We analyzed sea temperature as an environmental factor, in association with ENSO, affecting the migration of East Pacific black turtle, Chelonia mydas (=Chelonia agassizii Bocourt), to its foraging areas and its feeding ecology at San Andrés, Peru. A 19-year sea turtle landing database (1970-1988) was constructed to associate landing fluctuations with environmental variability represented by the Peruvian Oscillation Index. A positive correlation between them (r = 0.75, P ≪ 0.05) indicated that exceptionally large black turtle landings occurred in San Andrés port during El Niño episodes. Warmer waters (SST 22-28°C) approached near the Peruvian coast during El Niño episodes, thus facilitating black turtle access to this area. Furthermore, during El Niño 1987, large juvenile and adult black turtles, known to be primarily herbivorous, fed mainly on the scyphozoan jellyfish Chrysaora plocamia Péron & Lesueur, which was very abundant during this event. It is likely that black turtles exploited this resource opportunistically. Inter-annual environmental variability, driven by El Niño Southern Oscillation, has profound consequences for the ecology of the endangered black turtle, which should be considered when evaluating the effects of anthropogenic activities on its population dynamics. © Springer Science+Business Media B.V. 2010.