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.

To tease out the genetic basis of those differences, Anderson and colleagues teamed up with Yurok fishers living at the mouth of the Klamath River in northern California. Together, they examined 500 fish as they started their upstream journeys in all four seasons. The researchers measured the size of each fish, assessed its fattiness and reproductive status, and took samples of its DNA. The researchers gathered similar data from other rivers.

Past studies have shown that a small region of the salmon chromosome 28 contains two genes, GREB1L and ROCK1, that vary between spring and fall salmon. Anderson and his colleagues did further genomic studies, sequencing the whole genomes of 160 fish in the hopes of linking the two genes to the spring and fall traits. The data set is “impressive” and the work unique says Sheela Phansalkar Turbek, an evolutionary biologist at the University of Colorado, Boulder, who was not involved with the work.