Chinook salmon
The river also supports a remarkable diversity of fishes and recovering salmonid populations, especially chinook salmon ("Oncorhynchus tshawytscha") and steelhead trout ("Oncorhynchus mykiss"). In 2003 the Napa County Resource Conservation District began an ongoing salmon monitoring program, and have recorded a run of approximately 400 - 1000 fall-run Chinook salmon the past several years. The Chinook run begins in late October through January. Conclusive evidence of historical chinook salmon populations in the Napa River basin have not been established, but the river provides appropriate habitat for salmon and its location near the entrance to the Sacramento/San Joaquin Rivers make it likely that salmon would have at least ventured into the Napa River. In 2013, a genetics study of Napa River chinook salmon revealed that two adults migrated from the Klamath River and successfully spawned in the Napa River, since four juvenile chinook collected from the Napa River in 2010 were proved to be siblings from the close similarity of their DNA and that the latter was characteristic of Klamath River chinook. These findings have important implications for the protection of the federally endangered Coastal California Chinook Salmon ESU since the Napa River, nor any stream in the Bay Area, was included in this ESU.