Homing ability of adult cardinalfish is affected by elevated carbon dioxide

The levels of carbon dioxide (CO2) predicted for the oceans by the end of this century have recently been shown to impair olfactory discrimination in larval fishes. However, whether this disruption extends to olfactory-mediated behaviour in adult fishes is unknown. In many fishes, adult survival and reproduction can be critically dependent upon navigation to home sites. We tested the effects that near-future levels of CO2 (550, 700 or 950 ppm) have on the ability of adult five-lined cardinalfish, Cheilodipterus quinquelineatus, to home to their diurnal resting sites after nocturnal feeding. Cardinalfish exposed to elevated CO2 exhibited impaired ability to distinguish between odours of home- versus foreign-site conspecifics in pair-wise choice experiments. A displacement experiment demonstrated that fish from all CO2 treatments displayed a 22-31% reduction in homing success compared with control fish when released at 200 m from home sites. While CO2-exposed cardinalfish released directly back onto home sites exhibited similar site fidelity to control subjects, behaviour at home sites was affected, with CO2-exposed fish exhibiting increased activity levels and venturing further from shelter. This study demonstrates that the potential disruption of chemosensory mechanisms in fishes due to rising CO2 levels in the ocean extend to critical adult behaviours.