In response to a petition to list the Chinook salmon (Oncorhynchus tshawytscha) in the Upper Klamath and Trinity Rivers Basin as threatened or endangered and designate critical habitat under the Endangered Species Act, the National Marine Fisheries Service issued a finding (pdf) that "the petition presents substantial scientific information indicating the petitioned actions may be warranted."  The petition (pdf) was filed by the Center for Biological Diversity, Oregon Wild, Environmental Protection Information Center, and The Larch Company.  NMFS is soliciting information pertaining to the species and its habitat until June 13, 2011.

NMFS previously defined an evolutionarily significant unit (ESU) of Chinook salmon to include all spring-run and fall-run populations from the Trinity River and the Klamath River upstream from the confluence of the Trinity River.  This ESU is referred to as the Upper Klamath and Trinity Rivers Chinook salmon ESU.  The petition focuses on the status of the spring-run population as the basis for listing.