ABSTRACT

Many fish naturally encounter a daily cycle of hypoxia, but it is unclear whether this exposure hardens hypoxia-intolerant fish to future hypoxia or leads to accumulated stress and death. The rainbow trout (Oncorhynchus mykiss) is a putatively hypoxia-sensitive species found in rivers and estuaries that may routinely experience hypoxic events. Trout were exposed to one of four 135 h treatments in a swim-tunnel respirometer: (1) air-saturated control (20.7 kPa P_O2); (2) diel cycling O₂ (20.7–4.2 kPa P_O2 over 24 h); (3) acute hypoxia (130 h at 20.7 kPa P_O2 followed by 5 h at 4.2 kPa P_O2); and (4) the mean oxygen tension (12.4 kPa P_O2) experienced by the diel cycled fish. Some responses were similar in diel O₂ cycled and mean P_O2-treated fish, but overall, exposure to ecologically representative diel hypoxia cycles improved hypoxia tolerance. Diel hypoxia-induced protective responses included increased inducible HSP70 concentration and mean corpuscular hemoglobin concentration, as well as reduced plasma cortisol. Acclimation to diel hypoxia allowed metabolic rates to decline during hypoxia, reduced oxygen debt following subsequent exposures, and allowed fish to return to an anabolic phenotype. The data demonstrate that acute diel cycling hypoxia improves hypoxia tolerance in previously intolerant fish through the activation of cellular protective mechanisms and a reduction in metabolic O₂ requirements.