ABSTRACT
In response to petitions to list populations of steelhead (anadromous Oncorhynchus mykiss) under the U.S. Endangered Species Act (ESA), the National Marine Fisheries Service (NMFS) initiated a status review of steelhead from the states of Washington, Idaho, Oregon, and California. A Biological Review Team (BRT) of 12 scientists from NMFS and the Biological Resources Division of U.S. Geological Survey identified 15 evolutionarily significant units (ESUs), or “species” under the ESA, for steelhead and evaluated the risk of extinction for these ESUs. Identification of these ESUs was based on genetic, life history, biogeographic, geologic, and environmental information. These ESUs include steelhead populations from the U.S.-Canada border south to the southern limit of their range in southern California. The BRT considered qualitative and quantitative information to assess extinction risks faced by the steelhead ESUs. Available information indicated a pattern of greater risk for ESUs approaching the edges of the species’ range, to the south in central and southern California, and to the east in the Columbia and Snake River basins. Based largely on the findings of the BRT, and after considering proposed and implemented conservation measures, NMFS has taken several listing actions for steelhead. As of July 1999, nine ESUs have been listed (two as endangered and seven as threatened) and three ESUs have been designated as candidate species for which listing may be warranted in the future. Three ESUs have been determined not to be warranted for listing at this time.
