Salmon In The Orogenic Crucible
Mountain Uplift And Volcanism Impacted Early Salmonids
Millions of years worth of mountain building along North America's coast most likely forced early salmonids to develop age classes and straying so as to survive.
Ten to twenty million years ago, uplift of coastal mountains in Alaska and the Pacific Northwest slammed early members of the anadromous salmonids with massive habitat disruption. Because the fish uses river and lake systems as well as ocean ranges, the early salmonids effectively straddled both sides of the coast, which was at that time a geological time bomb.
Salmon, Rivers, and Mountain Building
Rising mountains forced many rivers to down cut in their quest for the sea, inevitably creating landslides large enough to form waterfalls or block streams entirely. Mountain building also created an unremitting cycle of habitat destruction and creation as river courses shifted with time. Many early salmon runs found themselves choked off from the ocean or their spawning grounds, while other populations went extinct as their habitat became untenable.
In addition, volcanic eruptions periodically knocked river ecosystems back to square one. Colliding tectonic plates caused molten rock to burn its way up through continental crust, erupting with explosive force and burying the land in scalding lava, mudslides and ash. The repeated lava flows decimated salmonid habitat time and time again. After each eruption, the habitat had to slowly rebuild itself anew: rivers recut courses, soil developed, and plants and animals from areas outside the damaged region recolonized the landscape.
Geological tumult tested the early salmonid races, but over time prompted them to evolve the life history strategies that serve them even today.
A Sheaf of Life Histories
An undisturbed river—and a salmon’s ecosystem—is clearly described by biologist W.F. Thompson and quoted within the book Salmon Without Rivers by Jim Lichatowich, as “a chain of favorable environments connected within a definite season in time and place.” Within a river system, Thompson wrote, there are “various possible combinations or chains of this sort which can be connected in time and space in a variety of ways.”
To this, Lichatowich lucidly adds: “Each of those chains is a life-history variant, a different pathway the salmon follow through the chain of favorable environments that make up an ecosystem.” Thus the survival of a species or race in any particular pathway will vary over time, as it responds to changes in the habitat. Some life histories may be favored by changing conditions, while others will not. Thus, over time, the survival value of any given life history path will pendulum—sometimes being more effective, and at other times, not.
Salmonid populations, with their multiple life histories, have evolved “insurance across time.” Most salmonid species possess multiple age structures, meaning not all fish from a run return in the same year. Mount St. Helens in Washington State offers a modern example of the temporal protection offered by age classes. When the volcano erupted, only part of each species stock was present in the rivers at the time of the Mount St. Helens disaster. The rest were at sea, which ensured that a reserve of genetic material from those stocks was still available. After the natural restoration of the river was sufficiently advanced, the salmon reestablished themselves in the region.
Straying
Salmonids have also developed“insurance in space.” Salmonids home accurately to their birthplace stream. The advantage of this is that successful gene complexes tested over generations for that particular river are maintained. However, this adaption carries a degree of risk. If a salmon population’s river is destroyed and rendered uninhabitable beyond the reach of even multiple age classes, then those specific stocks are threatened.
When this happens, sometimes it is best to simply leave. Species such as today’s pink salmon, which show almost no life history flexibility and therefore have little “insurance in time,” display an inclination to stray and spawn in streams other than where they first hatched. Thus they compensate for a lack of “insurance in time” with an increased “insurance in space.”
Pink salmon are not the only salmon displaying this tendency. There is evidence of relatively rapid colonization of recently deglaciated streams in Alaska by several other Pacific salmon species. This suggests that most if not all salmon species populations possess the innate ability to quickly colonize new habitat.
As stray salmon colonize new streams and the colonial population takes hold, the process of adaptation between fish and river begins anew. On an evolutionary time scale, local extinction of the original population offset by straying and recolonization has long served as an important survival strategy for Pacific salmon.
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.
