The eastern Pacific Ocean (EPO) covers ~ 55 million km2 of complex, dynamic habitats that can affect the distribution of ecologically and commercially important skipjack tuna, Katsuwonus pelamis, and its forage. The relationships between oceanography, ontogeny, and forage were quantitatively assessed to better understand skipjack tuna trophic ecology, and for parameterizing ecosystem models. Predator–prey dynamics of 320 skipjack tuna sampled from 1992 to 1994 and 2003 to 2005 were characterized—based on mean prey biomass (%W¯)—with respect to oceanographic biomes (Longhurst provinces), Oceanic Niño Index (ONI), year and skipjack tuna size, using a classification tree approach. Longhurst province was the most influential variable in explaining diet. Spatially, prey taxa richness was significantly lower, and taxonomic composition different, in predominant upwelling provinces compared to the primarily oligotrophic waters of the tropical offshore provinces. Anchovies comprised over 50%W¯ of diet composition in the ‘upwelling’ provinces whereas various epipelagic (51%W¯) and mesopelagic fishes (28%W¯) were important in the ‘central American coastal and tropical offshore’ provinces. An ontogenetic breakpoint in diet was identified by the classification tree at 547 mm fork length, with small skipjack tuna primarily consuming anchovies, Humboldt squid and krill and large skipjack tuna consuming epipelagic fishes and red crab. Identifying skipjack tuna foraging arenas by Longhurst province can help facilitate the development of a spatially explicit ecosystem model, while defining ‘life stanzas’ as at the 547 mm breakpoint in the EPO will provide more realistic representations of diet composition, consumption rates, and losses to predation and fishing in such models.