California is a pretty amazing place for many reasons, and one is that we have a lot of different species and subspecies of salmon! The Sacramento River, for example, is the only major river in the world that has four distinct subspecies, or runs, of Chinook Salmon!
Salmon are such amazing creatures! They are so important to the ecosystems in which they live, and they have been so important to human societies for so long that they get a lot of attention, and rightly so. And, when their populations are not doing well, ecosystems and human societies feel the impacts.
Coho Salmon (Oncorhynchus kisutch), sometimes called Silver Salmon, are one of the species found in California. They are a species that live in the ocean for most of their lives, but then migrate up rivers and streams along the north and central coast of California to breed and lay their eggs in gravel beds in the upper watersheds. In addition to being found in California, Coho Salmon are also found breeding in rivers and streams along the Pacific coast of North America all the way to Alaska, and the Pacific coast in northeastern Russia as far south as northern Japan.
When they do this epic migration, how long it takes, where they lay eggs, how long those eggs take to hatch, what the young salmon need to survive and for how long, when the young salmon migrate down river and into the ocean, and how long they live there before returning to the river of their birth are all important aspects of Coho Salmon biology that need to be understood if restoration efforts are to be successful.
The grant program I manage funds salmon recovery projects, so I am writing this blog post in part to help myself understand the life cycle of the Coho Salmon so that I can make the best decisions I can when deciding what projects to fund. So, with no further adieu…
The Coho Salmon life cycle in California:
In January, adult female Coho Salmon will lay between 1,500 and 3,000 eggs each in a gravel nest, called a redd, in a stream in the upper watershed of a river system. After mating and laying eggs, the adult Coho die (more on this later).
Many of the eggs are eaten by other aquatic animals, but generally between 200 and 300 hatch. This takes between 50 and 70 days depending on water temperature.
Once they hatch, these young Coho Salmon spend approximately a year living in shallow areas of streams where they can hide amounts rocks and large woody debris and prey on smaller fish and aquatic insects.
After about 14 months living and growing in these shallow streams, the 50 to 100 surviving young salmon, that are now called smolts, begin to migrate down stream in spring. Their color changes. They loose the bars and spots that helped them to camouflage in the upper watershed streams and turn onto a bright silver color. Internal changes also occur with their gills and kidneys transitioning to be able to survive in salt water.
Once they reach the ocean in summer, the smolts spend about a year-and-a-half feeding and growing into adult salmon. Adult Coho commonly grow to about 18 lbs in weigh and 24 to 30 inches in length.
Beginning in December, the now three year old Coho Salmon begin their migration back up the rivers and streams. This first involves a transition period to allow their gills and kidneys to transition back to tolerating freshwater. The adult Coho also start to become more and more red in color as they migrate up the rivers. Once the adults are accustomed to freshwater, they begin swimming upstream, guided by scent and triggered by stream flow volumes, back to the upper watershed streams where they hatched.
Then in January, the one to four adults that have survived all the way to this point from all those eggs that were originally laid find a partner, mate, the females lay between 1,500 and 3,000 eggs each in a gravel nest, called a redd, and then the adults die. Unlike some other salmon species, Coho Salmon breed one time and then die. No adult Coho preform a return migration to the sea.
While the completion of the life-cycle of a Coho Salmon with the laying of eggs and death is the end of the individual salmon’s life, it is just the beginning of the lives of many other organisms and has effects that ripple out across the ecosystem. The dead adult salmon are important food sources for aquatic animals (such as crayfish, caddisflies, and rainbow trout) and terrestrial animals (such as bear, river otters, raccoons, and eagles). The decomposing adult salmon bring nutrients from the ocean in the form of their own tissues, and deposit those nutrients in mountain streams and rivers. In this way, they serve as an important source of nutrients, basically fertilizing the soils of the forests of the upper watersheds.
Coho Salmon are amazing animals with an amazing life story. I am very happy to be doing work that will help to provide the habitats and water they need to allow them to continue it to the next chapter.
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For myself, I decided to set a challenge of birding within the city limits of my home town, West Sacramento, CA. Unfortunately, the wildfires that are burning across much of state made the air quality pretty bad, so I was not able to stay out and bird for the whole day. 
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