MEPS 462:261-271 (2012)  -  DOI: https://doi.org/10.3354/meps09851

ABSTRACT: Reductions in the availability of energetically valuable prey associated with the El Niño Southern Oscillation (ENSO) may force seabirds to switch to low-energy prey or smaller prey, with consequent effects on their fitness. Impacts of ENSO on seabird diet have been documented in several regions but remain unexplored in the warm eastern tropical Pacific. Analysis of 5 yr of data on the blue-footed booby Sula nebouxii, a seabird specialized on small pelagic fish, revealed that both sexes switch diet and capture fewer types of prey when warm El Niño conditions prevail during the breeding season. Moreover, ENSO-related alternations in the relative abundances of anchovies and herrings, the main prey of these boobies and important commercial fishes in the region, were associated with boobies feeding on more and larger anchovies and fewer and smaller herrings during warm El Niño events, and vice versa during cold La Niña episodes. Females consumed herrings more frequently than males across the 5 yr and consistently fed on larger anchovies and herrings than males, with these differences likely due to this booby’s reversed sexual size-dimorphism. We conclude that diet composition and prey size of boobies may be sensitive to variation in relative availabilities of different prey species associated with the ENSO, making these birds robust indicators of environmental fluctuations in the warm eastern tropical Pacific. Inter-annual variations in the availability of herrings, probably the most energetically valuable prey for these boobies, may explain the large ENSO-related variations in booby reproduction documented previously.

KEY WORDS: El Niño Southern Oscillation · Seabird diet · Eastern tropical Pacific · Fish abundance · Anchovies · Herrings · Sardine fisheries