Spatial and ontogenetic variation in the trophic ecology of skipjack tuna, Katsuwonus pelamis, in the eastern Pacific Ocean

The eastern Pacific Ocean (EPO) covers similar to 55 million km(2) 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) over bar)-with respect to oceanographic biomes (Longhurst provinces), Oceanic Nino 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) over bar of diet composition in the 'upwelling' provinces whereas various epipelagic (51%(W) over bar) and mesopelagic fishes (28%(W) over bar) 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.

Automated summary

The study reveals that different ecological regions in the eastern Pacific Ocean have a significant impact on the diet of skipjack tuna, leading to differences in prey species richness and composition under varying environmental conditions. Through analysis of skipjack tuna samples, an ontogenetic breakpoint in diet at 547 mm fork length was identified, highlighting the important influence of growth stages on feeding habits.