Researchers studying 11,500-year-old fish bones have discovered the oldest evidence of salmon fishing in North America. According to their findings, published in Proceedings of the National Academy of Sciences this week, salmon spawning runs had been established by the end of the last Ice Age. 

Paleoindians have been traditionally considered big-game hunters, though their use of fish and other resources have been difficult to determine because of the lack of preserved remains. These days, millions of salmon migrate from the ocean to spawn (and die) in their natal rivers and lakes. But many of those rivers used to be blocked by glacial ice, which severely restrict salmon ranges. However, there may have been a potential glacial refugium: Beringia, the mostly ice-free land bridge between Asia and Alaska.

Now, fish bones dating back 11,500 years have been discovered at the Upward Sun River site in interior Alaska around 1,400 kilometers (870 miles) upriver from the coast. The site is 50 kilometers (31 miles) downstream from the modern limit of major spawning areas. Based on the appearance of the vertebrae, researchers believe that they’re salmon from the genus Oncorhynchus. They recovered 308 Oncorhynchus specimens and the cremated remains of three-year-old child from the central hearth of a residence. A double infant burial with grave goods were found 40 centimeters (16 inches) below the hearth. And an additional 29 fragmented, mostly burned salmon specimens were found within the pit fill. 

By conducting ancient DNA (aDNA) analyses on two of the unburned vertebrae, a team led by Carrin Halffman from the University of Alaska Fairbanks identified the fish as chum salmon (Oncorhynchus keta). The team also conducted stable isotope analyses on one of the vertebrae, and they determined that the salmon had migrated upriver from the sea. Isotope ratios of carbon and nitrogen are typically higher in marine compared with freshwater food consumers. 

The location of the remains – at the margins of modern salmon habitat – suggests that salmon spawning runs were established at least as early as the end of the last Ice Age. These remains also predate any known human uses of salmon in North America – suggesting that salmon may have been a factor in human expansion into the continent’s northwest region.

^{Upward Sun River stratigraphy, chronology, and aDNA and stable isotope bone samples. C.M. Halffman et al., 2015 PNAS}