Ocean acidification affects somatic and otolith growth relationship in fish: evidence from an in situ study

Ocean acidification (OA) may have varied effects on fish eco-physiological responses. Most OA studies have been carried out in laboratory conditions without considering the in situ pCO₂/pH variability documented for many marine coastal ecosystems. Using a standard otolith ageing technique, we assessed how in situ ocean acidification (ambient, versus end-of-century CO₂ levels) can affect somatic and otolith growth, and their relationship in a coastal fish. Somatic and otolith growth rates of juveniles of the ocellated wrasse Symphodus ocellatus living off a Mediterranean CO₂ seep increased at the high- pCO₂ site. Also, we detected that slower-growing individuals living at ambient pCO₂ levels tend to have larger otoliths at the same somatic length (i.e. higher relative size of otoliths to fish body length) than faster-growing conspecifics living under high pCO₂ conditions, with this being attributable to the so-called 'growth effect'. Our findings suggest the possibility of contrasting OA effects on fish fitness, with higher somatic growth rate and possibly higher survival associated with smaller relative size of otoliths that could impair fish auditory and vestibular sensitivity.