ABSTRACT. We used a 21 yr time series of productivity for 6 seabird species nesting in large numbers at the Farallon Islands, 40 km offshore of San Francisco. California, USA, to assess proximate and remote factors leading to variation in the food supplies available to these predators. The latter sampled prey throughout a 3200 km2 area. Depending on foraging ecology and reproductive capacity, some specles were more sensitive to food web perturbation than others. A serious lack of food was indicated by negat.ivc reproductive anomalies during all warm-water events, some of which were classifled as tropical El Nifio and others which were not. Equally spectacular but positive anomalies occurred during years adjacent to the negative ones, particularly evident among the most sensitive species. Much of the annual variation, positive or negative, in seabird reproductive success was explained by variation in the Southern Oscillation and/or the Aleutian low pressure system, both of which affect sea-surface temperature and thermocline depth off California. Results indicate that perturbations in the marine food web of the Cal~fornia eastern boundary current system, as indicated by the availability of food to seab~rds, are much more complex than is generally appreciated, and are not conf~ned only to negative excursions from normalcy. ENSO is important, but other global atmosphere-ocean phenomena affect the California Current just as dramatically.
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 Nlno 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 wlth storm-petrels. Dunng spring, shearwaters and storm-petrels were important in both water masses w h~l e gadfly petrels were important only in the Equatorial Surface Water. Assemblage characteristics varled from year to year, but changed the most during El Nino and La Nlna 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 N~n o , assemblages changed more during autumn compared to spring The effect of La Nina was strongest during sprlng
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