Behavioral responses of the rocky intertidal mosshead sculpin (Clinocottus globiceps) and woolly sculpin (Clinocottus analis) to increasing environmental hypoxia (low PO₂) were observed in a seawater aquarium with a depth gradient. The experimental design allowed observation of the response of these animals to encroaching environmental hypoxia. We observed three behaviors by which the fish dealt with hypoxia: emergence from substrate (EFS), first avoidance response (FAR), and aquatic surface respiration (ASR). With respect to EFS and FAR, mosshead sculpins were more labile in their responses than woolly sculpins. However, the mean PO₂ at which ASR was initiated was statistically indistinguishable between species. We suggest that the differences we observed represent alternate behaviors by which these related species deal with the trade-off imposed by the simultaneous needs to avoid predation and optimize respiration. We further suggest that these alternate behaviors have evolved around a conserved physiological constraint, the PO₂ at which ASR must be performed.