Pacific salmon and steelhead are salmonids, of the scientific family Salmonidae. They are anadromous fish, which means that they migrate up rivers from the ocean to breed in fresh water. Pacific salmon are in the scientific genus Oncorhynchus, which includes pink, sockeye, chum, Chinook and coho salmon, steelhead and rainbow trout.

These fish have a complex life-cycle that spans a variety of fresh and saltwater habitats. Salmon are born in inland streams and rivers, migrate to coastal estuaries, and then disperse into ocean waters to grow. Once mature, they reverse their course, returning through the estuaries, fighting their way back upriver to the very streams where they were born, to reproduce, die and begin the cycle again.

In 1991, NOAA Fisheries Service received a petition to list Pacific Northwest salmon runs under the Endangered Species Act (ESA). In response, the Northwest Fisheries Science Center and the Southwest Fisheries Science Center launched a proactive, systematic review of all West Coast salmon runs. To do this, however, the agency first had to determine how a “species” of salmon was defined under the ESA.

The ESA allows listing of “distinct population segments” of vertebrates. NOAA Fisheries Service, through the scientific leadership and expertise of the Science Centers, developed a technical document to describe how it will apply this definition in evaluating Pacific salmon stocks for listing under the ESA. A policy (PDF 902KB) was then developed that establishes a group of salmon populations to be a distinct population segment if it is an “evolutionarily significant unit,” or ESU. Scientists established two criteria for ESUs: 1) the population must show substantial reproductive isolation; and 2) there must be an important component of the evolutionary legacy of the species as a whole.