To date the study of ocean acidification on fish otolith formation has been mainly focused on larval and juvenile stages. In the present pilot study, wild-captured adult Atlantic cod (Gadus morhua) were exposed to two different levels of pCO_2, 422µatm (ambient, low pCO₂) or 1091µatm (high pCO₂), for a period of 30 weeks (from mid-October to early April 2014-2015) in order to study the effects on otolith size, shape and CaCO₃ crystallization amongst other biological parameters. We found that otoliths from cod exposed to high pCO₂ were slightly smaller (- 3.4% in length; - 3.3% in perimeter), rounder (- 2.9% circularity and + 4% roundness) but heavier (+ 5%) than the low pCO₂ group. Interestingly, there were different effects in males and females; for instance, male cods exposed to high pCO₂ exhibited significant changes in circularity (- 3%) and roundness (+ 4%) compared to the low pCO₂ males, but without significant changes on otolith dimensions, while females exposed to high pCO₂ had smaller otoliths as shown for length (- 5.6%), width (- 2%), perimeter (- 3.5%) and area (- 4.8%). Furthermore, while the majority of the otoliths analysed showed normal aragonite deposition, 10% of fish exposed to 1091µatm of pCO₂ had an abnormal accretion of calcite, suggesting a shift on calcium carbonate polymorph crystallization in some individuals under high pCO₂ conditions. Our preliminary results indicate that high levels of pCO₂ in adult Atlantic cod might affect otolith growth in a gender-specific way. Our findings reveal that otoliths from adult cod are affected by ocean acidification, and we believe that the present study will prompt further research into this currently under-explored area.

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