Linkages between marine and riverine environmental phenology and adult Atlantic salmon return timing

Phenological changes are occurring around the world as climate change alters the timing of life history processes, such as animal migrations. This study demonstrates linkages between environmental change and Atlantic salmon (Salmo salar) migration dynamics. From 1983-2019, adult Atlantic salmon returned earlier from the ocean to the Penobscot River, Maine. Median return dates advanced 12 days per decade for one sea-winter (1SW) salmon and seven days per decade for multi-sea-winter (MSW) salmon. The strongest advances occurred late in the run, with the 95th percentile of adults returning 19 (1SW) and 23 (MSW) days earlier per decade. Late-stage advancement contracted run duration by 12 (1SW) and 17 (MSW) days per decade. Ocean seasonality—particularly the length of winter and summer based on sea surface temperatures—was most strongly associated with earlier returns. For some run segments, the timing of freshwater flows and the start of summer riverine temperatures influenced return dates. Findings reveal changes in Northwest Atlantic Ocean temperature phenology, link shifting environmental phenology to migration phenology, and identify concerns about Atlantic salmon adaptiveness.