ABSTRACT: We assessed the spatial scales at which variability in ocean climate could affect habitat associations of tropical seabirds and whether species-environmental variable relationships at the smallest scale provide insight into occurrence patterns at larger scales. In the eastern tropical Pacific (10°N to 10°S and 140° to 90°W), seabirds were censused on 16 cruises that followed the same tracks every spring and autumn, 1984 to 1991. This study area possesses closely spaced, well-defined water types and the time period was sufficient to include 2 ENSOs (El Niño-Southern Oscillation) and 1 cold water event (LNSO), thus insuring maximum environmental variability. On the cruises, we measured physical features such as sea-surface temperature (SST) and salinity (SSS) at a coarse scale (1 to 100 km) as well as, at the large scale, the identity of surface water masses and current systems. During spring, at the coarse scale, seabirds were associated mainly with a canonical axis consisting of SST and SSS. During autumn, coarse scale seabird/environment associations were defined by a canonical axis composed of SST and thermocline depth; SSS was of secondary importance. Using SST and SSS optima from the coarse scale analysis, we could predict only the water mass associations for individual species during spring. Consistent between-year associations with water masses were not found in either season. Regardless of season, seabirds at the large scale were associated most consistently with current systems rather than water masses. At the coarse scale, neither the canonical axes nor the seabird associations with these axes were affected by ENSO/LNSO events. At the large scale, seabird associations with currents remained the same during the ENSO/LNSO events for sooty tern, black-winged and white-winged petrels, and wedge-tailed shearwater. Juan Fernandez petrel, wedge-rumped storm-petrel, and Leach's storm-petrel had different associations with currents during the ENSO/LNSO events. We hypothesize that the effect of ENSO/LNSO events on individual species is mitigated by the ability of species to persist in low productivity waters during the non-ENSO/LNSO years.

KEY WORDS: Seabirds · Eastern tropical Pacific · Water masses · Currents · El Niño-Southern Oscillation · Species-habitat relationships

Published in MEPS Vol. 156 (1997) on September 25
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997