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To be considered for protection under the U.S. Endangered Species Act (ESA), a group of organisms must first qualify as a "species" as defined by the ESA. To be considered a "species," the group must represent an evolutionarily significant unit (ESU) of the biological species (Waples 1991). If this requirement is met, the status of the ESU is evaluated in terms of its qualifications for a listing as threatened or endangered.

The National Marine Fisheries Service (NMFS) Species Definition Paper (Waples 1991) provides a guide for evaluating the petitions for the three forms (spring-, summer-, and fall-run) of Snake River chinook salmon (Oncorhynchus tshawytscha). Those groups that are reproductively isolated from groups with different run-times should be considered separately under the ESA; fish of different run-times for which reproductive isolation cannot be established should be considered as a unit. In the Snake River, there is compelling evidence that fall chinook salmon are reproductively isolated from the other two forms. However, the key information necessary to understand the reproductive and evolutionary relationship between spring- and summer-run fish is lacking. Because the possibility of substantial gene flow exists between the two forms in streams where they co-occur, it is inappropriate at this time for ESA evaluations to assume the two forms represent independent evolutionary lineages. Therefore, NMFS will consider fall chinook salmon separately and spring and summer chinook salmon together in ESA evaluations. 

To be considered an ESU under the ESA, a population (or group of populations) must be reproductively isolated from conspecific populations, and must contribute substantially to the ecological/genetic diversity of the species (Waples 1991). Snake River spring and summer chinook salmon as a group meet both criteria. Although there are some indications that more than one ESU may exist within the Snake River Basin, the data presently available are not sufficient to clearly demonstrate the existence of multiple ESUs or to define their boundaries. Therefore, at present, the NMFS Northwest Region Biological Review Team concludes that the Snake River spring and summer chinook salmon are to be considered a single ESU.

There is no official NMFS policy regarding thresholds for determining threatened or endangered threshold status. Therefore, a variety of factors were used to evaluate the status of the population: historical, current, and projected abundance; trends in abundance; and the spatial and temporal distribution of fish. In addition, the stochastic extinction model of Dennis et al. (1991) was employed to gain some insight into the likely persistence of the ESU in the future if corrective actions are not taken. Collectively, the information suggests Snake River spring and summer chinook salmon are in jeopardy, but not in imminent danger of extinction throughout a significant portion of their range. However, they are likely to become endangered in the near future if corrective measures are not taken.

ACKNOWLEDGMENTS
The status review for Snake River spring and summer chinook salmon was conducted by the NMFS Northwest Region Biological Review Team (BRT). The extensive public record developed pursuant to this petition and discussions of that record by the ESA Technical Committee formed the basis for the review. Members of the BRT for spring and summer chinook salmon were: Brian Beckman, David Damkaer, Thomas Flagg, Elizabeth Garr, Orlay Johnson, Robert Jones, Conrad Mahnken, Gene Matthews, Desmond Maynard, George Milner, Gerald Monan, Ben Sandford, Michael Schiewe, George Swan, Grant Thompson, Merritt Tuttle, William Waknitz, Robin Waples, John Williams, Gary Winans, and Waldo Zaugg.
INTRODUCTION
Spring, summer, and fall chinook salmon (Oncorhynchus tshawytscha) are native to the Snake River, the largest tributary of the Columbia River. Historically, these fish were abundant throughout most of this large, complex drainage. From the latter 1800s until the present, a variety of factors (including overfishing, irrigation diversions, logging, mining, grazing, obstacles to migration, hydropower development, and questionable management practices and decisions) have led to the current depressed status of these populations. This situation led Oregon Trout, Oregon Natural Resources Council, Northwest Environmental Defense Center, the Idaho and Oregon Chapters of the American Fisheries Society, and American Rivers to petition the National Marine Fisheries Service (NMFS) to list all three forms of Snake River chinook salmon as threatened or endangered "species" under the Endangered Species Act (ESA). This report summarizes the review of the status of Snake River spring and summer chinook salmon conducted by the NMFS Northwest Region Biological Review Team (BRT). For reasons that will be explained below, the Snake River spring and summer chinook salmon petitions are considered together in this report.

KEY QUESTIONS IN ESA EVALUATIONS
Two key questions must be addressed in determining whether a listing under the ESA is warranted:1) Is the entity in question a "species" as defined by the ESA?2) If so, is the "species" threatened or endangered?

The "Species" Question
The ESA of 1973, as amended in 1978, allows listing of "distinct population segments" of vertebrates as well as named species and subspecies. However, the Act provides no guidance for determining what constitutes a distinct population, and the resulting ambiguity has led to a variety of criteria being used in listing decisions over the past decade. To clarify the issue for Pacific salmon, NMFS published an interim policy describing how the definition of "species" in the Act will be applied to anadromous salmonid species (Federal Register Docket No. 910248-1048; 13 March 1991). A more detailed description of this topic appears in the NMFS "Definition of Species" paper (Waples 1991). 

The NMFS policy stipulates that a salmon population will be considered "distinct" for purposes of the Act if it represents an evolutionarily significant unit (ESU) of the biological species. To qualify as an ESU, a population (or group of populations) must be a) reproductively isolated from conspecific populations and b) represent an important component in the evolutionary legacy of the species. Types of information that can be useful in determining the degree of reproductive isolation include incidence of straying, rates of recolonization, degree of genetic differentiation, and the existence of barriers to migration. Insight into evolutionary significance can be provided by data on phenotype, protein, or DNA characters; life history characteristics; habitat differences; and the effects of stock transfers or supplementation efforts.

For the spring and summer chinook salmon ESA evaluations, it is also necessary to consider races of fish that have traditionally been differentiated on the basis of run-timing. Following the framework of the "Definition of Species" paper, it first must be determined whether spring-, summer-, and fall-run chinook salmon in the Snake River are separate, reproductively isolated groups. Those groups that are reproductively isolated from groups with other run-times should be considered separately for ESA purposes; fish of different run- times for which reproductive isolation cannot be established should be considered as a unit. 

Thresholds for Threatened or Endangered Status
There is no official NMFS policy regarding thresholds for considering ESA "species" as threatened or endangered. An unofficial policy paper on this topic, drafted in 1980, is being revised. Written comments on that document and extensive discussions in ESA Technical Committee meetings stressed the importance of incorporating the concepts of Population Vulnerability Analysis (PVA) into threshold considerations. Although such a procedure exists, the concept is rapidly evolving and a definitive policy position on this issue is not expected in the near future. Furthermore, most of the PVA models developed to date require substantial amounts of life-history information that often will not be available for Pacific salmon stocks. 

Accordingly, our approach was to consider a variety of information to determine threatened or endangered status. Important factors included absolute numbers of fish and their spatial and temporal distribution; current abundance in relation to historical abundance and current carrying capacity of the habitat; trends in abundance, based on indices such as dam or redd counts or on estimates of spawner-recruit ratios; natural and human-influenced factors that cause variability in survival and abundance; possible threats to genetic integrity (e.g., from strays or outplants from hatchery programs); and recent events (e.g., a drought or improvements in main-stem passage) that have predictable short- term consequences for abundance of the "species" in question. Because a more comprehensive PVA model is not now available for Pacific salmon, we used the stochastic extinction model of Dennis et al. (1991) to provide some idea of the likely distribution of outcomes (population abundance over time) for petitioned stocks. This model is useful for identifying outcomes that are most likely if no protective measures are taken because it assumes that future fluctuations in population abundance are determined by parameters of the population measured in the recent past.

Threshold determinations will focus on threats to the ESU, which are defined in terms of wild fish (1) in the "Definition of Species" paper. The focus on wild fish is consistent with the mandate of the Act to conserve threatened and endangered species in their native ecosystems.

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