Potential limits to local adaptation of Coho Salmon populations from eggs to fry under naturally varying and warming temperature conditions

Objective Fish survival responses to stressful conditions from climate change involve physiological adaptation and/or behavioral avoidance through shifts in phenology or distribution. How coldwater-adapted, anadromous salmonids will respond in this regard is a major research and management question owing to their genetic and phenological diversity, their life history complexity, and the human reliance on location-based hatcheries and fisheries. Methods We experimentally evaluated physiological adaptation of two Coho Salmon Oncorhynchus kisutch populations across the first 9 months of early development while the fish were exposed to naturally varying stream thermal regimes (Plus0 treatment) with warming scenarios of +2°C (Plus2) and +4°C (Plus4). The more interior population of Coho Salmon spawns and rears in conditions within the range of our temperature treatments, whereas the coastal population generally experiences temperatures that are 6°C lower than our Plus4 temperature treatment. Results We found evidence of local adaptation, as the interior population outpaced the coastal population in terms of growth and had the largest body sizes in Plus4. The interior population, and to a lesser degree the coastal population, also exhibited higher growth per accumulated thermal unit at lower temperatures, indicating either compensatory responses at low temperatures and/or thermal stress at high temperatures. However, we found minimal differences between populations and temperature treatments in embryonic development stages (eyed egg to emergence). Fry hormone measurements (cortisol, heat shock protein 70, and insulin-like growth factor 1) indicated that both populations were physiologically stressed in the Plus4 treatment (daily mean ∼20°C). Conclusions Results identified beneficial growth responses by the interior population to warm temperatures but a potentially narrower window for plasticity and adaptation than the coastal population. Evidence of physiological adaptation for these two populations holds promise for some capacity of salmonids to withstand warming conditions, although fish populations that are already adapted to warmer conditions may be limited in further phenotypic responsiveness.