Student Lesson 3: THE SALMON LIFE CYCLE: IN DEPTH

Recommended for grades 4-8

This lesson covers:

  1. The anadromous salmonid life cycle
  2. The trout life cycle
  3. Predators of salmon throughout their life cycle
  4. Salmonids at the hatchery

Learning Activities

A Natural Anadromous Salmonid Life Cycle

Eggs
A typical lifecycle of a salmon, steelhead, or trout begins when eggs are deposited and fertilized in the gravel of a stream. Thousands of eggs are deposited in gravel nests, which we call redds.

Successful reproduction depends on a reliable supply of cold, oxygen-rich, clean water to percolate through the redd, whose gravel must be free of most sediment. The gravel protects the eggs during incubation from direct sunlight and strong current. For the next month or so after spawning, the eggs are very fragile. Even a slight disturbance could destroy them.

About a month after being deposited (the amount of time depends on water temperature), head and body regions develop in the eggs and eyes begin to show. Good water flow delivers oxygen and carries away waste products. Water temperatures must stay between 40 degrees to 65 degrees F. Salmonids are cold-water fish and do not easily tolerate temperatures above 68 degrees F. The greatest mortality in a salmon’s life cycle occurs during this time, referred to as the egg-to-fry stage. Only about 10 percent of the eggs deposited in the redd survive. Those eggs hatch within a few months and are called sac fry.

Sac fry
In late winter or spring, the eggs hatch. The young, tiny fish are called sac fry, or alevin. They have huge eyes and a large yolk sac protruding from their belly. The orange sac contains a completely balanced diet of proteins, sugars, minerals and vitamins. Sac fry remain in the gravel and grow rapidly, living off their yolk sac for one to three months.

By remaining under the gravel, sac fry are protected from most predators and other hazards. A good, reliable flow of cool, clean, oxygenated water through the gravel continues to be critical for survival. When the yolk sac is used-up, the fish, now called fry, emerge from the gravel.

Fry
Sac fry absorb their yolk sacs and emerge from the gravel as fry in late spring and summer. About an inch long, they are easy prey for other fish. Chinook, coho and sockeye salmon, steelhead trout, and some cutthroat trout spend half a year or more in fresh water before starting their migration. Pink and chum salmon, on the other hand, begin their migration to the ocean soon after emerging from the gravel. Other trout remain in the watershed where they were conceived for their entire lives.

Fry feed on plankton and small insects. They depend on streamside cover for protection from predators, to keep water temperatures cool, and to attract insects which they rely on for food. Fry stay in shallow pools near the edge of the stream where the current is not too strong. They are easy prey for trout and other large fish. Some birds also devour fry.

When the young fish reach the size of a human finger, they are called fingerlings. Vertical dark marks (parr marks) along their silvery sides help hide them from predators. The fry continue to eat and grow rapidly.

Smolts
At some point, depending on the species, the young salmon, steelhead, and sea run cutthroat trout undergo a change in their behavior and start toward the ocean. These 4- to-6-inch fish, known as smolts, head to the sea with fall or spring freshets, when rainfall or snowmelt provide a rush of freshwater into the stream. During smoltification, the bodies of young salmonids undergo internal changes to allow them to transition from the fresh to saltwater. In addition, the parr (first reference to “parr marks”? marks disappear as the smolt becomes silvery and more distinctly counter-shaded (dark above, light below) to survive ocean conditions. Interestingly, most smolt drift backward with their heads upstream as they migrate downstream. They travel mostly at night to avoid predators who hunt by sight.

Water flow continues to be a critical factor as they migrate downstream. High flows mean higher survival rates. On some streams, dams alter the natural flows of river systems, slowing flows as they store spring runoff. Decreased flows will increase the amount of time it takes smolts to reach the ocean and affect their ability to adjust to saltwater conditions. Delays can increase their susceptibility to predators and disease. Smolts are often lost when they pass into unscreened irrigation ditches and becoming stranded agricultural fields. Smolts also can be injured or killed as they pass through hydroelectric facilities or fail to find fish passageways at dams.

Pink and chum salmon actually begin their migration to the ocean soon after emerging from the gravel as fry. Chinook, coho and sockeye salmon, steelhead (rainbow) trout, and some cutthroat trout spend half a year or more in fresh water before starting their migration to the sea. All other trout remain in freshwater their entire lives.
Salmon face many predators in the ocean
Salmon face many predators in the ocean

Salmon at Sea
Once at sea, anadromous salmonids spend up to five years growing to maturity, depending on the species. Plankton make up their first food source. If ocean temperatures are too warm, plankton becomes less abundant, and smolts struggle to survive. As the surviving fish grow, their diet evolves to shrimp, anchovies and herring. They must avoid predators, including sharks, other marine mammals, and fishermen.

Adult Salmonids
Usually in early summer of their maturing year, salmon begin to head back to their home streams. They have one purpose – to get back home to spawn. While their exact method of navigation is not fully understood, researchers believe salmonids navigate by electromagnetic signals, the moon and stars, the smell of their home stream, or a combination of these factors. This is called homing. Salmon stop feeding when they enter fresh water and live on stored body fats for the rest of the trip. Steelhead and sea- run cutthroat trout also cease feeding during their upstream journey.

Using every last ounce of energy to complete this journey, the salmon’s body once again undergoes profound changes. The skin thickens and bones soften. The aging that occurs during these few days of a salmon’s life is equivalent to 20-40 years in a human’s life.

Adult fish change in form and color as they return to fresh water to spawn. Spawning adults deposit eggs in gravel nests (redds). Salmon die shortly after spawning, while some steelhead and sea-run cutthroat may return to spawn again.

Anglers, natural predators, and other hazards continue to reduce salmonid numbers on their way to the spawning beds of their home stream. Hydroelectric dams can block passage upriver. Most dams now have fish passages but finding them uses part of the salmon’s limited energy supply. Log jams, landslides or other obstructions occasionally restrict passage. Waterfalls, road culverts and velocity and temperature barriers also create migration problems for salmon.

When flows are too low for upstream movement, water temperatures can become quite warm in the holding pools and cause conditions promoting disease outbreaks. Low flows can delay salmon movement for too long, reducing their chances for successful spawning when they finally reach the spawning beds.

Resident trout (those that don’t migrate to the ocean) often move between the streams and lakes of their home watersheds. While they do not face the long journeys of their anadromous cousins, they still must avoid predators, find food and shelter, and withstand high water flows and temperature fluctuations. All salmonids have similar spawning processes.

When salmon finally arrive within a few yards of their birthplace, they are ready to spawn. The female searches for a place suitable for spawning and digs a nest, or redd. Using her tail, the female (or hen) digs a 12- to 18-inch depression in the porous gravel of the streambed. She then deposits from 3,000 to 5,000 (varies with species) bright pink eggs in the redd. After the male (or buck) fertilizes the eggs with milt, a milky white substance containing sperm, the female moves upstream from the redd. With her tail, she digs more gravel which the current carries downstream to cover and protect the eggs.

With the completion of the reproductive task, the adult salmon’s life is complete. Within a short time it dies, and the carcass drifts downstream, decaying and contributing its nutrients to the stream that gave it life. Not all steelhead and sea-run cutthroat trout die after spawning, but because of their weakened condition, the percentage of adults that live to return to spawn again is small.

The Trout Life Cycle

Growth begins in a hatchery tray or gravel bed of a stream or lake.

Sac Fry
The young trout receive their food from the attached egg sac for one to six weeks after hatching. The yolk sac is absorbed into the belly providing nourishment until the young trout can feed.

Fry
Feeding begins after the egg sac is absorbed.

Fingerlings
When 2 to 5 inches, most hatchery trout are stocked into lakes at this size. One-year-old hatchery trout are usually 8- to 10-inch “legal.” Wild trout are usually several inches smaller.

Adults
Relatively few fish are needed to support hatchery production compared to the numbers required to sustain wild populations. Some hatchery trout are stocked as 10-inch legal-sized fish. These contribute little to future generations. A few hatchery trout are held for future egg production. Spawning for most trout begins at three years of age. Eggs are taken from brood trout reared in a hatchery or from wild fish trapped from a stream or lake. Fertilized eggs are placed in hatchery or streamside incubator trays.

Consumers of Salmon during their Life Cycle

Salmonids at the Hatchery

Mature adult salmon spawners are captured and held at a hatchery until they are ripe. When the females are ripe (the eggs are loose in the body cavity and ready to exit), a few eggs will freely drop out of their vent. Hatchery personnel usually select fish from all parts of the run for their broodstock. Some are taken early in the run, some at the peak, and others near the end. This is to ensure a diversity of inherited traits (genes) in the offspring.

To provide for healthy processing, the captured salmon are killed with a sharp blow to the head – this is of little consequence to the fish as all Pacific salmon naturally die shortly after they spawn. The adults are gently rinsed and the excess water is wiped off.

The female salmon is cut in front of the tail or at the gills to allow her blood to escape. This is done so that the blood doesn’t mix with the eggs and interfere with the fertilization or egg development processes.

The eggs are removed with a special knife, called a Wyoming or Zack knife, which is inserted in the vent and slit up the belly of the fish (the technique varies from hatchery to hatchery). The eggs drop into a bucket.

The male’s milt is removed by applying pressure (by hand) on the underside of the fish near the head and along the length of the fish towards the tail, which forces the sperm out.

The number of eggs taken from the fish is equivalent to that of natural spawning.

Hatcheries that raise trout capture broodstock from nearby streams or lakes to be used in the spawning process at the hatchery. Because trout normally don’t die after spawning, care has to be taken to avoid injury to the brood trout. This is done using a process called “air spawning.”

Females ready for spawning are anesthetized and a hypodermic needle is pushed in the egg cavity in the abdomen. A low pressure air burst (4-5 pounds per square inch) is pumped into the fish, which gently forces the eggs out the vent. The fish is then allowed to recover from the anesthesia. Milt from the male is then gently squeezed onto a sanitary bucket to fertilize the eggs, similar to the way salmon are handled.

Caring for the Eggs
When milt or sperm is added to the egg fertilization technically begins. Water or iodine solution is then added to “harden” the eggs. Once the eggs are rinsed clean, they are placed in incubators that allow a constant flow of water around the eggs. Heath trays are the most common type of container for eggs at hatcheries.

Egg development is closely related to water temperature. For instance, eggs that are placed in the Heath trays with water 8C (about 46F) would hatch in about 54 days. The warmer the water the faster the eggs will hatch. Incubation temperatures at the hatchery usually range from 2C-10C (35.6F-50F).

For the first six weeks the eggs are very sensitive to mechanical shock, light and extreme fluctuations in water temperature. At the end of this period the eggs become “eyed.” This means that the embryo has eyes that may be seen inside the egg. At this point, the eggs are less sensitive and can be handled. The dead eggs, which are easily recognized by their opaque white color, must be removed because fungus can grow on them. This fungus can spread to the live eggs and kill them. The eggs are usually counted at this stage.

Sac Fry
Once the eggs hatch, they are called sac fry. Sac fry have a large yolk sac attached to their belly. This is the main source of food at this stage. The yolk sac is gradually used up by the developing sac fry, and at age 24 weeks all that remains is a tiny slit. This stage is called buttoning-up, because the slit looks like a buttonhole. This process occurs in wild environments as well.

Fry
The salmon are now called fry and are placed in ponds according to species. While the fish are being reared, or raised, at the hatchery they are monitored for growth, health and mortalities. Environmental conditions within the ponds are carefully controlled.

One important part of the rearing program is feeding the fish. The fish food is a carefully manufactured diet developed by biologists to ensure maximum growth. The main ingredient is ground up ocean fish such as herring, but the food also contains essential vitamins and minerals necessary for fish health. Both dry and wet feeds are used; the wet foods are kept frozen until used. Feeds are regularly tested to ensure high quality.

After the fry are placed in ponds, they are fed a food called starter diet. As they grow, the size and amount of food is steadily increased. The starter diet is fed by hand. For each gram of food fed to the rearing fry, the fish will gain one gram or more of weight. Coho grow from .4 to 25 grams during their rearing period at the hatchery.

Fish are reared at the hatchery between 90 days and 16 months, depending on the species. 

Releasing the Fish into the Natural World
Releases are carefully planned to reduce interaction and competition with wild species. Young trout destined for large bodies of water stay at the hatchery until they reach the fingerling stage. They are released when they are about the size of your finger – 150 of them weigh only 1 pound. Trout destined for rivers and streams spend about 16 months at the hatchery and are released when they are about 10 inches long – it takes only three of these fish to weigh one pound.

For fall chinook salmon, release is done after 90 days of rearing; spring chinook, 12 – 18 months; and coho are released after about 14 months.

Releasing fish is carefully planned to avoid interaction and competition with the wild fish already in the stream, river, or lake.

Compare Egg Survival Rate: Natural vs. Hatchery-Raised Chinook Salmon

For Issaquah Creek Fall Run Chinook

Naturally-Produced Issaquah Creek fall Chinook

  • Adults enter the Ballard Locks from Puget Sound in July-August, then swim through and hold in the deeper, cooler waters of Lake Union and Lake Washington as their bodies transform to spawning stage.
  • Adults continue up the Sammamish River and through Lake Sammamish and into Issaquah Creek in late August and early September, with the end-of-summer rains.
  • In late September and early October, adults search for suitable spawning gravel with adequate water supply and pair up with a suitable mate for spawning.
  • In November-December, fry hatch and remain in gravel.
  • Fry emerge in February-March and fish spend approximately 3-4 months in fresh water before migrating downstream to the ocean.
  • Estimated survival of naturally produced salmon (egg to smolt): Less than 5 percent.

Chart:

Diagram - Compare Egg Rate Survival - Natural vs. Hatchery
Click to enlarge

Hatchery-Produced Issaquah Creek fall Chinook

  • Adults enter the Ballard Locks from Puget Sound in July-August, then swim through and hold in the deeper, cooler waters of Lake Union and Lake Washington as their bodies transform to spawning stage.
  • Adults continue up the Sammamish River and through Lake Sammamish and into Issaquah Creek in late August and early September, with the end-of-summer rains.
  • The adults are diverted out of the stream at the weir and jump up the 11 steps of the fish ladder to hatchery holding ponds, where they are protected against predators.
  • Starting in late September and continuing through the first half of October, hatchery staff and volunteers randomly select salmon pairs and spawn them manually.
  • Eggs are treated to control fungus and placed in incubators in the hatchery building.
  • In the spring, fry are transferred to the outdoor rearing ponds (raceways). (2-3 percent loss at this point).
  • Fingerlings are protected from predators by fences and netting, given intensive feeding and provided disease control treatment. Adipose fin-clipping is accomplished both with automated machinery and by hand. Fish reared to smolts and released in early summer.
  • Estimated survival of hatchery produced salmon (egg to smolt): 90-95 percent.

Compare Egg Survival Rate: Natural vs. Hatchery-Raised Chinook Salmon

For Columbia River Chinook salmon that follow Tyee’s Journey

Naturally-Produced Columbia River Spring Chinook

  • Adults enter the Columbia River in January-February, cross the Willamette Falls in April and enter the McKenzie River in May. Adults move upstream and hold in deep pools until spawning. Adults need to find suitable spawning gravel with adequate water supply, as well as a suitable mate to spawn with.
  • Fry hatch and remain in gravel.
  • Fish spend approximately one year in fresh water before migrating downstream to the ocean.
  • Estimated survival of naturally produced salmon (egg to smolt: 4-10 percent).

Chart:

Diagram - Compare Egg Rate Survival - Natural vs. Hatchery
Click to enlarge

Hatchery-Produced Columbia River Spring Chinook

  • Adults enter the Columbia River in January February, cross the Willamette Falls in April and enter the McKenzie River in May. The adults move upstream into the hatchery with controlled water supply and incubators. The adults are inoculated against bacterial diseases, treated to control fungus. and protected against predators. Salmon pairs randomly selected by hatchery personnel.
  • Eggs are treated to control fungus and placed in a protected structure.
  • Fry are placed in starter troughs (5-10% loss at this point).
  • Fingerlings are moved outside to rearing ponds and are protected from predators, given intensive feeding and provided disease control treatment. Fish reared to smolts and released.
  • Estimated survival of hatchery produced salmon (egg to smolt: 80-85 percent).

Learning Activity: Compare Wild vs. Hatchery-Raised Salmon

Materials:

  • Use the provided “Life Cycle Comparison” and “Salmon Life Cycle” diagrams. BE SURE TO INSERT THESE.

Activity:

  1. In your journal: draw a diagram of the life cycle of a wild salmon.
  2. In your journal: draw a diagram of the life cycle of a hatchery-raised salmon.
  3. Write down some of the differences and similarities between the two life cycles.
  4. Discuss with your class or small group why the chance of survival for egg-to-smolt stage salmon is so much higher in the hatchery than in the wild. What are the advantages and disadvantages of each?
  5. Write in your journal any questions you have. You can ask them during your hatchery visit.
  6. Do some hatchery math: Out of 100,000 eggs, how many survive to become smolts at the hatchery, on average? How many survive to become smolts in the wild, on average?