What is an Evolutionarily Significant Unit (ESU)?
An ESU is a collection of one or more salmon populations that share similar genetic, ecological, and life history traits but differ in important ways from salmon in other ESUs. Salmon ESUs are considered to be “distinct population segments” under the federal Endangered Species Act (ESA). Because a species, as defined in the ESA, can include subspecies, salmon ESUs can be listed under the ESA if they are threatened or endangered. Because the ESA does not explain or define how “distinct population segments” should be identified, the NMFS has developed a policy to do this for salmon based on the ESU concept.
