The Salmon's Ancient Origins
Did Pacific Salmon Come from Salt or Fresh Water Salmonids?
Onchorhynchus's ancestors first appear in the fossil record 50 million years ago, offering a vital clue as to where this line of fish originated.
Five species of Pacific salmon (Onchorhynchus spp.) occur in Alaska and the Pacific Northwest: chinook, coho, sockeye, chum, and pink. Pacific trout within the Oncorhynchus genus include the anadromous steelhead and sea-run cutthroat trout. Together they represent an unusual life history strategy that takes advantage of both freshwater and saltwater environments.
Life History
Modern species of Pacific salmon have developed a wide array of life history paths and distributions. In general Pacific salmon spawn in gravel beds in rivers, streams, or along lakeshores. After an early freshwater stage, varying in length depending on the species, the young fish migrate down stream. This life-history strategy of being born in fresh water, migrating towards the ocean to feed and mature, then returning to freshwater to spawn and die, is called anadromy.
While some populations of certain species remain in fresh water, others migrate to nearby coastal waters or head further out to sea. Many populations undergo extensive migrations covering thousands of miles. As such, different species of Pacific salmon are widely distributed throughout the North Pacific and Bering Sea.
Most salmon species and stocks mature after one-to-seven years. They then begin the long journey home, returning to their home rivers and natal breeding grounds. The salmon spawn sometime between mid summer and early winter, after which all of the fish die (with the exception of steelhead and cutthroat and some salmon species occurring in Asia).
This anadromous pattern, with its temporal staggering of habitat use by multiple stocks of fish, is the main reason every salmon species has evolved into a collection of reproductively isolated and genetically distinct subpopulations. It is the foundation upon which Pacific salmon’s genetic variety is built.
A question of origin remains, however—did this life history strategy begin with early salmonids originating in freshwater, or in saltwater?
Early Lineage
55 million years ago, a new type of fish had risen to dominance in the coasts, lakes and streams of western North America. These were the teleosts, fishes whose skeletons are made of cartilage rather than bone.
Teleosts today include almost all jawed fishes. Like previously dominant forms, they gained the upper fin over their contemporaries through improvements in body form and physiology. Theirs was a more efficient respiratory system, while their shape and musculature—changed relative to earlier fishes—allowed for more rapid and complex movements. These changes enhanced the teleost ability to survive in a world of “hunt and be hunted.”
Freshwater or Salt?
The early ancestors of salmon were teleosts, and the fossil record suggests that they originated in freshwater. The first known salmonid ancestor, Eosalmo driftwoodensis, looked much like today’s grayling and lived in primeval lakes in western Canada approximately 40 to 50 million years ago. The fossil record has filled in marginally since then, with additional ancient trout, graylings, and whitefishes showing up in freshwater sediments.
This freshwater origination is significant, especially when coupled with the fact that primitive salmonids, like graylings and whitefishes that are evolutionarily older than salmon yet still exist today, also live in freshwater (though some graylings do venture into estuaries.) If both extinct members of the family and older existing salmonids dwell primarily in freshwater, it seems likely that the early salmon, which evolved from these ancestors, first lived out their lives in lakes and rivers too.
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.
