The lack of avoidance of total dissolved gas supersaturation in juvenile rainbow trout

We investigated whether individual rainbow trout (Oncorhynchus mykiss; 9–53 g) avoid a potentially lethal level of total dissolved gas (TDG) supersaturation using lateral movements during an acute exposure. As there is no mechanism by which fish can detect and avoid TDG supersaturation in shallow water, we hypothesize that rainbow trout do not directly detect TDG supersaturation. Most previous studies have tested TDG avoidance in groups of fish, which may confound responses because many fishes, such as juvenile rainbow trout, display territorial behavior. We placed rainbow trout individually into a flume and allowed them to swim freely between two channels for 6 min: one channel contained water at 145% TDG and the other contained air-equilibrated water. We then switched treatments between channels and tested fish in the same way for an additional 6 min. We quantified the duration spent by fish in each channel during the last 2 min of each 6 min exposure. Fish spent a mean duration of 11s longer in 145% TDG water compared to air-equilibrated water, and the inclusion of TDG treatment as a factor improved the model of the duration spent by fish in a channel. These results indicate that fish did not avoid a potentially lethal level of TDG. This suggests that some fishes in shallow water may be unable to avoid harmful TGD supersaturation generated by events such as spilling from hydroelectric dams.