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Chinook salmon population declines span their geographic range with climate hypothesized as a major driver. Concerns of warming freshwater temperatures in their northern range gained urgency during 2019 when a heatwave coincided with premature mortality. This study examined heat stress during the 2019 heatwave compared to subsequent years and described water temperatures in western Alaska to understand the degree to which freshwater temperatures may be a stressor. Heat stress was prevalent among Chinook salmon captured in the 2019 heatwave (Kuskokwim tributaries: 90% in Kwethluk and 63% Takotna river), and variable in subsequent years (~8% to 60% across Kuskokwim tributaries and Norton Sound rivers). A review of water temperature data indicated potentially stressful temperatures (≥18°C) were most common and prolonged in the Yukon River, moderately common and prolonged in the Kuskokwim River, and relatively rare in the Norton Sound region. Water temperatures in 2019 broke several records for overall maximum and frequency of temperatures ≥18°C. Migration water temperatures and heat stress in northern Pacific salmon habitats varies more widely than previously recognized (up to 25°C). 

Abstract In marine habitats, Atlantic salmon (Salmo salar) imbibe seawater (SW) to replace body water that is passively lost to the ambient environment. By desalinating consumed SW, the esophagus enables solute‐linked water absorption across the intestinal epithelium. The processes underlying esophageal desalination in salmon and their hormonal regulation during smoltification and following SW exposure are unresolved. To address this, we considered whether two Na⁺/H⁺exchangers (Nhe2 and −3) expressed in the esophagus contribute to the uptake of Na⁺from lumenal SW. There were no seasonal changes in esophagealnhe2or−3expression during smoltification; however,nhe3increased following 48 h of SW exposure in May. Esophagealnhe2, −3, andgrowth hormone receptor b1were elevated in smolts acclimated to SW for 2.5 weeks. Treatment with cortisol stimulated branchial Na⁺/K⁺‐ATPase (Nka) activity, andNa⁺/K⁺/2Cl⁻cotransporter 1(nkcc1),cystic fibrosis transmembrane regulator 1 (cftr1), andnka‐α1bexpression. Esophagealnhe2, but notnhe3expression, was stimulated by cortisol. In anterior intestine, cortisol stimulatednkcc2, cftr2, andnka‐α1b. Our findings indicate that salinity stimulates esophagealnhe2and−3, and that cortisol coordinates the expression of esophageal, intestinal, and branchial solute transporters to support the SW adaptability of Atlantic salmon.