Decadal-scale Comparisons of Predation by a Generalist Pelagic Predator: Ecosystem Implications in the Eastern Tropical Pacific

One of many challenges for ecosystem-based fisheries science is to improve an often rudimentary understanding of food-web relationships so that community ecology may contribute to the process of setting management policy.  Changes over time in the structure of pelagic open-ocean food webs are difficult to quantify; fisheries-independent trawl surveys are expensive and biased against larger micronekton.  Tropical tunas, however, are opportunistic, non-selective predators due to their broad diet and high energy requirements in oligotrophic habitats.  Assuming yellowfin tuna is an effective biological sampler of middle-trophic-level communities, comparative diet studies across temporal scales have the potential to provide valuable clues about changes in species distributions and ecological relationships within the food web.  Stomach samples from 6810 yellowfin tuna (Thunnus albacares) were taken from 433 purse-seine sets in the eastern tropical Pacific during 2 two-year periods separated by a decade.  Classification trees were used to explore the relationships of spatial, temporal, biological, and environmental predictor variables to the prey composition by weight for 3122 stomachs that contained prey remains.  Large changes in the diet composition were apparent in the core region of the fishery.  Diet proportions of epipelagic fishes declined from 1992-1994 to 2003-2005, while diel vertically-migrating mesopelagic fishes increased in dietary biomass.  Cephalopods gained in occurrence in the stomach contents during the decade, as well as during the previous 50 years in the eastern tropical Pacific (M. Hunsicker, pers. comm.).  Moreover, daily ration (energy consumption) and prey-predator size relationships declined considerably.  Ecological implications of these results and circumstantial evidence of a shift to a more oligotrophic system will be discussed.