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

—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. The invasion of cold streams and lakes by brook trout Salvelinus fontinalis has contributed to the decline of native fishes, amphibians, and invertebrates throughout much of the United States and Canada (Leary et al. 1993; Carlisle and Hawkins 1998; Dunham et al. 1999; Knapp and Matthews 2000). Preventing further invasion has become a major concern for many managers, particularly as mandates to preserve native fauna have increased (Kruse et al. 2001). Invasion can be viewed as a cycle with three * Corresponding author: [email protected] 1 Present address: U.S. Forest Service, Southern Research Station, 1000 Front Street, Oxford, Mississippi 38655, USA. 2 Present address: Pacific Rivers Council, PMB 219, 1 Second Avenue East, Suite C, Polson, Montana 59860, USA. Received May 22, 2001; accepted November 21, 2001 phases: (1) arrival (including dispersal), (2) establishment, and (3) integration (Vermeij 1996). Establishment is defined as the presence of a locally reproducing population (Vermeij 1996). With positive population growth and additional dispersal, the cycle continues, and invasion progresses either continuously or intermittently. Different factors operating over increasingly long time scales can influence success or failure in each phase (Carroll and Dingle 1996; Moyle and Light 1996; Vermeij 1996). We use the term ‘‘distribution expansion’’ to describe situations in which brook trout had arrived since previous surveys but were not es-