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The Salmon Files

Salmon Life Cycle

Both Atlantic and Pacific salmon, along with several related species of trout, are anadromous fish, living part of their lives in salt water and spawning in fresh. The details vary by species, but typically adult fish swim upstream to spawn in the same river or creek where they were born, often in the very same stretch of water. When the females locate a suitable spot, they use their tails to create a hollow in the stream bed gravel, then lay hundreds of eggs. Nearby males emit sperm into the water to fertilize the eggs, and the females rearrange the gravel to cover the eggs. Females typically repeat the process, creating several nests in the same general area.

In a matter of weeks or months, the eggs hatch. Again varying by species, the juvenile fish spend anywhere from a few months to two or more years in fresh water, developing through several stages to small fish recognizable as salmon. At the stage known as smolt, the fish migrate downstream to the ocean, changing their body chemistry to live in salt water and taking on the silvery color typical of salmon in the ocean.

Over the next year to four years, salmon range widely in the ocean, growing to adult size. At some point, guided by instinct and generations of adaptation to local conditions, salmon of the same age begin to form large schools near the coast, and then migrate up their natal stream to spawn. In this stage, they may change dramatically in appearance, losing their silvery ocean color in favor of darker skin colors and often growing garishly elongated jaws as they move upstream. Once at the spawning grounds, the females compete for prime nesting locations, and the males for positions near the spawning females, each fish trying to maximize its odds of passing on its genes to the next generation.

For most Pacific salmon, spawning is a one-time event. The enormous energy required to swim often hundreds of miles upstream, and irreversible physical changes as the fish convert their energy reserves and tissues into eggs and milt, leaves them literally exhausted, and they die shortly after spawning. Dead and dying salmon provide a seasonal feast for birds and bears, but much of the biomass is recycled by the stream itself, as the decaying fish bodies provide nutrients that feed the entire food chain of the stream and the surrounding forest (including their own offspring after hatch). Atlantic salmon, as well as some populations of Pacific steelhead (the seagoing form of rainbow trout), may make several spawning runs in a lifetime, returning to the ocean in between.

Like most fish, salmon rely on a large number of eggs to ensure that enough offspring survive to reproductive age to maintain the population. Every stage of the salmon life cycle, from egg to adult, is potential prey for some other creature, and every pair of adult salmon that reach the spawning grounds represents the final survivors of thousands of eggs laid years before.