Krill swarm structure drives trophic partitioning in sympatric baleen whale species on the Northern California Current foraging grounds

For sympatric species, niche differentiation may manifest through resource selection or spatiotemporal partitioning of shared resources. Traits including body size, physiology, and morphology influence the tactics that predators, such as baleen whales, use to meet their energetic needs. Blue, fin, and humpback whales require rich prey patches, like the extensive swarms the zooplankton krill form, to offset the energetic burden incurred by their large body sizes. Krill swarm characteristics (e.g., length, height, biomass) may influence whale foraging efficiency and therefore habitat selection. We used a Random Forest model to classify blue, fin, and humpback whales based on characteristics of nearby krill swarms acoustically detected in the Northern California Current. Swarm characteristics associated with whale sightings – including depth, krill species, proportion of the swarm occupied by krill rather than water, and internal heterogeneity – indicated species-specific habitat selection and breadth of each whale species’ trophic niches. Despite overall good classification performance (model accuracy = 0.79), erroneous predictions revealed the difficulty of differentiating the niches of generalist humpback and fin whales, while the niche of blue whales, a specialist predator, was clearer. Our results suggest krill swarm characteristics contribute to resource partitioning between baleen whale species on shared foraging grounds.