Krill swarm biomass, energetic density, and species composition drive humpback whale distribution in the Northern California Current

Prey abundance and quality are dynamic in time and space, impacting predator ecology. We examine variation in species-specific krill quantity and quality as prey for humpback whales in the Northern California Current region, using generalized additive models to assess metrics including biomass and energy density derived from an integrated dataset of concurrent active acoustics, net tows, and marine mammal observations (2018–2022). Overall, prey metrics were positively correlated with humpback whale presence, with increasing trends modified by seasonal (early versus late foraging season) and spatial (continental shelf versus offshore) variation (model deviance explained 36.3%–40.8%). Biomass and energy density had strong positive effects on humpback whale presence, suggesting whales target high-quality swarms that offer more energy per lunge. Elevated Thysanoessa spinifera abundance near humpback whales suggests that they target this species, particularly in the late season when they are energetically richer than Euphausia pacifica, the region’s other abundant krill species. Environmental change may decrease krill abundance and quality, impacting humpback whales’ ability to meet energetic requirements and potentially driving changes in their distributions and exposure to anthropogenic threats. Clarifying drivers of humpback whale krill patch selection can improve species distribution models and aid managers in mitigating risk to whales.