Sub‐regional differences in groundfish distributional responses to anomalous ocean bottom temperatures in the northeast Pacific

: Although climate-induced shifts in fish distribution have been widely reported at the population level, studies that account for ontogenetic shifts and subregional differences when assessing responses are rare.In this study, groundfish distributional changes in depth, latitude, and longitude were assessed at different size classes by species within nine subregions. We examined large, quality-controlled datasets of depth-stratified-random bottom trawl surveys conducted during summer in three large regions-the Gulf of Alaska and the west coasts of Canada and the United States-over the period 1996-2015, a time period punctuated by a marine "heat wave." Temporal biases in bottom temperature were minimized by subdividing each region into three subregions, each with short-duration surveys. Near-bottom temperatures, weighted by stratum area, were unsynchronized across subregions and exhibited varying subregional interannual variability. The weighted mean bottom depths in the subregions also vary largely among subregions. The centroids (centers of gravity) of groundfish distribution were weighted with catch per unit effort and stratum area for 10 commercially important groundfish species by size class and subregion. Our multivariate analyses showed that there were significant differences in aggregate fish movement responses to warm temperatures across subregions but not among species or sizes. Groundfish demonstrated poleward responses to warming temperatures only in a few subregions and moved shallower or deeper to seek colder waters. The temperature responses of groundfish depended on where they were. Under global warming, groundfish may form geographically distinct thermal ecoregions along the northeast Pacific shelf. Shallow-depth species exhibited greatly different distributional responses to temperature changes across subregions while deep-depth species of different subregions tend to have relatively similar temperature responses. Future climate studies would benefit by considering fish distributions on small subregional scales.