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.

From 1994 to 1999, NOAA Fisheries Service, through biological review teams (BRTs) convened by the Science Centers, reviewed the ESA status of all anadromous salmon species on the West Coast. (BRTs are groups of federal agency scientists with expertise in the species being reviewed. They solicit and review all pertinent data and assess risks to the viability of the species.) During these reviews the BRTs identified 52 ESUs, and evaluated whether they were at risk of extinction and should be considered for listing as threatened or endangered under the ESA.

The final BRT reports provided a solid scientific foundation for NOAA Fisheries Service to make ESA listing determinations. Before beginning the coastwide status review, the agency had listed two salmon populations in the Snake River basin and one in California’s Sacramento River. Following the reviews, the agency listed a total of 26 salmonid ESUs, with five listed as endangered and 21 as threatened.

In 2003, the Northwest and Southwest Fisheries Science Centers convened another BRT to update the status reviews of all 26 listed salmon and steelhead ESUs, plus one candidate species population. This review was precipitated by the 2001 court ruling in Alsea Valley Alliance v. Evans (Alsea decision), that set aside NOAA Fisheries Service’s 1998 ESA listing of Oregon coast coho salmon. The court ruled that the ESA does not allow the agency to list a subset of a distinct population segment: it cannot list the natural fish in an ESU without listing the hatchery fish that are part of the same ESU; it cannot list the anadromous fish in an ESU without listing the resident fish that are part of the same ESU.

Although the court’s ruling affected only one ESU, it raised interpretive issues that called into question nearly all Pacific salmonid listing determinations (including NOAA’s listings of only the steelhead portion of O. mykiss ESUs, which include resident rainbow trout). In reevaluating the status of listed salmonid ESUs, the agency reexamined the role that hatchery-produced salmon play in listing determinations. This new policy - termed the Hatchery Listing Policy - is intended to ensure that salmon ESA listing determinations are in compliance with the Alsea decision. The new policy was released in June 2005.

NOAA Fisheries Service completed the status review updates (large file -- PDF 6.3MB) for 16 of the ESUs under review in June 2005. Despite improvements in abundance for several salmon ESUs during the past few years, NOAA Fisheries Service still believes that salmon listed in the 1990s warrant protection under the ESA. The updated listings maintained existing listings for 16 salmon ESUs, listed an additional ESU, and added to the listings those hatchery fish that are part of each ESU. The agency announced updated listings for 10 steelhead populations on Dec. 23, 2005.