High latitude, high tolerance: Northern juvenile Chinook salmon exhibit substantial thermal plasticity and acute heat tolerance

In a rapidly warming North, Chinook salmon are facing rising river temperatures, increasing frequencies of heat waves, and greater annual thermal variation. These factors have putatively contributed to substantial declines in Alaska Chinook salmon abundance; understanding the thermal sensitivity and acclimation capacity of Chinook salmon at the northern extents of their range is therefore imperative. To this end, we acclimated juvenile Yukon River Chinook to five ecologically relevant summer temperatures (12.5, 15, 17.5, 20, and 22.5{degrees}C) under two feeding treatments of 100% or 40% satiation. We then assessed whole organism heat tolerance at rest and while swimming, along with the thermal sensitivity of heart rate and cardiac electrical activity. Yukon River Chinook substantially increased whole organism heat tolerance at rest and under exercise with warm acclimation. Cardiac heat tolerance also increased, and fish showed marked thermal compensation of maximum heart rate, resetting it to lower levels with warm acclimation. Feed treatment had a limited effect outside of peak heart rate and the transition temperatures of Q10. Warm acclimation was also associated with a shortening of the PR and QRS intervals and a lengthening of the QT interval. Across all acclimation groups, the PR interval plateaued to an extent far greater than heart rate itself, suggesting that limitations to AV conduction may limit cardiac function at high acute temperatures. Altogether, high latitude juvenile Chinook display remarkable heat tolerance and thermal plasticity, which may be preserved in association with the large seasonal, weather dependent, and spatial thermal variation Yukon River Chinook face during freshwater rearing and along with their extensive seaward and return migrations. The acute heat tolerance of Yukon River Chinook, which is comparable to or outperforming lower latitude Chinook and other salmonids, emphasizes the need for population-specific work for informed conservation efforts.