SCALE-RELATED SEABIRD-ENVIRONMENTAL RELATIONSHIPS IN PACIFIC EQUATORIAL WATERS, WITH REFERENCE TO EL-NINO SOUTHERN-OSCILLATION EVENTS

We assessed the spatial scales at which variability in ocean climate c ould affect habitat associations of tropical seabirds and whether spec ies-environmental variable relationships at the smallest scale provide insight into occurrence patterns at larger scales. In the eastern tro pical Pacific (10 degrees N to 10 degrees S and 140 degrees to 90 degr ees W), seabirds were censused on 16 cruises that followed the same tr acks every spring and autumn, 1984 to 1991. This study area possesses closely spaced, well-defined water types and the time period was suffi cient to include 2 ENSOs (El Nino-Southern Oscillation) and 1 cold wat er event (LNSO), thus insuring maximum environmental variability. On t he cruises, we measured physical features such as sea-surface temperat ure (SST) and salinity (SSS) at a coarse scale (1 to 100 km) as well a s, at the large scale, the identity of surface water masses and curren t systems. During spring, at the coarse scale, seabirds were associate d mainly with a canonical axis consisting of SST and SSS. During autum n, coarse scale seabird/environment associations were defined by a can onical axis composed of SST and thermocline depth; SSS was of secondar y importance. Using SST and SSS optima from the coarse scale analysis, we could predict only the water mass associations for individual spec ies during spring. Consistent between-year associations with water mas ses were not found in either season. Regardless of season, seabirds at the large scale were associated most consistently with current system s rather than water masses. At the coarse scale, neither the canonical axes nor the seabird associations with these axes were affected by EN SO/LNSO events. At the large scale, seabird associations with currents remained the same during the ENSO/LNSO events for sooty tern, black-w inged and white-winged petrels, and wedge-tailed shearwater. Juan Fern andez petrel, wedge-rumped storm-petrel, and Leach's storm-petrel had different associations with currents during the ENSO/LNSO events. We h ypothesize that the effect of ENSO/LNSO events on individual species i s mitigated by the ability of species to persist in low productivity w aters during the non-ENSO/LNSO years.