More than 200 tributaries drain into Lake Superior from a relatively small (by lake surface area) watershed. The large volume of the lake (12,100 km3) and relatively small discharge account for a water residence time of approximately 191 years making the lake particularly sensitive to pollutants. Nipigon Bay and Black Bay are large, shallow, warm, and highly productive environments unique to the western portion of the lake. They provide important habitat for many fish species that are not typically found at greater depths and attract recreational and commercial fishing. Offshore shoals and associated currents and upwellings near the outer islands also make these areas attractive for fishing.