Determining the drivers of prey selection in marine predators is critical when investigating ecosystem

structure and function. The newly recognized Rice’s whale (Balaenoptera ricei) is one of the most

critically endangered large whales in the world and endemic to the industrialized Gulf of Mexico. Here,

we investigated the drivers of resource selection by Rice’s whales in relation to prey availability and

energy density. Bayesian stable isotope (δ13C, δ15N) mixing models suggest that Rice’s whales feed

primarily on a schooling fish, Ariomma bondi (66.8% relative contribution). Prey selection using the

Chesson’s index revealed that active prey selection was found to be positive for three out of the four

potential prey identified in the mixing model. A low degree of overlap between prey availability and

diet inferred from the mixing model (Pianka Index: 0.333) suggests that prey abundance is not the

primary driver of prey selection. Energy density data suggest that prey selection may be primarily

driven by the energy content. Results from this study indicate that Rice’s whales are selective

predators consuming schooling prey with the highest energy content. Environmental changes in

the region have the potential to influence prey species that would make them less available to Rice’s

whales.