Seasonal changes in distribution and habitat differences among dolphins in the eastern tropical Pacific

Large-scale patterns of dolphin distnbution and oceanography were s t u l e d from research-vessel surveys conducted in the pelagic eastern tropical Pacific during June to November 1982November , 1986November and 1987.Substantial changes were observed in relation to previously reported winter dstribut~ons for spotted and/or spinner dolphin schools (Stenella attenuata and/or S. 1ongirostris)and for striped dolphin schools (S. coeruleoalba).These dolphin species were sighted in abundance west of 120" W along 10°N coincident w ~t h seasonal shoaling of a thermocline ridge.No seasonal distribution changes were observed for common dolphin schools (Delphlnus delphls); as in the winter, they occupied upwelling-modified waters of the region.Highest-density areas for the 3 school types were clearly separated spatially, and the thermocline depths and s ~g m a -t ' s of sighting localities were statistically different between spotted/spinner dolphin schools and common dolphin schools.Striped dolphin schools could not be discriminated from the other 2 types based on these habitat var~ables, indicating other factors or processes contribute to the observed spatial separat~on of the 3 distributions.