Interannual variability of dolphin habitats in the eastern tropical Pacific. I: Research vessel surveys, 1986-1990

we used canonical correspondence analysis (CCA), an eigenvector ordination technique that includes directgradient analy­ sis, to investigate habitat use by spotted, Stenella attenuata, spin­ ner, S.longirostris, striped, S. coeruleoalba. and common, Del­ phinis delphis, dolphins in the east­ ern tropical Pacific during 1986­ 90. Data were collected during an­ nual research vessel cruises con­ ducted in August-November of each year. Environmental vari­ ables included in the analyses were surface temperature, salinity, sigma-t, and chlorophyll, and ther­ mocline depth and thickness. The dominant pattern in the species-en­ vironmentrelationship (1st canoni­ cal axis) separated common dol­ phins from spotted and spinner dolphins, based on their associa­ tions with cool upwelling habitat and warm tropical habitat, respec­ tively. The second axis separated whitebelly spinners from eastern spinner dolphins. Both occurred in tropical water, but were separated primarily by thermocline topogra­ phy. The species-environment cor­ relations were 0.67 on the first axis, 0.42 on the second. Overall, the environmental data explained 15% of the variance in the species data. For individual school types this ranged from 36% for common dol­ phins to 6% for striped dolphins. Interannual variability in the spe­ cies data was small but wasjudged sigQificant by a Monte Carlo ran­ domization test. Residual inter­ annual variance was insignificant after removingvariance associated with environmental variables.