Sublethal responses of fish to low dissolved oxygen (DO) include changes in cardiac function, increased respiratory and metabolic activity, alterations in blood chemistry, mobilization of anaerobic energy pathways, upset in acid-base balance, reduced growth and decreased swimming capacity. Lowered final temperature preferenda and avoiding low DO are behavioral responses, and under extreme conditions, fish may take advantage of oxygen-rich surface film water. Low DO can affect invertebrate communities by causing selective mortality or inducing drift, which may affect fish production. The presence of pollutants can exacerbate responses to low DO with the effect of raising the threshold DO at which such responses occur. Based on published literature, northern Alberta riverine fishes are grouped into four categories of acute lethal sensitivity. However, chronic DO requirements are far more important to long-term maintenance of healthy fish communities than acute tolerances. Defining chronic DO criteria for northern fishes is complicated by long periods of winter ice cover, possible presence of pulp mill effluent, and lack of information on many regional species. Although previously recommended criteria should provide a reasonable level of protection for fish, any reduction in DO below saturation will cause some production impairment within the aquatic community.