Mapping suitable thermal migration corridors for western Alaska chum salmon in the North Pacific

It is evident that warming North Pacific Ocean (NPO) temperatures are impacting salmon fitness and survival. Record-low western Alaska chum salmon run sizes were recorded in the Yukon River during 2020 and 2021. Based on recent analyses of Pacific salmon species-specific ocean temperature preferences, chum salmon have the widest thermal preference range; we focus on chum because they may have the greatest resilience to temperature variability and if their range is impacted, it is likely other species will respond to observed warming as well. Thermal suitability was mapped along the seasonal migration based on swimming rates to examine potential interannual range or distribution shifts. Two individual-based models of salmon migration driven by surface temperatures and geostrophic currents were run to test the impact of ocean physics on migration and distribution. We found that in the last decade ocean temperatures have similar magnitude and variability as projected through 2050. Since around 2013, the high suitability migration corridor has shifted northwards into the Bering Sea, but the seasonal migration may not be driven primarily by temperature or ocean currents. Warmer ocean temperatures, marine heatwaves, and loss of seasonal sea ice are likely to have the greatest impacts on western Alaska chum salmon where thermal suitability is lowest–in the Bering Sea in winter, and in the eastern Gulf of Alaska in summer. The impacts at the margins of their habitat range may lead to poor condition, a northward domain shift, and more fish entering the Arctic Ocean. • The chum salmon migration corridor was further north 2013-2023 than previous decades. • In warm years, chum salmon have a reduced range of high thermal suitability habitat. • Regional surface temperatures from 2019–2022 are similar to future projected ranges. • Evolution may primarily drive chum migration, except at the low suitability margins.