Pelagic habitat of seabirds in the eastern tropical Pacific: effects of foraging ecology on habitat selection

A central tenet of ecology is the quantification of species-habitat relationships in order to explain spatial variability in the distribution and abundance patterns of animals, We quantified habitat preferences for 6 tropical seabird species representing a phylogenetically and ecologically diverse group within the eastern tropical Pacific (ETP). Seabird censuses were conducted aboard NOAA research vessels from August to November of 1989 and 1990 using 300 m strip transect methods. Simultaneously with the seabird censuses and from the same ships, oceanic habitat was quantified by measuring sea surface temperature, salinity, and chlorophyll, as well as thermocline depth and strength. We quantified seabird habitat using generalized additive models fitted with forward and backward selection algorithms based on minimizing the Akaike information criterion (AIC). In general, seabird habitat models performed well in explaining spatial variability in abundance, reducing the null deviance to ranges between 42.8 and 73.5%, and predicting seabird density patterns with an accuracy ranging from 56.6 to 80.5%. Relative abundance of the species modeled affected the outcome: the higher the abundance, the more robust the fit. Seabirds feeding on fishes and squids associated with habitats characterized by deep (> 100 m) and strong (> 2 degrees C 10 m(-1)) thermoclines, while planktivorous species preferred habitat characterized by shallower thermoclines with cooler surface temperatures (< 25 degrees C) and a less stratified water column. Thus, seabirds in different feeding guilds have clear and distinct habitat preferences within the ETP. We propose thermocline topography as a key variable in predicting distribution and abundance of seabirds in this area, probably due to its influence on the availability of seabird prey.