By the Numbers: Freshwater Fish at Risk in the U.S.; December 1997; Scientific American Magazine; by Doyle; 1 Page(s)

Of all places on earth, rivers and lakes are the most dangerous for wildlife. Their natural ecology is segmented by dams and locks, their waters are diverted, and they are the principal depositories of civilization¿s wastes. It is therefore not surprising that aquatic species in the U.S. are at far greater risk of extinction than mammals and birds are. Of the 822 fish species native to American rivers and lakes, as many as 21 have become extinct since the time of the first European settlement, according to the Nature Conservancy in Arlington, Va., and its partners in the Natural Heritage Network. Their data show that another 297 species--36 percent of the total--are currently at risk of extinction. Other freshwater animals are in an even more perilous condition: 38 percent of amphibian, 50 percent of crayfish and 56 percent of mussel species are in jeopardy. Another 12 percent of mussel species are already extinct.

The three most important threats to freshwater fauna are agricultural runoff, dams and water diversion, and interference from exotic species (such as the flathead catfish, which was introduced in the Southwest and many other places for recreational fishing). Such alien species compete with native species and generally upset the balance of local ecologies.