Why fall-run fish are bigger

For the Yurok people, who have lived at the mouth of the Klamath River for generations, the spring run of Chinook salmon (Oncorhynchus tshawytscha) is a welcome—and nutritious—relief from winter. But as the fish have dwindled to just a fraction of their original numbers, Indigenous groups there are pushing to have them protected by the Endangered Species Act. New research, which suggests genes play only a small role in distinguishing the spring salmon from their fall-run cousins, may call into question the need for such a designation.

The rivers of the Pacific Northwest used to teem with two waves of Chinook: those that arrived in March or April, and those that came 6 months later, swimming from the sea to their upriver breeding grounds. Although technically the same species, the spring-run and fall-run fish have some “iconic differences,” says Eric Anderson, a molecular geneticist at the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center. Spring salmon are smaller, fattier, and less sexually mature than fall fish. They also swim further upriver to breed.  

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