Humpback-krill relationships are strongest at fine spatial scales in the Northern California Current region

Abstract

Understanding scale-dependent variability in predator-prey relationships is essential to ecosystem management. The Northern California Current (NCC) ecosystem is an important foraging ground for humpback (Megaptera novaengliae) and other rorqual whales, but it is also an area that presents diverse anthropogenic threats. Clarifying the most meaningful spatial scale to analyze relationships between whales and krill, a key prey item, is important to understanding ecosystem function and informing research and management efforts. To examine spatially explicit relationships between humpback whales and krill in the NCC, we analyzed concurrent whale sightings and acoustically detected krill abundance estimates collected in May and September between 2018 and 2022. We used generalized additive mixed models to predict humpback whale occurrence at a series of ecologically relevant nested spatial scales: 1, 2, 5, and 20 km. We found that relative krill abundance at a spatial scale of 5 km had the greatest influence on humpback whale occurrence. Our results suggest that whale and krill relationships at the 5 km scale may be both energetically profitable for whales to optimize foraging efficiency and also most detectable using our traditional methodological approaches (paired observer and echosounder surveys). We recommend that zooplankton prey data at the 5 km scale be incorporated into future humpback whale distribution models and considered for management applications in this region.