Salmon stocks are declining in the Yukon. Could global warming be connected? For Grist, Max Graham talks to elders, harvesters, fishery officials, and scientists to learn more about this complicated problem and its potentially devastating repercussions.

There have been salmon in the Yukon, the fourth-longest river in North America, for as long as there have been people on its banks. The river’s abundance helped Alaska earn its reputation as one of the last refuges for wild salmon, a place where they once came every year by the millions to spawn in pristine rivers and lakes after migrating thousands of miles. But as temperatures in western Alaska and the Bering Sea creep higher, the Yukon’s salmon populations have plunged.

Salmon are vital to the river’s Yup’ik and Athabascan communities as a source of nutrients and a symbol of cultural identity. Dense with protein and fat, Yukon kings are highly nutritious. To swim as many as 2,000 miles upriver, against the current — the world’s longest salmon migration — the fish put on huge stores of fat, some bulking up to 90 pounds. (Their journey is equal to running an ultramarathon every day for a month without stopping for a snack.)

But salmon are notoriously difficult to study. They spawn in fresh water, then spend most of their lives far out in the Pacific, an area dubbed the “black box” because it’s so vast and poorly understood. Most salmon research — in Alaska and along the entire Pacific Coast — is focused on streams and lakes, where it’s easier to study their habitat, sample the water, and count stocks.