EFFECTS OF EL-NINO AND LA-NINA ON SEABIRD ASSEMBLAGES IN THE EQUATORIAL PACIFIC

Spring and autumn cruises in Equatorial and Subtropical Surface Waters were conducted from 1984 to 1989 in the eastern Equatorial Pacific. Assemblage characteristics of species richness and diversity during El Nino 1986-87 and La Nina 1988 were compared with the other years. The 3 genera that dominated the assemblages, storm-petrels (Oceanodroma), gadfly petrels (Pterodroma), and shearwaters (Puffinus), differed markedly in relative importance depending on season and water mass. During autumn, on the basis of biomass, gadfly petrels dominated assemblages in both water masses; on the basis of abundance, gadfly petrels shared dominance with storm-petrels. During spring, shear-waters and storm-petrels were important in both water masses while gadfly petrels were important only in the Equatorial Surface Water. Assemblage characteristics varied from year to year, but changed the most during El Nino and La Nina. Either event was manifested by a decrease in richness and a disappearance of genera and species, particularly those of medium-abundance. Generally, the common genera and species were not affected. For El Nino, assemblages changed more during autumn compared to spring. The effect of La Nina was strongest during spring.