No abstract

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.