Following completion of a comprehensive status review of west coast steelhead (Oncorhynchus mykiss, or O. mykiss) populations throughout Washington, Oregon, Idaho, and California, NMFS published a proposed rule to list 10 ESUs as threatened or endangered under the Endangered Species Act (ESA) on August 9, 1996. One of these steelhead ESUs, the Northern California ESU, was proposed for listing as a threatened species. Because of scientific disagreements, NMFS deferred its final listing determination for five of these steelhead ESUs, including the Northern California ESU, on August 18, 1997. After soliciting and reviewing additional information to resolve these disagreements, NMFS published a final determination in March 1998 that the Northern California ESU did not warrant listing under the ESA because available scientific information and conservation measures indicated the ESU was at a lower risk of extinction than at the time of the proposed rule. Because the State of California did not implement conservation measures that NMFS considered critically important in its decision to not list the Northern California steelhead ESU, NMFS completed an updated status review for the ESU and reassessed the State and Federal conservation measures that were in place to protect the ESU. Based on this reconsideration, NMFS proposed to list the Northern California steelhead ESU as a threatened species under the ESA on February 11, 2000. After considering public comments on the proposed determination, NMFS now issues a final rule to list the Northern California ESU of steelhead as a threatened species. Within the Northern California ESU, only naturally spawned populations of steelhead (and their progeny) residing below naturally occurring and man-made impassable barriers (e.g., impassable waterfalls and dams) are listed. NMFS has examined the relationship between hatchery and natural populations of steelhead in this ESU and concludes hatchery populations are not essential for recove