Physiological and behavioral sensitivity to hypoxia in two historically overfished rockfish species (Sebastes spp.) with divergent habitat partitioning

In the eastern North Pacific, the oxygen minimum zone (OMZ) has shoaled in recent decades, potentially affecting the habitat and distribution of commercially important fish species such as rockfishes (genus Sebastes ). Respirometry and behavioral experiments were used to determine the hypoxia sensitivities of 2 rockfish species that inhabit deep-water, rocky-reef habitat along the upper boundaries of the OMZ, but that exhibit contrasting movement patterns and habitat use: cowcod S. levis are strongly benthic-oriented, while bocaccio S. paucispinis are more pelagic and active above the reef. These species thus serve as excellent models for examining the potential range in hypoxia sensitivity among rockfishes and similar species. Standard metabolic rate, critical oxygen threshold ( P crit ), and hypoxia-induced loss of equilibrium (LOE) were determined for each species at 2 seasonal and depth-relevant temperatures (9 and 12°C). Additionally, a shuttle box system was used to determine the dissolved oxygen (DO) level eliciting a behavioral avoidance response. Results show that cowcod and bocaccio are both resilient to low-oxygen environments, although cowcod generally demonstrated greater tolerance to low oxygen levels and changes in temperature. In particular, cowcod tolerance to low oxygen ranks especially high among fishes and likely facilitates a sit-and-wait approach to episodic low-oxygen exposure, whereas more mobile bocaccio are more likely to behaviorally avoid hypoxia and seek favorable conditions. Our findings indicate that DO levels typical of depths occupied by bocaccio are nearing their critical metabolic limits. Thus, the shoaling of the OMZ is likely to push bocaccio and similar species into shallower habitats.