Behavior of Resident Fish Relative to Total Dissolved Gas Supersaturation in the Lower Clark Fork River

Abstract

The behavior of resident fish exposed to total dissolved gas (TDG) supersaturation in Pacific Northwest rivers greatly influences the degree of supersaturation these fish actually experience. Because TDG supersaturation is a physical condition that is moderated by hydrostatic pressure, the depths occupied by fish during supersaturation conditions determine the biological effects experienced by members of the exposed population. Data obtained from fish equipped with depth-sensing radio tags showed that many of the fish spent sufficient time at depths of several meters or more, where they are not exposed to TDG supersaturation. These depths also provide an opportunity to recover from the short-term exposure to supersaturation experienced by the fish during the periods they occupy shallower depths. Most species tagged had median and average depth distributions of about 2 m or more, providing compensation for TDG supersaturation in the range of 120% of saturation or more. Tagged rainbow trout Oncorhynchus mykiss generally remained in the river for only brief periods before returning to Lake Pend Oreille or to the tributaries of the lower Clark Fork River, where they were no longer exposed to TDG supersaturation.