FISHERY BULLETIN: VOL. 83. NO. 4 



tral America and Subtropical Water off Peru. The 

 equatorial distribution is in Equatorial Surface 

 Water (Wyrtki 1966, 1967), a transitional water mass 

 straddling the Equator and characterized by 

 salinities between 34 and 35"/oo, upwelling, and a 

 relatively weak thermoclina These waters are 

 markedly cooled from June to December (southern 

 winter-spring) by increased upwelling and by advec- 

 tion from the Peru Current. In the Subtropical Water 

 habitats of striped and common dolphins, both off 

 Peru and Baja California, there are also large 

 seasonal changes in temperature structure and ef- 

 fects from upwelling. Finally the Tropical Water 

 habitat in the Central American Bight is notably 

 variable (below). 



The waters we call the "Central American Bight" 

 (roughly, the near coastal waters from Guatemala 

 to Ecuador) constitute the most important area of 

 overlap for spotted, spinner, striped, and common 

 dolphins, but this overlap is not balanced among the 

 species. These waters are tropical, but they are the 

 most variable within the Tropical Surface Water pro- 

 vince The Equatorial Countercurrent, flowing 

 eastward between lat. 4°N and 10°N, terminates and 

 turns there, creating a complex circulation. The an- 



nual north-south migration in these latitudes of the 

 Intertropical Convergence Zone, where north and 

 south trade winds meet, bring southerly winds, rain, 

 reduced salinity, and an intensified Countercurrent 

 during the second half of the year (Bennett 1966; 

 Wyrtki 1967, 1974; Forsebergh 1969). Later during 

 the northern winter (January-March), northeaster- 

 ly winds blow across Central America from the 

 Atlantic, producing coastal upwelling, wind stir- 

 ring, and more complex temperature patterns. The 

 Costa Rica Dome, a localized, offshore upwelling 

 at about lat. 8°N, long. 90°W (Wyrtki 1964b), 

 also may be seasonally intensified (Hofmann et al. 

 1981). 



These variable Central American Bight waters ap- 

 pear to have more abundant schools of striped and 

 common dolphins than of spotted and spinner 

 dolphins (cf. Figs. 3-7). It seems that all areas with 

 greater concentration of striped and common 

 dolphins have highly variable oceanographic features 

 that are "upwelling-modified". 



In spite of the rather strong overlap in distribu- 

 tion among the four dolphin species in the Central 

 America Bight, the biogeographic distinction, in- 

 cluding the relationships to environment, between 



170' 

 30" 



160* 



— r- 



1«0' 



— r 



130* 



— I— 



70' 

 30' 



20- 



Chriatmat Island * 



COMMON AND STRIPED DOLPHIN 

 SCHOOLS PER 100 MILES SEARCHED 



RESEARCH CRUISES JAN. -MAR. 

 1976, 1977, 1979, 1980 



/ 



Figure 7.— Distribution of relative abundance of striped and common dolphins, inferred from data of the January-March research cruises. 



630 



