It’s a good thing that severely hypoxic salmon (Salmo salar) havea limited capacity to increase heart rate when warmed

With climate change, fish are facing rising temperatures, an increasein the frequency and severity of heat waves and hypoxia, sometimesconcurrently. However, only limited studies have examined thecombined effects of increases in temperature and hypoxia onfish physiology and survival. We measured the cardiorespiratoryphysiology of 12°C-acclimated Atlantic salmon when exposedacutely to normoxia [100% air saturation (sat.)] versus 75 and 50%air sat., and then warmed to their critical thermal maximum (CT max ) at2°C h−1 . Fish exposed to 50% air sat. became bradycardic, wereunable to increase heart rate ( fH ) when warmed, and had lowervalues for metabolic scope and CTmax (21.3 vs 26.1°C in normoxicfish). The effects of 75% air sat. on cardiorespiratory parameters andCTmax were intermediate. We then used atropine (1.2 mg kg−1) and 8-cyclopentyltheophylline (CPT; 50 nmol kg−1) to investigate whatrole(s) cholinergic tone on the heart and cardiac adenosinergiceffects, respectively, play in preventing severely hypoxic salmon(40% air sat.) from increasing fH when warmed. CPT had no/limitedeffects on salmon cardiorespiratory parameters and thermaltolerance. However, atropine increased fH in hypoxic fish andallowed it to rise with temperature, and this resulted in salmonthat were much less tolerant to warming. Collectively, these results:(1) show that fish in severely hypoxic environments will bevery susceptible to climate change-associated heat waves; and(2) suggest that cholinergic tone on the heart is not removed whenseverely hypoxic fish are exposed to rising temperatures to protectthe heart’s pumping capacity.