On The Trails: Variations in salmon life histories

Kokanee salmon schooling in a stream near Lake Tahoe, California. (Public domain photo by Hemming1952 published under a CC-BY-SA-4.0 license)

By Mary F. Willson

First, a word about definitions: we use the word "anadromous" to refer especially to fish that run up streams from the sea to spawn. That word comes from the Greek words meaning "to run up." However, since some salmon live in freshwaters and run up from lakes into the spawning streams, they also fit the definition of anadromous. So here, I will use the term "sea-run" for the familiar types of salmon.

Coho or silver salmon are typically sea-run, coming up from the sea to spawn in fresh waters; the fry rear there for a year or two and then go back to sea. In marine waters, they normally spend two to four years feeding, growing, and maturing before returning to freshwaters to spawn. Sometimes, however, some individuals spend less than a year at sea before returning to the spawning streams. These are called "jacks." Having fed less in the sea, they are smaller than the usual size, but they are capable of spawning.

There is another variation on the standard sea-run life history: Some coho do not go to sea at all, but spend their whole lives in fresh water. They take longer to mature than the sea-run forms, because the food resources are less in freshwater than in the sea. A check of the otoliths in at least one specimen told observers that the fish was six years old. These so-called "residuals" are the offspring of sea-run parents, and they have been found in northern B.C, in the Berners Bay area, and in the Dredge Lakes system in Juneau. 

Distinguishing residuals from sea-run individuals is apparently done typically by using size and morphological features.  One way to confirm the difference might be to analyze the body for marine-derived nutrients, which could tell if the fish had been to sea. But that does not seem to have been done. 

It would be interesting to learn what causes some individuals to have a life history different from the more usual sea-run pattern. More questions: Do the residuals breed with the normal ones and with each other? Do the offspring remain as residuals or do some of them go to sea?

That variability of life history may have contributed to the success of introducing coho to the Great Lakes. The first introductions were in the 1920s, but really major introductions began in 1966, using fish taken from the Pacific Northwest, mainly Oregon. By 1970, all the big lakes had established populations of coho. These populations are maintained from hatcheries in the area.  The introduced coho do spawn in natural (not hatchery) steams, but that is not enough to sustain the population.

A similar story is told for king (chinook) salmon: residuals are known in natural populations, and the species has been introduced, with limited success, in the Great Lakes.

Sockeye salmon have the usual sea-run life history but, in addition, have numerous populations that live their entire lives in freshwater. They spawn in streams but live their lives in lakes. The freshwater form is known as kokanee, found in many lakes in Southeast Alaska and the Pacific Northwest. It is distinguished from the sea-run form by small size, different number of gill rakers, and some physiological differences. They are sometimes considered to be a separate species. Kokanee have been introduced to fresh waters in many parts of the world. 

Pink salmon (or humpies), with their two-year life cycle, are known for having odd- and even-year populations that do not usually interbreed. They are commonly sea-run, but there is one known freshwater population. That’s in the Great Lakes, where the species was introduced in the 1950s.

The remaining species of our five Pacific salmon is chum or dog salmon. As far as I can tell from the information available to me, chum are not known to have residual individuals that stay in freshwater.

Salmon are thought to have evolved from freshwater ancestors. It seems that their genome may contain genes that allow this kind of variation.

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent. For a complete archive of Mary Willson’s “On the Trails” essays, go to https://onthetrailsjuneau.wordpress.com.