Heat stress in migrating Yukon River Chinook salmon

Migrating adult Pacific salmon (Oncorhynchus spp.) are sensitive to warm water (> 18 °C) with a range of consequences from decreased spawning success to early mortality. We examined the proportion of Yukon River Chinook salmon (O. tshawytscha) exhibiting evidence of heat stress to assess the potential that high temperatures contribute to freshwater adult mortality in a northern Pacific salmon population. Water temperatures greater than 18 °C have occurred almost annually in the Yukon River and correspond with low population abundance since the 1990s. Using gene transcription products and heat shock protein 70 biomarkers validated by field experiment we identified heat stress in half of Chinook salmon examined (54%, n = 477) across three main-stem locations and three tributaries in 2016–2017. Biomarkers tracked wide variation in water temperature (14–23 °C) within a tributary. The proportion of salmon with heat stress differed between years at four of the six locations, with more prevalent heat stress in the warmer year. This work demonstrates that warming water temperatures are currently affecting northern populations of Pacific Salmon.

This research may contribute to preventing a Chinook salmon failure in the future. If evidence of heat stress is present in Yukon River Chinook salmon and heat stress can be linked to increased prespawn mortality during future studies, management plans can be adjusted to account for the additional increase in adult mortality. If freshwater mortality is underestimated in warm years, there is potential for overharvest and ultimately fishery failure. By sampling throughout the Yukon River watershed, we will identify areas of the watershed where heat stress is more common. Sampling at the Eagle test fishery on the Canadian boarded will provide an indication of whether fish escaped to Canada are heat stressed or not.