Pollution, dams, and introduction of exotic species have decimated some native fish populations. Efforts to mitigate these problems and restore Atlantic salmon to the Connecticut River watershed have had very little success. The other widespread native salmonid, the brook trout, persists in cold upland streams, particularly above waterfalls and other barriers that exclude introduced brown and rainbow trout. American shad runs have retained at least a fraction of their former abundance, and smallmouth bass, sunfish, and pike populations are healthy enough to support angling.