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

Large-scale patterns of dolphin distribution and oceanography were studied from research-vessel surveys conducted in the pelagic eastern tropical Pacific dunng June to November 1982, 1986 and 1987. Substantial changes were observed in relation to previously reported winter distributions 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 with seasonal shoaling of a thermocline ridge. No seasonal distribution changes were observed for common dolphin schools (Delphinus delphis); 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 sigma-t's of sighting localities were statistically different between spottedlspinner dolphin schools and common dolphin schools. Striped dolphin schools could not be discriminated from the other 2 types based on these habitat variables, indicating other factors or processes contribute to the observed spatial separation of the 3 distributions.

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