The Sebasticook River is the largest tributary (985 square miles) to the Kennebec and thus plays an important role in the restoration of both the anadromous and resident aquatic fisheries of the Kennebec basin and the Gulf of Maine ecosystem. Due to its relatively close proximity to the lower Kennebec, large drainage area, and low gradient, this watershed historically contributed a major percentage of available spawning and nursery habitat for anadromous runs of alewife, blueback herring, American shad, rainbow smelt, and striped bass associated with the Kennebec River watershed and Gulf of Maine ecosystems. To a lesser extent, the river provided habitat for Atlantic salmon.