Effects of acute hypoxia exposure and acclimation on the thermal tolerance of an imperiled Canadian minnow

Elevated water temperatures and low dissolved oxygen (hypoxia) are pervasive stressors in aquatic systems that can be exacerbated by climate change and anthropogenic activities, and there is growing interest in their interactive effects. To explore this interaction, we quantified the effects of acute and long-term hypoxia exposure on the critical thermal maximum (CT_max) of Redside Dace (Clinostomus elongatus), a small-bodied freshwater minnow with sparse populations in the Great Lakes Basin of Canada and designated as Endangered under Canada's Species at Risk Act. Fish were held at 18°C and acclimated to four levels of dissolved oxygen (>90%, 60%, 40%, and 20% air saturation). CT_max was measured after 2 and 10 weeks of acclimation and after 3.5 weeks of reoxygenation, and agitation behavior was quantified during CT_max trials. Aquatic surface respiration behavior was also quantified at 14 weeks of acclimation to oxygen treatments. Acute hypoxia exposure decreased CT_max in fish acclimated to normoxia (>90% air saturation), but acclimation to hypoxia reduced this effect. There was no effect of acclimation oxygen level on CT_max when measured in normoxia, and there was no effect of exposure time to hypoxia on CT_max. Residual effects of hypoxia acclimation on CT_max were not seen after reoxygenation. Agitation behavior varied greatly among individuals and was not affected by oxygen conditions. Fish performed aquatic surface respiration with low frequency, but performed it earlier when acclimated to higher levels of oxygen. Overall, this work sheds light on the vulnerability of fish experiencing acute hypoxia and heat waves concurrently.