Winter preconditioning, mesoscale variability and geomorphology influence the distribution and abundance of krill in the California Current System

Krill are important prey for many marine organisms due to their global distribution, high biomass and energy content. Krill form large, dense aggregations that move both passively and actively, making them extremely patchy in space and time. Understanding krill habitat associations is essential for forecasting range shifts or variability in the future, and for determining the response of krill predators. In the central California Current Ecosystem, we used a boosted regression tree statistical model to relate 17 years (2002-2018) of krill data collected from a shipboard mid-water trawl survey conducted during May/June to environmental variables to identify the factors related to the two dominant krill species (Thysanoessa spinifera and Euphausia pacifica). Their distribution and abundance were influenced by geomorphic features (e.g., depth and rugosity), coastal upwelling dynamics during the prior winter, and mesoscale oceanographic features at the time of the trawl, and revealed the nearshore distribution of T. spinifera compared to the offshore, slope distribution of E. pacifica. Both species were influenced by mesoscale variability with E. pacifica being especially negatively impacted by offshore transport. Both species also responded negatively (positively) to warm (cold) climate events. Krill-predators (seabirds, dolphins and whales) were often present in regions of high krill abundance compared to where they were absent, which corresponded to known prey preferences and life histories. Our framework demonstrates species-specific habitat relationships for keystone species and their response to large-scale climate events, which can inform ecosystem-based management of krill hotspots and conservation strategies for their predators.