- 

 ages of spotted, spinner, and rough-toothed dolphin 

 schools in Area I than in Area II (Fig. 10). Risso's 

 and bottlenose dolphins were important species in 

 both areas, and their percentage values did not dif- 

 fer significantly between the areas. The percentages 

 due to striped and common dolphins and pilot whales 

 increased in Area II relative to Area I. Though 

 reduced, the spotted dolphin remained important in 

 the Area II dolphin community. The increase in per- 

 cent composition of the common dolphin in Area II 

 was not quite significant, reflecting the inclusion in 

 Area I of that species' distributional lobe off Baja 

 California. Overall, the species composition differed 

 significantly between the two areas, as determined 

 by chi-square contingency test of the frequency of 

 species other than spotted, spinner, striped, and com- 

 mon, i.e, the species not initially considered when 

 delimiting Areas I and II (x^ = 74.4, df = 5, P < 

 0.005). 



Additional evidence for the distinctiveness of the 

 equatorial and subtropical portions of the Area II 

 community is provided by observations along 

 equatorial transects and transects south and south- 



30 r 



25 



(0 



§< 



W< 20 



•^ lU 

 11. H 

 O Z 



UJ 



Q. 



15 



10 



27.5* 



Area I 

 [ 1 Area II 



120' 110° 100" 90* 



Spotted Bottlenose Spinner Risso's Rough- 

 toothed 



Striped Common 



DELPHINID SPECIES 



Pilot 

 Whale 



Figure 10.— Percent species composition of some important cetaceans, by Area I and II. See 



Ikble 4. 



634 



