Pacific Salmon Develop Anadromy
Early Salmon Traded Fresh for Salt Water in Search of Food
In the face of geological turmoil and climate change, ancient ancestors of today's Pacific Salmon took advantage of cooler, more productive oceans.
Early ancestors of Pacific salmon are believed to have originated in a massive system of inland freshwater lakes at least 40 million years ago. What was it that led them to develop a life strategy that allowed a fish, over the course of its life, to cross from freshwater to saltwater and back again?
Why the Sea?
The west coast of North America from 10 to 50 million years ago was geologically violent. Volcanoes filled the sky with ash and flooded the land with molten rock. Earthquakes raised cliffs and broke hillsides. Rivers were periodically blocked and re-channeled. A large, persistent inland lake system formed. The salmonids that would become salmon were more or less isolated within the lakes.
At the same time, gradual shifts in climates and periodically reopening water- ways to the sea caused fluctuations in the salinity and temperatures of the lakes. As generation after generation of isolated salmonids bred in the lakes, the fish that were best able to withstand fluctuations in salinity and temperature survived and produced young with the same hardiness. Over time, populations of fish with the ability to move between environments of varying salinity evolved.
This flexibility served the emerging salmon races well, because their environment was continuing to change. At the same time that salmon lineages were evolving in the lakes, ocean temperatures were changing, laying the foundation for a dramatic shift in salmon life history.
Ocean Productivity and Climate
The oceans off the Pacific Northwest were not always as cold and productive as they are today. Forty million years ago, when Eosalmo driftwoodensis was swimming in western Canada, the oceans were about 18 degrees Fahrenheit warmer. About 25 million years ago they began to cool, reaching their current temperature regime approximately 8 million years ago. As the seas cooled, their productivity increased, since cold water regions produce more “biomass” pound- for-pound than warm water regions.
The water column, from surface to seafloor, presents variation to its inhabitants. The ocean varies in things like nutrient concentration, temperature, density, and salinity. In colder climates, the ocean is relatively uniform in these areas because storms and winds mix a body of water more easily when the water is the same density. Nutrients released into the water by dead plants and animals that have sunk mix up into the sunlit surface waters where photosynthetic plankton can continue to use them. This provides a strong food base for subsequent trophic levels.
In warmer climates, on the other hand, those nutrients are not available for use because they sink to depth and are not easily mixed back into the surface waters. The nutrients are distributed this way in warmer regions because warmer climates produce layering in the sea. Surface waters heat up in the sun and ride like a lens on the cold waters beneath. The two layers resist mixing because it takes great energy, in the form of storms and winds, to make liquids with different densities mix. As a result of this layering, nutrients released by dead, sinking animals and plants stay at depth and are removed from the surface production chain.
From Fresh to Salt and Back Again
Anadromous salmon, according to current theory, abandoned the streams and lakes for the food-rich waters of the North Pacific. The fish grew quickly as they ranged the cold blue waters, and upon returning to the streams of their birth, their larger size at maturity enabled them to fight deeper into the continent, expanding their distribution. Anadromous salmon also produced more offspring, because the larger females carried more eggs to the spawning ground, and were capable of digging deeper nests, or redds, to shelter the eggs.
The development of anadromy, then, was instrumental in the early salmon's rise to ecological eminence along the coasts of North America. And it ultimately helped power the fishes radiation to other regions around the North Pacific Rim as well.
References
Alaska Geographic. 1994. Prehistoric Alaska. Vol. 24, no. 4. Alaska Geographic Society, Anchorage, AK.
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.
