The Fascinating Journey of Coho Salmon in the Great Lakes

English names for Pacific salmon are relatively modern in the context of history. When Lewis and Clark made their historic journey in 1805, they were among the first English speakers to interact with the Pacific salmon species that are native to North America. From their writings, we know they encountered fish species in the Columbia River Basin that they called “red charr,” likely referring to sockeye salmon, “white salmon-trout,” possibly describing coho salmon’s silvery hue or white gum line, and simply “salmon,” which was probably the Chinook salmon. It’s unclear if they ever encountered pink or chum salmon during their expedition.

In modern English usage, each species of Pacific salmon is known by multiple common names, depending on the region and species. If you were to go fishing in Lake Michigan, you might encounter two well-known species: the Chinook (or king) salmon and the coho (or silver) salmon. But how did these species, which Lewis and Clark did not even encounter until they were near the end of their journey west, end up in the Great Lakes? Join me as we dive into how these Pacific salmon were introduced to the Great Lakes and what their life is like here in the Midwest.

Coho Salmon in the Great Lakes Ecology

Coho salmon were introduced to Lake Michigan in 1966 as part of an effort to control the population of an invasive fish species called the alewife (Alosa pseudoharengus). It is possible that this small species of herring was already present in Lake Ontario, but it was only able to access the other Great Lakes after the completed Welland Canal allowed them to access Lake Erie, bypassing Niagara Falls by the very route built to allow barges to do the same thing. The alewives might have run into trouble once they encountered lake trout, but lake trout populations were declining thanks to overfishing combined with the invasion of the sea lamprey. The lack of predators allowed alewife numbers to explode, beginning a pattern where the growing population would deplete food resources, leading to mass die-offs, with the survivors suddenly finding more food available, and the cycle would repeat. The large die-offs resulted in masses of dead fish washing up in windrows along the beaches of Lake Michigan. These piles of dead fish rotting on the lakeshore, sometimes several feet deep, were an aesthetic concern that also posed environmental and health issues for local communities.

Coho salmon, initially stocked by the state of Michigan, had some success as predators of the alewife. Their voracious appetite for small silvery fish provided pressure on the alewife population, and they also became an important game fish for recreational anglers. In addition to alewives, coho salmon consume other small fish such as smelt, an invasive species also abundant in the Great Lakes. This diet leads to rapid growth, like what these salmon would experience in their original oceanic habitat, and their predatory role helps maintain a balanced ecosystem. However, the ecosystem of the Great Lakes is complex, with new and established invasive species affecting food availability for many species, including coho salmon.

Some natural reproduction is occurring in streams that feed into Lake Michigan, but the continued presence of coho salmon is ensured by hatcheries in the surrounding states. These hatcheries collect eggs, rear the young fish, and release them into the tributary streams or directly into Lake Michigan harbors to ensure the continued presence of the salmon population. In Illinois, this work is done by the employees at the Jake Wolf Memorial Fish Hatchery, operated by the Illinois Department of Natural Resources Division of Fisheries and supported by anglers through fishing license purchases and the cooperative work of the Great Lakes Fishery Commission.

The Coho Salmon Life Cycle

What does life look like for a coho salmon? The life cycle of coho salmon is an intricate journey filled with remarkable changes, survival challenges, and contributions to the ecosystem. In their native habitat, coho salmon are born in freshwater streams, where they go through several stages before migrating to the vast waters of the ocean, where they grow to adulthood, eventually returning to their natal streams to spawn. Here in the Midwest, the Great Lakes provide the habitat for the otherwise oceanic life stages, and hatcheries provide additional support for large numbers of these salmon in the early life stages.

1. Eggs
   In their natural habitat, coho salmon spawn in gravel beds within freshwater streams and rivers. In the fall, adults leave the ocean or lake where they have been living, find the river where they were born and begin the long swim upstream to spawning grounds. It is at this point that fish hatcheries will collect adult salmon to spawn the next generation, using weirs to collect the fish as they swim upstream. In the wild, salmon will lay their eggs in nests that they dig with their tails in the gravel on the bottom of a stream. Gravel nests allow a fresh supply of water to continually flow through all the eggs, providing fresh oxygen for developing eggs. Hatcheries provide the same oxygenated water for salmon eggs by incubating them in trays with a constant flow of water. Whether in the gravelly bottom of a stream or in a hatchery tray, these eggs will develop through the winter months. Depending on the water temperature, the eggs can hatch within 30 to 70 days—warmer water temperatures lead to faster incubation times. In a hatchery, workers check the eggs regularly, monitoring their development and removing any eggs that have stopped developing and begun to decay.
2. Alevin
   After hatching, young salmon are called “alevin.” The term “alevin” comes from the French word alever, meaning “to raise offspring.” This stage can last for several weeks, during which the young fish do not swim much, spending their time absorbing nutrients from their yolk sacs. If the alevin are in a gravel nest in the wild, they will remain in the nest to stay hidden from predators while developing muscles and digestive systems needed for their survival. In a hatchery, the alevin typically stay in the trays where they hatched. They are not ready to swim around and being in a bigger tank would be stressful for them. Once the yolk sac is fully absorbed, the fish are ready to swim up and try finding food on their own.
3. Fry
   No, doesn’t have anything to do with the neighborhood fish fry! These fish are still too tiny to taste like much of anything to you or me, but they do have to be wary about other predators. Calling fish at this stage “fry” is thanks to a respelling of the Middle English word “frie,” which likely comes from the Old Norse frjó, meaning seed. Although these little fish are small enough to be easy prey for a lot of different animals, they are protected by strong survival instincts. They can respond to changes in water flow and light intensity to find places of shelter, and they are able to find food. At this age, they are small and will spend their time in the shallow, calm areas of streams and rivers. They feed on insects and other small aquatic organisms. Coho fry can remain in streams for more than a year, giving them time to grow before migrating to larger bodies of water. In a hatchery, with steady access to food and protection from predators, this timeline can be significantly reduced, with these fish moving to the next phase of their lives in a matter of months.
4. Smolts
   Once the fry are ready to move to open water, they undergo a physiological transformation called smoltification. The word smolt is an Old Norse word that still means “young salmon.” This change is accompanied by a new, shiny silver appearance, and that color shift is a good indicator that these fish are now ready to enter open water. In their native range, this is also the stage where they undergo physiological shifts as they transition from freshwater to saltwater. In the Great Lakes, they remain in fresh water, but the lake presents an entirely different set of food resources and challenges. Regardless of their eventual destination, at this stage these fish migrate in schools downstream to whatever open water they might find at the end. This migration usually occurs in the spring. For hatchery-raised fish, this is often when they are released. In Illinois, the coho salmon are stocked directly into Lake Michigan. If you find yourself near one of the harbors at the beginning of May, there’s a chance you will see one of the stocking trucks, full of coho salmon.
5. Adults
   Coho smolts grow rapidly once they reach open water. In their native range, they would find themselves in the ocean at this stage, with an abundance of small fishes like herring, anchovies, and sardines to feed their voracious appetites. In Lake Michigan, their diet consists mainly of small fishes like alewife and smelt, but researchers have found that these salmon also eat small freshwater crustaceans and terrestrial insects. After less than two years in open water, they mature into adults. Adult coho salmon typically weigh between 6 to 9 pounds and can reach lengths of up to 2 feet.
6. Spawners
   When coho salmon are ready to reproduce, they make the remarkable journey back to the freshwater stream where they were born. This migration, known as “homing,” is one of the most extraordinary aspects of the coho life cycle. After navigating the vast open waters of the Great Lakes, the coho make their way back to the stream their parents went to when they were spawned. Hatcheries staff catch them in weirs, bringing adults into the hatchery where their eggs are collected and fertilized. Salmon are semelparous, meaning that they will spawn one time, and that spawning event marks the end of their lives. In their native range, their decomposing bodies contribute vital nutrients to the ecosystem, enriching the stream and surrounding environment.

Conclusion

Coho salmon have become an integral part of the Great Lakes ecosystem, playing a vital role in controlling invasive species while providing exceptional recreational fishing opportunities. Their life cycle, from spawning in freshwater streams to maturing in the vast waters of the Great Lakes, is a fascinating journey that continues to captivate anglers and environmentalists alike. Through ongoing management and conservation efforts, and continued support from advocacy groups, coho salmon will remain a valued species in the region’s aquatic landscape for years to come, providing economic value and maintaining the ecological balance of one of the world’s largest freshwater ecosystems.