“…These iconic fish are essential to the culture, food security and economy of the North Pacific, and are threatened across their range, with rising stream temperatures highlighted as a major contributor to this decline (Crozier et al, 2021). The vulnerability of Pacific salmon to high temperature stems from their anadromous life history, which promotes local adaption of reproductively isolated populations to historical streams conditions as a result of their return to natal streams to spawn (Eliason et al, 2011;Farrell et al, 2008;Zillig, FitzGerald, et al, 2023). As cold-water species, even modest increases in temperature can have severe consequences on population viability by reducing cardio-respiratory performance, which has been widely suggested to constrain thermal tolerance in salmonids (Clark et al, 2008) (Eliason et al, 2013), however the mechanistic basis of this vulnerability is an open question.…”