Salmon begin their lives in freshwater, migrate downstream to estuaries and, eventually, the ocean, where they live for a while before returning to their natal streams to spawn. (It is estimated that less than 1 percent of salmon survive long enough to return.)
Before the 20th century, an estimated 10 million to 16 million adult salmon and steelhead trout returned annually to the Columbia River system. The current return of wild fish is 2 percent of that, by some estimates.
One of the largest factors inhibiting salmon recovery is habitat loss, Mr. Neatherlin said. A growing human population has led to development along the shoreline, and the addition of bulkheads, or sea walls, that encroach on beaches where salmon generally find insects and other food. More pavement and hard surfaces have contributed to an increase in toxic storm water runoff that pollutes Puget Sound.
More than 20,000 barriers — including dams, roads and water storage structures — block salmon migration paths in Washington alone, according to the state’s Department of Fish and Wildlife.
The waters they swim in are warming — in 2015, unusually warm water killed an estimated 250,000 sockeye salmon — and there is less cold water to feed streams in the summer, the report said, because slowly rising temperatures have accelerated the melting of glaciers.
Loss of habitat compounded by climate change has created a dire situation for salmon, Mr. Neatherlin said.
“Salmon have demonstrated during the past 10,000 years that they can adapt to a changing environment,” the report said. But “climate change has exacerbated these problems. When combined with fewer natural buffers, degraded habitats, and lost genetic diversity within the salmon themselves, salmon are challenged to change quickly enough and their survival is at stake.”