Salmon Life Cycle

The salmon life cycle encompasses freshwater and marine environments, up to thousands of miles of migration, and a unique life history. The following descriptions provide some basic information on the various phases of the salmon life cycle.

salmon life cycleEggs

In the hatchery the eggs, once fertilized, are called embryos and are incubated in heath trays. Each tray, holds up to 8,000 embryos. The heath trays simulate a redd. Redds are depressions excavated in the streambed by the adult female salmon by aggressively flapping their tails while lying on their side. The female salmon will then spawn with a male and deposit her eggs in the depression. Afterwards the female will dig upstream to wash gravel over the eggs to protect them. Whether in the wild or the hatchery, the embryos are very sensitive during the first two or three weeks of incubation, and any jostling or disturbance can kill them. The heath trays are set up as a flow-through system, which means that the water entering at the top tray percolates through each lower tray until it reaches the bottom. Therefore, the hatchery staff must do everything possible to keep dead embryos and unfertilized eggs out of the trays.

A way to tell the difference between normal embryos and dead eggs is to look for the color. Normal salmon embryos are a semi-transparent pink–orange color, but dead ones are opaque and pink–white. This happens because the membrane around the egg breaks and lets in water, the eggs become lighter, and the proteins are transformed. It's like frying an egg and seeing the clear part turn white.

Water quality is very important during incubation: salmon embryos need clean water to survive. Dechlorinated municipal water is used in the hatchery. The incubation trays are designed to create an upwelling where the eggs are located. This upwelling keeps fresh, oxygenated water flowing over the eggs and takes away wastes. In the wild, many fish select a redd site for spawning that has upwelling, so that eggs benefit from the water flow.


After about 20 days, the embryos will have well-developed eyes. About 20 days later they will begin to hatch. At this stage, they are called alevin. The alevin will remain in the incubators for another 30-40 days during which time they transform into fry. The alevin don't need to eat while they still have their yolk sac, but once they use that up, they need food pretty quickly. At this point they will begin swimming up to the top of the incubator, which is an indicator that they are ready to be “ponded” into a tank and begin feeding. At this time in the wild, the emerging fry will begin to swim up out of the gravel and start feeding on aquatic insects. 

Freshwater Rearing

When fry emerge from the redd or incubator, they must find food immediately. Fry eat a variety of foods in the wild, mostly aquatic insects. When fry are raised artificially, they are fed an artificial diet. UW Hatchery fry are kept in tanks and raceways and are fed several times each day until they are large enough to mark in order to distinguish them from wild populations. The salmon are marked by removing their adipose fin, which is the small fin located on the dorsal surface of the fish between the tail and the dorsal fin. Some fish are also coded-wire tagged. This involves inserting a very small wire tag into the nose of the fish that can be removed later to identify the fish.

After being marked, the fry are released into the pond. This typically occurs between late March and early April. The fry will be kept in the pond until May, when they have reached the appropriate size, or water temperature forces us to release them. Like their wild counterparts, the fry will imprint or "remember" the smells of the pond, their rearing habitat. This imprint will help them "home" back into the pond when they return as adults. These smells come from the surrounding soil, gravel, cement, pipes and other hatchery equipment. The combination of these scents creates a unique signature that the growing salmon fry remember. 

As the fry grow in the pond, some of them begin to transform into smolts. During this phase, their coloring changes from brown/green to blue/green along the back and the bands of shading on the side called parr marks fade, as the sides become silver. Physiological changes also occur that will allow the smolts to adjust to a saltwater environment. When the fry have reached the appropriate size, we release them into Portage Bay so they can begin their migration to sea. The fry migrate from Portage Bay to the Puget Sound by passing through the Hiram M. Chittendon locks via fish passage devices.

Estuary Rearing

Estuaries are very important in the life cycle of salmon. Estuaries are areas where fresh and salt water mix to form brackish water. When fry enter the brackish water, the smoltification process begins. Smolts must undergo many physiological changes to live in salt water, and estuaries allow salmon to undergo these changes gradually. This gradual change increases survival of the smolts.

If a flood occurs on a river or stream, parr or smolts may be washed out to sea before most of the physiological changes occur. This can cause high mortality of juveniles.  Estuaries are also areas of high predation of juvenile salmon. The physiological changes that occur cause the smolts to become less active, making them more susceptible to predation. There are many predators in estuaries including wading birds, kingfishers, and other larger fish.

Ocean Growth

Salmon gain most of their weight in the ocean. There is lots of food for the salmon in the ocean and growth is rapid. Salmon feed on krill, squid, herring and other small fishes. The first year of life in the ocean is the most critical year for the immature salmon. There are many large predators in the ocean that like to eat salmon as much as humans do. Since the salmon haven't grown to their full adult size in the first year, it is harder to avoid predators.

Ocean Migration

Salmon make huge migrations in the ocean. They can migrate thousands of miles during their residence in the ocean. Jacks, or salmon returning in their first year of ocean residence, may not even leave Puget Sound. Larger salmon may migrate as far north as Alaska, and as far south as California.

Spawning Migration

After spending some time in the ocean, many salmonid species return to their place of birth, devoting all their energy for spawning purposes. When the salmon enter the fresh water from the sea, they don't eat. The energy they need for migration and spawning is derived from fat stores accumulated while in the ocean. As the salmon begin their migration, they start to display some sexual dimorphic characteristics such as large humps and hooked jaws. Most species exhibit radical color changes. The physical metamorphosis of spawning salmon is caused by changes in their fat composition, blood chemistry, hormones, enzymes and skin pigmentation. Their arteries become clogged with severe artherosclerosis, their muscles soften, and their skin thickens. At this stage their immune system becomes compromised and they are more vulnerable to disease.

Sockeye, chinook, coho salmon, as well as steelhead, migrate to Lake Washington and its tributaries. At the beginning of the 20th century people recognized the importance of fish passage facilities and built a 10-step fish ladder, located south of the spillway dam at the Hiram M. Chittendon locks. A new fish ladder was finished in 1976 in order to match the modern fish ladder standards. This ladder includes an increased amount of attraction water and 21 steps, which allow fish to swim upstream on a gradual incline. Attraction water (water moving swiftly in a direction opposite the fish) helps fish find the ladder. Six lighted windows in the fish viewing room provide visitors with an underwater view of the migrating fish.

The fish returning to the pond—their place of birth—are two- (sometimes three-) year-old coho and three- (sometimes four- to five-) year-old chinook salmon. Frequently there are also other species that wander into the pond, like sockeye and rainbow trout. The returning fish swim up a three-step fish ladder to reach the pond from Portage Bay.


salmon miltSpawning in the wild usually occurs in streams, and the salmon die soon after spawning. Here at the UW Hatchery, spawning is done artificially, as seen in this picture. First, the pond is seined to collect the fish. Ripe fish are euthanized by severing the spinal cord. Workers on shore weigh and measure the fish and collect other data that are used for various research projects. Eggs are removed from the female and put into a bucket. Then milt (or sperm) is added to the eggs. Fertilization does not occur until water is introduced to the bucket. The water activates the sperm and fertilization occurs within a minute.  The eggs are then allowed to water-harden in a disinfectant solution of water and PVP iodine. After soaking in the disinfectant, the fertilized eggs are transferred to the incubators.

salmon bitmap