Ecological considerations for reintroducing Chinook salmon upstream of Shasta Dam

Abstract In this assessment, we reviewed existing literature to assess the ecological risks, benefits, and uncertainties associated with reintroducing Chinook salmon ( Oncorhynchus tshawytcha ) into the McCloud River, a tributary of the Sacramento River, California. Two source populations are being considered for reintroduction by co‐managers in the basin (the Winnemem Wintu Tribe, California Department of Fish and Wildlife, and National Oceanic and Atmospheric Administration): existing winter‐run Chinook salmon (WRCS) from the mainstem Sacramento River, and Chinook salmon from New Zealand (NZCS), which originated from eggs shipped from tributaries of the Sacramento River in the late 19th and early 20th century. Reintroduction has the potential to reduce extinction risks for the Sacramento River WRCS evolutionarily significant unit, which is currently listed as endangered under the U.S. Endangered Species Act. The primary ecological benefits identified were enhancement of spatial diversity and access to cold‐water refuge for WRCS. Primary risks included depletion of source populations and potential disease transmission from New Zealand and the mainstem Sacramento River into the McCloud River. Predation or competition between reintroduced Chinook salmon and resident fish in the McCloud River is also possible but likely to be biologically negligible. There are many uncertainties regarding how WRCS and NZCS may interact with each other and with other species in the McCloud River, several of which may depend on Chinook salmon life history expression. The ecological risks, benefits, and uncertainties described here are important considerations for planning and implementing Chinook salmon reintroduction, along with other key considerations such as dam passage and cultural concerns.