The Endangered Species Act (ESA) allows listing of "distinct population segments" of vertebrates as well as named species and subspecies. The policy of the National Marine Fisheries Service (NMFS) on this issue for Pacific salmon and steelhead is that a population will be considered "distinct" for purposes of the ESA if it represents an Evolutionarily Significant Unit (ESU) of the species as a whole. To be considered an ESU, a population or group of populations must 1) be substantially reproductively isolated from other populations, and 2) contribute substantially to ecological/genetic diversity of the biological species. Once an ESU is identified, a variety of factors related to population abundance are considered in determining whether a listing is warranted. NMFS received a petition in May 1992 asking that winter steelhead of Oregon's Illinois River be listed as a threatened or endangered species under the ESA. In May 1993, NMFS published a Federal Register notice concluding that Illinois River winter steelhead did not by themselves constitute a species as defined by the Endangered Species Act (ESA). At the same time, NMFS indicated that it would undertake a broader status review to determine the boundaries of the Evolutionarily Significant Unit (ESU) that contains Illinois River winter steelhead and determine whether this broader group was threatened or endangered. This report summarizes biological and environmental information gathered in that status review.
Based on genetic, life history, zoogeographic, geologic, and environmental information, we conclude that the ESU that contains Illinois River winter steelhead extends from the vicinity of Cape Blanco in southern Oregon to the Klamath River Basin (inclusive) in northern California. These are essentially the boundaries of a prominent geologic feature known as the Klamath Mountains Province.Keyword Tags: