For the purposes of the Endangered Species Act (ESA), a "species" is defined to include "any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature" (Waples 1991). The Act, however, does not specify how to determine a "distinct population segment" (Johnson et al. 1994). To clarify the issue for Pacific salmon, the National Oceanic and Atmospheric Administration National Marine Fisheries Service (NOAA Fisheries) developed a policy to determine how the agency will interpret the term "distinct population segment." The NOAA Fisheries policy stipulates that a salmon population (or group of populations) will be considered "distinct" – and therefore a "species" under the ESA – if it represents an Evolutionarily Significant Unit (ESU) of the biological species (Waples 1991).
"An ESU is defined as a population that 1) is substantially reproductively isolated from conspecific populations and 2) represents an important component in the evolutionary legacy of the species" (Johnson et al. 1994). Information used to determine degree of reproductive isolation includes degree of genetic differentiation, incidence of straying, recolonization rates, and physical or ecological barriers to migration. Research on genetics and life history, habitat differences, and the effects of stock transfers or supplementation efforts can provide information about evolutionary significance. For coastal managers and other planning professionals, NOAA provides guidance for determining the conservation status of populations and larger groupings – like ESUs – of Pacific Salmonids (McElhany et al. 2000.).
Seventeen ESUs have been designated and mapped in the Pacific salmon range in California, Washington, Oregon, and Idaho. Each unit generally includes a major river basin such as the Snake or Sacramento Rivers or a section of coastline that may include several river basins as in the California Central Coast ESU. Maps of the ESUs are available at NOAA Fisheries Protected Resources Web site.
Johnson, O.W., R.S. Waples, T.C. Wainwright, K.G. Neely, F.W. Waknitz, and L.T. Parker. 1994. "Status Review for Oregon's Umpqua River Sea-Run Cutthroat Trout." U.S. Department of Commerce. NOAA Technical Memorandum NMFS-VWFSC-15. View on-line source.
McElhany, P., M.H. Ruckelshaus, M.J. Ford, T.C. Wainwright, and E.P. Bjorkstedt. 2000. "Viable Salmonid Populations and the Recovery of Evolutionarily Significant Units." U.S. Department of Commerce. NOAA Technical Memorandum NMFS-NWFSC-42, 56 pp. View document (PDF).
Waples, R.S. 1991. "Definition of 'Species' Under the Endangered Species Act: Application to Pacific Salmon." U.S. Department of Commerce. NOAA Technical Memorandum NMFS F/NWC-194. View on-line source.