Decadal diet shift in yellowfin tuna Thunnus albacares suggests broad- scale food web changes in the eastern tropical Pacific Ocean

Widespread climate-induced habitat compression and reductions in biological production are beginning to alter food webs in tropical and subtropical oceans, but the effects on midtrophic level micronekton communities that support commercially important pelagic fishes are unclear. The predation habits of yellowfin tuna Thunnus albacares, a wide-ranging generalist predator with high energy requirements, provide rare insights into the distribution and availability of prey communities in pelagic regions. We used a modified classification tree approach to analyze spatial, temporal, environmental, and biological covariates explaining the predation patterns of 3362 yellowfin sampled across the eastern tropical Pacific Ocean (ETP) during two 2 yr periods occurring a decade apart. Persistent zoogeographical assemblages of prey were important only in relatively small subtropical regions at the extreme northern and southern ranges of the purseseine fishery for tunas. Prey biomass patterns for the majority of the samples over most of the ETP (6 degrees S to 17 degrees N, coast to 150 degrees W) were best explained by a tree partition distinguishing samples by sampling period, 1992 to 1994 and 2003 to 2005. The classification tree revealed that a major diet shift had transpired in the heart of the ETP. Yellowfin predation had changed from primarily larger epipelagic fish prey in the 1990s to a diverse array of smaller mesopelagic species and a crustacean that apparently had expanded its range in the 2000s. Partial dependence plots from the tree model showed range expansions previously described for some prey and unknown for other prey. Diet analysis of selected marine predators offers a practical means of monitoring prey communities poorly sampled by conventional methods.