Changes in Distribution of Nonnative Brook Trout in an Idaho Drainage over Two Decades

Abstract

Invasions of brook trout Salvelinus fontinalis are believed to threaten the aquatic fauna native to the cold streams and lakes of western North America. Although invasion is widespread, documentation of historic versus recent invasion rates throughout drainages is lacking. We compared brook trout distributions in 1996 to those as early as 1971 for 17 streams in the upper South Fork Salmon River, Idaho. In 1996, brook trout occurred in 11 of the streams. Adult ranges expanded at least 0.5 km upstream in 8 streams, and upstream invasion (1.2-2.4 km) occurred in 3 of those. No recent invasion was apparent in 10 streams that lacked dispersal barriers within 1 km of the South Fork Salmon River confluence or of previous distribution limits, including 4 streams containing at least some adult brook trout. We also compared distributions between 1993 and 1997 in two streams of the Weiser River drainage, Idaho, and found no changes in upstream distribution limits over that shorter time interval. Although invasion may be ongoing in some streams, brook trout do not appear to be relentlessly invading every accessible stream.