Gas Bubble Disease in Resident Fish of the Lower Clark Fork River

Gas bubble disease (GBD) occurs in the resident fish of the lower Clark Fork River that are exposed to total dissolved gas (TDG) supersaturation produced by the spill at upstream hydroelectric projects. This report describes the incidence and severity of GBD observed in fish routinely collected by electrofishing and other techniques during periods of high supersaturation from 1997 to 2000. These data include GBD observations for 1997, a year of extremely high runoff resulting in TDG levels approaching 150% of saturation, and for 1999, a year of moderately high TDG levels (typically 120–130% of saturation). Although electrofishing only samples that portion of the fish populations present near the river surface (upper 2 m), in a deep stream (3–25 m) like the lower Clark Fork, the observed incidence and severity of GBD was substantially lower than anticipated for the levels of TDG measured. It appears that the majority of fish are spending sufficient time at depths that avoid or mediate both the incidence and severity of GBD when TDG supersaturation is in the range of 120–130% of saturation. Fish also have access to a number of tributaries that have little or no supersaturation and to Lake Pend Oreille, where fish commonly occupy depths that provide hydrostatic compensation, which, in turn, eliminates the effects of exposure to supersaturation.