Are fish deterred by carbon dioxide because they detect the CO2 itself and/or the carbonic acid it creates?
It is well established that many species of fish avoid carbon dioxide (CO2) enriched waters. However, carbon dioxide is known to create carbonic acid when it dissolves, so it is possible that either the gas itself and/or the acidity of the carbonic acid (pH) could be responsible. Notably, some fish have external taste systems that can detect both dissolved CO2 and pH. Because CO2 has been suggested for use as an invasive species deterrent, it is important to understand the basis of this response. To investigate this, we tested behavioral responses of rainbow trout, a model fish with external taste systems. Groups of four trout were placed into a tank with flowing water which had a gas/fluid injection system placed perpendicularly across its mid-line. Fish were then observed for a 15 min pre-test, followed by a 15 min test period when one of 4 treatments were administered: 1) nothing (control); 2) CO2 gas; 3) atmospheric air (a control for the injection of gas), or 4) air plus hydrochloric acid (HCl) to create a pH plume equivalent to that created by CO2/carbonic acid. The frequency of mid-line crossings and the relative distribution of fish in the tank (% time in the upstream region with a stimulus) was recorded and analyzed using a Kruskal-Wallis test. While the addition of air had no apparent influence on trout behaviors (p>0.05), CO2 both reduced mid-line crossings (52% drop; p<0.05) and increased time spent in the upstream region (85%; p<0.05). The addition of air + HCl caused no change in mid-line crossings (p>0.05), but did evoke a notable increase in time spent in the upstream region (96%; p<0.05). Comparing CO2 vs HCl, we found that the former was better at blocking fish movement, while their abilities to deter were similar. We conclude that CO2 and pH are independently discerned by the rainbow trout, and suggest that future studies examine the role of the taste system directly.