On August 20, 2010, the United States Court of Appeals for the Ninth Circuit held that the National Marine Fisheries Service ("NMFS") did not violate the law when it omitted resident rainbow trout from the Distinct Population Segment of California Central Valley steelhead ("CV Steelhead"), despite the fact that rainbow trout and steelhead are the same species and can interbreed.  The court affirmed NMFS’s listing of the DPS Steelhead as a threatened species under the Endangered Species Act ("ESA").

In order to be listed as a threatened species, the ESA requires that, based on the best scientific information available, a species will, within the foreseeable future, likely be in danger of extinction throughout all or a significant portion of its range.  The ESA defines the term "species" to include "any subspecies of fish or wildlife or plants, and any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature."

 In 1991, in an attempt to define what would qualify as a DPS for Pacific salmon, NMFS promulgated a policy stating that a salmon stock will qualify as an evolutionary significant unit ("ESU") if (1) it is "substantially reproductively isolated from other nonspecific population units"; and (2) it "represents an important component in the evolutionary legacy of the species."  In 1996, however, NMFS and the Fish & Wildlife Service ("FWS") adopted a joint policy setting forth multiple factors that must be considered when determining whether any animal, bird or fish species will qualify as a "distinct population segment."  And, unlike the 1991 ESU policy, which required that a salmon species be "substantially reproductively isolated," the 1996 DPS policy authorized placing a species in its own distinct population segment so long as it was "markedly separated from other populations of the same taxon as a consequence of physical, physiological, ecological, or behavioral factors."  Despite the promulgation of the 1996 DPS policy, however, both NMFS and FWS agreed to apply the 1991 ESU policy to Pacific salmon.

In 1997, applying the ESU policy, NMFS determined that interbreeding steelhead and rainbow trout should be classified in the same ESU because they were not "substantially reproductively isolated."  However, this determination created difficulties for NMFS, because while the rainbow trout was thriving in many areas of California, certain steelhead populations were plummeting rapidly.  Further, given the thriving status of the rainbow trout, FWS opposed listing any ESU containing both steelhead and rainbow trout.  In an effort to sidestep this controversy, whenever an ESU contained both rainbow trout and a dwindling steelhead population, NMFS made a practice of only listing the steelhead population within the ESU as a threatened or endangered species. 

In 2001, however, a federal district court invalidated this practice, holding that the ESA requires an agency to list an entire DPS or ESU.  In response to the district court’s ruling, starting in 2004, NMFS proposed listing entire salmon ESU’s as threatened or endangered, regardless of whether the rainbow trout population was thriving.  Shortly thereafter, FWS sent NMFS a letter recommending that NMFS stop applying the ESU policy, and start applying the joint 1996 DPS policy so that steelhead could be treated as a separate DPS from rainbow trout.  Around the same time, new scientific reports emerged concluding that while rainbow trout and steelhead may interbreed, it was unlikely that rainbow trout could reestablish a steelhead population.  In response to this information, NMFS reported in early 2006 that it would abandon use of the ESU policy for steelhead, and instead list the CV Steelhead as a DPS.

A group of central valley irrigation districts filed a lawsuit in federal court challenging both NMFS’s policy shift and the listing determination, arguing that because steelhead and rainbow trout interbreed and can look virtually identical, the Service was required to treat them as a single species.  The Ninth Circuit rejected the challenge, finding that NMFS’s interpretation of the ESA was entitled to Chevron deference, and concluding that "under the ESA, interbreeding is not alone determinative of whether organisms must be classified alike where, as here, they develop and behave differently."  While the Ninth Circuit held that NMFS was required by law to provide an explanation for its policy shift, the court went on to find that the "decision to adopt the DPS Policy . . . was supported by the record and sufficiently explained."