Climate-driven variability in Euphausia pacifica size distributions off northern California

We analyzed an 11-year data set of length measurements for Euphausia pacifica, resolved by developmental stages ranging from early furcilia through adults, to characterize temporal variability in size distributions in relation to environmental and climate forcing in coastal waters of the northern California Current. Our analysis reveals that coastal size distributions of this linchpin species are related to temperature and chlorophyll a concentration, and indicate that warm climate events disrupt and suppress typical seasonal dynamics, resulting in persistent shifts towards populations dominated by smaller juveniles and adults. The 2014-16 marine heatwave (MHW) resulted in sharp and sustained transition in the size structure of E. pacifica off northern California; we captured numerous mature individuals smaller than any previously reported in the literature and larger size classes were conspicuously absent or rare when the MHW most strongly impacted coastal waters. Early life history stages exhibited a contrasting response, shifting instead towards larger size distributions during warm years. Advection appears to be the dominant mechanism driving rapid shifts in E. pacifica size structure off northern California, which implies alongshore and cross-shelf gradients in life history expression (e.g., changes in size-at-maturation) related primarily to temperature. Variability in food availability (chlorophyll concentration) and physiological responses (e.g., changes in growth rates) likely contribute to seasonal variability, but appear insufficient to account for sharp, climate-driven transitions. Climate-driven shifts in individual and total biomass of this critical and dominant forage species have important ecological implications, particularly for growth and survival of species at higher trophic levels. Given that warm water events are likely to increase with climate change, the effects of the 2014-16 MHW on E. pacifica in coastal waters may serve as a harbinger for future change. Detailed information on size structure of euphausiid populations is fundamental to understanding population responses to and ecosystem consequences of climatic and environmental forcing, and has been incorporated in assessments that support ecosystem-based approaches to fisheries management.