Placing CO2 Effects on Fish Otoliths in a Natural Variation Context

Numerous studies have observed hypercalcification of otoliths in fishes exposed to elevated CO₂ levels associated with projected ocean acidification. Most of these experiments have been short-term exposures in larvae. Few studies have evaluated whether these effects persist into the juvenile stage or attempted to put the effect into the context of natural variation in otolith size. This study examines the effects of a 32-week exposure to elevated CO₂ levels (to 2900 µatm) on otolith characteristics in juvenile walleye pollock (Gadus chalcogrammus). Hypercalification of the otolith in response to increased CO₂ did occur in walleye pollock juveniles with the magnitude of the effect increasing with CO₂ concentration. This suggests that the effect is not restricted to larvae and does not appear to be compensated for in prolonged exposures to elevated CO₂. Interestingly, the effect of CO₂ was restricted to otolith size; there were no significant effects of CO₂ level on shape or degree of asymmetry. Otolith sizes of similarly-sized, wild-caught fish confirmed that elevated CO₂ levels result in larger otolith sizes than observed in natural populations. While the sensory and ecological impacts of otolith hypercalcification remains speculative, this work places these effects into a broader context natural patterns of variation.