Ever wonder why there are five different species of Pacific salmon — six if you count steelhead — and only one species of Atlantic salmon? It turns out it is because the wild West Coast has been a much more dynamic place to bring up a species.

Genera Salmo, (Atlantic salmon and brown trout) diverged from genera Onchorhynchus (Pacific salmon, steelhead, and western trouts) 15 to 20 million years ago, as the Arctic Ocean began to cool.

Salmon are genetically nimble. An evolutionary event about 80 million years ago essentially doubled the proto salmon’s DNA, allowing them to adapt to changing environments more quickly, and protecting them against localized inbreeding. But Atlantic salmon have not diversified much. That’s because on the east side of our continent things have been geologically stable, relatively speaking, for the past 70 million years.

The western salmon, on the other hand, have faced a far more dynamic environment. Forty million years ago the region that is now the Washington Cascade Mountains was a wide coastal plain. Between 6 million and 17 million years ago basaltic volcanic eruptions flowed like molten rivers across the central Columbia River basin. Tectonic forces lifted up the Cascade, Pacific, and Coast mountain ranges. And the Pacific Northwest endured at least four major ice ages starting about 1.8 million years ago.

It was only 16,000 years ago that the continental ice sheet extended down into Puget Sound, the Okanogan River basin and the upper Columbia River basin. During this time wide regions that are now salmon habitats were sometimes buried under hundreds of feet of glacial ice.

Advancing glaciers dammed up Puget Sound and turned the floes of mighty rivers to the south. Inland, in present day Idaho and Montana, an ice dam blocked the flow of the Clark Fork River above the upper Columbia River drainage. Like an icy Godzilla, Glacial Lake Missoula was a vast body of frosty water trapped up in the fingers between the mountains behind a long, tall wall of ice.

Over a span of several hundred years this monster burst through the ice dam around 100 times, about every 30 to 70 years. These catastrophic ‘megafloods’ broke across the Columbia Plateau, rushed down the Columbia gorge and finally piled into the Pacific Ocean. About 15,000 years ago one such breach blasted the landscape with 500 cubic miles of water that all rushed down to the sea over a 48-hour period. It scoured the earth down to the bedrock as underwater tornadoes swept 200 ton boulders downstream. The resulting scarred countryside is now known as the “scablands.”

We don’t know how many full-blown species of salmon came and went during these turbulent times. We do know that all these radical environmental and physical habitat changes had the effect of isolating surviving populations of Pacific salmon from one another. While Atlantic salmon had little in the way of changing environments to spur diversity, Pacific salmon found themselves hunkering down in environmental bunkers throughout the Pacific Northwest, where they adapted to local conditions.

The ones that made it to the recent ice ages found glacial refuges in ice-free areas along the Bering Sea, the southern Puget Sound, the Snake River basin, the lower Columbia River, the Olympic peninsula, and areas to the south, where populations evolved separately. This is thought to be why we have distinct fall- and spring-run Chinook salmon, for instance, and odd- and even-year pink salmon whose idea of ideal conditions are very different. 

Over the past 5,000 years or so salmon habitat in the Pacific Northwest has been sailing along smoothly, from a natural perspective. Depending on the species, between 1,250 and 2,500 generations of salmon have now evolved in relatively stable habitats. The glaciers are long gone, and the land has sprung back. Rivers have had time to cut their way through the glacial sediments to form wide, braided mouths. The vegetation and climate have not changed much in the past few thousand years. Yes, the last five-thousand years have been remarkably stable for Pacific salmon. Except for one recent addition to their environment.

Volcanoes continue to erupt, and floods and landslides damage salmon habitat every year. But eventually rivers flow around and through ash and blockages, and in time flood-damaged habitat does rebound. The biggest long-term challenges to salmon habitat today are man-made.

The problem is that human-driven changes to salmon habitats tend to be just as persistent as humans themselves. Natural disturbances — even catastrophic ones — are followed by gradual recovery. Even titanic volcanic ejections and mountainous landslides eventually yield to the power of flowing water, and are washed away.

Dams, canals, locks and levees, by contrast, inhibit salmon migration indefinitely, year after year. Plus, human obstructions to spawning salmon tend to be spread across the entire landscape. Whole drainage systems and entire stretches of coastline are now tamed by human civilization. The most mundane and basic infrastructure — storm drains and culverts — are at the heart of the problem. A recent study of steelhead in Idaho showed that the cumulative loss in genetic diversity due to blockage caused by culverts was far higher than losses due to natural disturbances like landslides and wildfires.

A case-in-point of an unobstructed river system is Bristol Bay. The striking ongoing success of Bristol Bay sockeye salmon has been led by different rivers in different years. Scientists think Bristol Bay’s interlocked drainage system of unobstructed rivers allows for environmental flexibility for the stock as a whole, since a bad year in one river can be balanced out by a good one where conditions were better.

It certainly seems to argue for the value of Alaska’s wild river systems. Pacific salmon have survived tsunamis, megafloods, molten lava, and crushing ice. Hopefully, they will survive us, too.


Terry Haines was a commercial fisherman in Kodiak for more than 30 years. He now produces the Alaska Fisheries Report for KMXT and is a member of the Kodiak City Council. He can be reached at thaines@city.kodiak.ak.us

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