Modern Salmon And The Ice Age
Salmon Life History Is Tested First by Mountain Building, Then Ice
Millions of years of evolutionary adaptation forged by mountain building and volconism led to modern day salmon species, who in their turn survived an onslought of ice.
The salmon’s ancestral genus Oncorhynchus is believed to have evolved during the Meiocene Epoch, perhaps five to twenty-five million years ago, at the height of mountain building in western North America. Pacific salmon species as they are known today sprang from this line three million years later—just in time for testing by a series of ice ages.
Appearance of Modern Salmon
Mitochondrial DNA analysis suggests the genus subsequently broke into three branches approximately two million years ago. One branch produced Chinook salmon, coho, and rainbow trout, while the second produced sockeye salmon. A third line diverged approximately 1.25 million years ago, yielding pink and chum salmon.
If the DNA analysis’s placement in time is correct, the ancestors of today’s salmon were shaped by anywhere between 3 and 20 million years of violent geology. Many valuable life history variations had been woven into the species by this time. Yet stabilizing watersheds in the Pacific Northwest and Alaska, two million years ago, did not mark the end of Pacific salmon’s trials. A period of intense glaciations now descended, drastically altering the regions climate, rivers, and habitat.
Salmon and the Ice Age
During the past two and a half million years, there have been many periods of glacial climate. The most recent ice age, called the Wisconsin, crested approximately 18,000 years ago, covering North America with two immense ice sheets. The Laurentide ice sheet blanketed most of north-central and eastern North America. The Cordilleran ice sheet covered the Pacific Northwest, British Columbia, and Southeast Alaska. In some places the ice reached a thickness of 4,000 feet.
With vast stretches of salmon habitat entombed in ice, salmon shifted to ice-free rivers in Oregon and California in the south, and Berengia in the north. Berengia consisted of a continuous unglaciated landmass running from the Yukon territory in Canada, across interior and western Alaska, over the Bering Land Bridge—part of the Bering Sea floor between North America and Asia exposed by lowered sea levels—and west into eastern Russia.
At the end of the Wisconsin Ice Age, the climate cycle warmed significantly following the retreat of the glaciers. Pacific salmon reestablished themselves in the lands freed from ice. Salmon from the Ice Age refugia of the Columbia River basin repopulated streams as far north as the Southeast Alaska. Colonizing salmon also moved south from hideouts in Berengia.
Salmon recolonization did not come easy. The newly freed rivers ran unpredictably; sometimes swift, sometimes sluggish as they braided their way across a moonlike landscape of boulders, gravel, and glacial outwash. Huge sediment loads sixty times that of many modern rivers choked the water. A warm drying climate from about 14,000 years ago to 5,000 years before present, made it difficult for vegetation to take hold on the riverbanks. The stunted trees that did grow could not hold the rocky soil in place nor stabilize the streams in consistent channels.
Despite this, salmon continued to enter the rivers. Time after time colonial salmon fought inland to spawn, only to have the eggs suffocated by silt or crushed by gravel rolling in turbulent waters. Summer baked newly hatched fry between unshaded river banks. Survivors slowly starved from the lack of insects to eat, or were killed by birds stalking the shallow glacial stream braids.
Present Day
Gradually the tide turned. Somewhere between 5,000 and 4,000 years ago, the climate shifted towards the cool, wet maritime regime seen today. This change enabled true forests to grow, ultimately leading to old growth stands surrounding stable, clear watered, well-shaded streams. The life and death cycle of the forest produced plenty of downed snags in river channels, creating pools and side channels and diversifying the habitat. Salmon populations once again grew. Salmon-run nutrient flow from the sea to the rivers resumed, first in a trickle, then in a flood. Eventually, the complex, relationship between old growth forest and Pacific salmon stabilized and flourished—
—until salmon's next, and as yet unresolved, challenge loomed: habitat destruction by humankind.
References
Brown, Bruce. 1990. Mountain in the Clouds: In Search for the Wild Salmon. University of Washington Press. Seattle, WA.
Frissel, C.A. 1989. Evolution of the Salmonid Fishes: Zoogeography and the Fossil Record, Department of Fisheries and Wildlife, Oregon State University, Corvallis.
Groot, C. and L. Margolis. 1991. Preface. In Pacific Salmon Life Histories. Edited by C. Groot and L. Margolis, pp ix – x. UBC Press, Vancouver, Canada.
Lichatowich, Jim. 1999. Salmon Without Rivers: A History of the Pacific Salmon Crisis. Island Press, Washington D.C.
