Introduction
Evolutionarily Significant Units (ESU) are defined by NOAA Fisheries under their Endangered Species Act (ESA) status reviews. They are defined as a population that is substantially reproductively isolated from cospecific populations and represents an important component in the evolutionary legacy of the species. The ESU policy for Pacific salmon defines the criteria for identifying a Pacific samon population as a distinct population segment (DPS), which can be listed under the ESA.