Freshwater hydrology emerges as the strongest predictor of phenological variability in a Southeast Alaska steelhead trout (Oncorhynchus mykiss) population.

Phenological variation, such as the migration trends of salmonids, may provide resilience in changing environmental scenarios, influencing individual survival and population productivity. Phenological shifts may be particularly consequential for anadromous species that rely on seasonal cues for migration between freshwater and marine habitats and may reflect underlying environmental changes. Steelhead trout Oncorhynchus mykiss), the anadromous life history variant of rainbow trout, may undergo these migrations multiple times in their life as an iteroparous species. Using 21 years of steelhead demographic, phenological, and morphometric data in a Southeast Alaska system (Sashin Creek), we found a shift towards earlier return dates over time for adult steelhead. We integrated local environmental and regional climate data into models to investigate potential predictors of migration date. Freshwater temperature and sex were the strongest predictors of adult upstream migration; adults returned earlier in years with higher freshwater temperatures, and females returned later than males. For steelhead smolts, freshwater temperature and snowfall were the strongest predictors of outmigration; smolts outmigrated earlier in years with higher freshwater temperatures and later in years with higher snowfall. With projected increases in temperature and precipitation and projected decreases in snowfall in Southeast Alaska, continued shifts towards earlier adult and smolt steelhead migration timing are expected. The potential fitness consequences of such shifts will depend on many factors, including the degree of environmental change and possible disruption of trophic synchronies. This work highlights that steelhead phenology is dynamic and should be considered when evaluating their resilience to environmental change.