There are currently 22 native and 9 exotic fish species inhabiting the Imnaha subbasin (Table 13; Mundy and Witty 1998).
Table 13. Fish Species present in the Imnaha River Subbasin (Mundy and Witty 1998)
3 Fish species abundance based on average number of fish per 100m2: A=abundant, R=rare, U=uncommon, C=common, and I=insufficient data
Naturally occurring anadromous species in the Imnaha subbasin include spring/summer and fall chinook salmon (Oncorhynchus tshawytscha), summer steelhead (Oncorhynchus mykiss), and Pacific lamprey (Lampetra tridentata). Naturally reproducing Imnaha chinook populations were listed by the National Marine Fisheries Service as threatened on May 22, 1992 (Federal Register, Vol. 57, 14653) (National Marine Fisheries Service 1997). Similarly, wild Snake River summer steelhead were federally listed as threatened on August 18, 1997 (Federal Register, August 18, 1997, Vol. 62, 43937). (National Marine Fisheries Service 1997). Pacific lamprey are a federally listed species of concern, but considered extinct in the Imnaha subbasin.
Non-anadromous salmonids endemic to the Imnaha subbasin include interior redband trout (O. mykiss spp.), rainbow trout (O. mykiss spp.), bull trout (Salvelinus confluentus), and mountain whitefish (Prosopium williamsoni). As of April 20, 2000, redband trout were listed as a sensitive species in Oregon and managed similarly as steelhead when occurring in anadromous waters. Bull trout, which are under the jurisdiction of the U. S. Fish and Wildlife Service (USFWS) are also listed under the ESA. On July 10, 1998, the USFWS listed the Klamath and Columbia River bull trout as threatened (Federal Register, June 10, 1998, Vol. 63, 31647). Bull trout are also listed as a species of critical concern in Oregon. White sturgeon (Acipenser transmontanous ), a federally listed species of concern (CBFWA 1999), occasionally utilize lower portions of the mainstem Imnaha (Wallowa County and Nez Perce Tribe 1993) but do not likely inhabit the river for extended periods (D. Bryson, NPT, personal communication, March, 2001). Although Pacific lamprey are currently considered to be extinct, there is likely a population of the non-anadromous brook lamprey in portions of the subbasin.
Exotic species inhabiting the Imnaha subbasin are comprised primarily of centrarchids, ictalurids and cyprinids. Their distribution and abundance has increased following the construction of major hydroelectric projects on the Snake River (Wallowa-Whitman National Forest 1999). For example, the introduction of smallmouth bass (Micropterus dolomieu) into the Hells Canyon Hydroelectric Complex was accompanied by a subsequent expansion of the fishery into free flowing reaches of the Snake and Imnaha rivers (Wallowa-Whitman National Forest 1999). The development of the smallmouth population in free flowing environments warrants concern due to the piscivorous and competitive behavior smallmouth exhibit toward salmonid species.