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.