Atlantic Salmon Migration Timing Determined by Ocean Temperature Phenology

The migration phenology of Atlantic salmon (Salmo salar) adults returning to the Penobscot River, Maine, has changed dramatically over recent decades. The median date of the returning run of Atlantic salmon has advanced by over a month since 1978, with even stronger advances in dates of later portions of the run. As such, the majority of the run (between 5^th and 95^th percentiles of returning fish) now occurs in a much more condensed period of time than it has in the past. These seasonal advances in run timing are associated with shifting ocean temperature phenology—particularly the length of winter—in Atlantic salmon overwintering habitat around Newfoundland. In this area, calculations with daily Optimum Interpolation Sea Surface Temperature time series using a location-specific temperature threshold (i.e., Thomas et al. 2017) show the duration of winter has become shorter since 2000. Potential consequences of the advance in migration timing include a narrowing of population diversity, as unique traits and growth patterns are no longer represented among late-returning fish.