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Life Cycle of Pacific Salmon


A Salmon's Life: An Incredible Journey

Salmon make an incredible journey downstream from the fresh water where they are born, to the ocean, and then back upstream again as adults, finding the exact location where they began several years earlier.

Salmon Eggs
Salmon Eggs

Dave Alf, USFS

Salmon lay their eggs in many of the streams and rivers of the Columbia Basin. Depending on the species, a female salmon will lay anywhere from 1,500 to 7,000 eggs in a nest or redd she has created by making a shallow depression in the stream bottom. The male fertilizes the eggs and then both fish push gravel over them to protect them.

Hatching Salmon
Hatching Salmon

Dave Alf, USFWS

Young fish or alevins hatch in late winter or early spring, looking more like worms than fish. These tiny fish depend on a yolk sac to provide them with nourishment until they are mobile enough to wiggle out of the gravel and find their own food. At this stage, the young salmon are called fry. Feeding on tiny plants and animals, the fry cluster in groups and develop into juveniles.

Salmon fry
Salmon fry cluster in groups

Dave Alf, USFWS

When juveniles are ready to migrate to the ocean, they undergo a physiological transition from freshwater to saltwater fish. Only about 10 percent of the fry make it to this stage and are called smolts.

Salmon smolt development
Smolt development. Only 10 percent make it to this stage.


Smolts are especially vulnerable. They are frequently injured or killed by turbine blades and the extreme pressure changes experienced when passing through the turbines of dams. In addition, smolts float downstream tail-first and depend on the river's current to flush them to the ocean quickly. The slack water created behind the dams is very difficult for the young fish to negotiate, making them vulnerable to predators. Dams also slow the migration considerably. A trip that used to take one to three weeks can now take one to three months, depending on the beginning point of the trip. The smolts have limited energy stored in their bodies and may run out before they are able to reach the ocean. Up to 90 percent of the salmon hatched never reach the ocean.

Kokanee Salmon
Kokanee Salmon spawning in the North Fork of the Payette River, Idaho


The smolts that complete the journey downstream spend several weeks in estuaries where the river meets the ocean, feeding on small fish and shrimp. Eventually, they disappear into the ocean where they grow to adulthood.

After two to five years, the adult salmon are ready to migrate upriver to spawn in the streams where they were hatched. It is believed that salmon are guided to the rivers by currents, stars, and the Earth's magnetic force. Once in the river, the fish find their home streams by scent.

The journey upriver is a difficult one. Salmon do not eat during this time, but live on fat stored in their body. They may travel as far as 1,440 km in fresh water to their spawning grounds. Obstacles encountered upstream are many and varied. Dams, waterfalls, anglers, bears, uncertain stream conditions, and habitat degradation are among the most common challenges for the salmon in the Columbia River Basin ecosystem.


Flooding Helps Cottonwoods

One of the most diverse and beautiful ecosystems in the state of Idaho is found in the river corridor where the South Fork and Henry's Fork of the Snake River merge, about 32 km outside of Idaho Falls. Found here is the largest cottonwood forest in the western United States. The area attracts a variety of wildlife populations including more than half of Idaho's bald eagles, a growing population of peregrine falcons, trumpeter swans, and 126 species of migratory birds (about 75 percent of which are neotropical, migrating from Central and South America each year). The area also attracts moose, elk, deer, mountain lions, and one of the largest native cutthroat trout populations outside of Yellowstone National Park.

Cottonwood Forest
A healthy cottonwood forest. The understory is silverberry.

Karen Rice, BLM

Cottonwood trees are found throughout the western United States and are important to riparian ecosystems. They tend to grow close to the water, providing shade that cools the water to temperatures required by native fish. Tree limbs and dead trees provide protective habitat areas for young fish.

Cottonwood forest along riverbanks in the fall
Cottonwood forests grow along many rivers and streams in the West. They are part of a dynamic system that depends on periodic flooding.

BLM—Medicine Lodge Resource Area—File Photo

Cottonwood trees evolved on dynamic river systems that flood periodically. Floods tear down riverbanks and carry sediment downstream to form new gravel bars. Cottonwood seeds need this fresh gravel, which provides a moist, sunny environment on which to grow. New cottonwoods grow in new areas with fresh gravel bars, while old cottonwoods are replaced by juniper and sagebrush. Cottonwood forests are therefore part of a dynamic ecosystem dependent on natural flooding.

Recent changes in stream structures (dams, for example) have changed the dynamic nature of many streams, threatening the natural regeneration of the cottonwood forest. The Idaho floods in 1996, however, provided a tremendous volume of water that created large gravel bars on which cottonwood seedlings could sprout. Now, hundreds of seedlings are thriving, ensuring healthy cottonwood populations in the area for years to come.


Lodgepole Pine Forests

Lodgepole pine is the third most abundant western tree after Douglas fir and ponderosa pine. It was named by explorers Meriwether Lewis and William Clark during their historic 1804­1806 journey, when they observed Native American Indians using the straight, slender poles for their lodges or tepees. Today, lodgepole pine is a principal source of poles for log houses as well as posts.

Lodgepole pine forests are found in five of the seven states in the Interior Columbia River Basin. They cover about 80 percent of the forested land in the northern and central Rocky Mountains on the lower reaches of the subalpine fir zone and within the Douglas fir stands. They may also be found in the salt spray zone of the Pacific Coast ranging from sea level to an elevation of 2,400 m.

To understand a lodgepole forest, stand back and look at it from a distance. Foresters describe the tall, thin, dense forests as "dog-hair" stands. Lower branches die out with the shade and leave the trunk with only a topknot of live branches. Inside the stand, shade-loving seedlings thrive —Douglas fir, Engleman spruce, or subalpine fir.

Elk in a lodgepole pine stand in Idaho
Elk in a lodgepole pine stand in Idaho

Larry Ridenhour, BLM

Lodgepole pines are especially adapted to fire. Some of the cones are sealed by a resin that melts when heated by a forest fire. Cone scales open and seeds fall onto the bare ground that is rich with nutrients in the ash. Exposed to the sky, which supplies rain and sun, the seeds have perfect conditions for germination and growth. Because post-fire seedling establishment is often very successful, the "dog-hair" stands appear again. If there are no fires and conditions are right, the lodgepole will die out, leaving behind the spruce and fir community. Pure stands of lodgepole may persist if the site is too dry, moist, steep, or fire-prone for spruce and fir.

lodgepole pine stand

Lodgepole pines are characterized by their long straight poles.

Larry Ridenhour, BLM

Lodgepole pines are especially appealing to the mountain pine beetle. When the larvae hatch, they girdle the tree as they eat their way around its circumference. Although this is destructive, it creates a habitat for other forest dwellers. Dead trees or snags may stand for more than 20 years and provide perches for hawks and cavities for owls, chickadees, and squirrels. Downed trees provide a haven for red-backed voles, toads, salamanders, western garter snakes, and rubber boas.

Live tree stands are home to a variety of insect and seed-eating birds such as yellow-rumped warblers, ruby-crowned kinglets, mountain and black-capped chickadees, white-and red-breasted nuthatches, pine siskins, western and mountain bluebirds, Townsend solitaires, and blue grouse. Mule deer and elk use the stands for cover and browse on lichen.

As with many ecosystems, lodgepole pine forests have been drastically modified by aggressive fire suppression. As more fires are curtailed and acres of trees die from beetle kill, fuels accumulate, setting the stage for larger and more severe fires.


The Grizzly Bear: A Debate

A symbol of wildlands—and one of the largest North American land mammals—the grizzly bear is a threatened species under the Endangered Species Act. Confined to only two percent of their original range in the lower 48 states, grizzlies need large blocks of diverse wildlands to thrive. Some of the most suitable habitat for grizzly bears is in the mountainous western Montana and central Idaho portions of the Interior Columbia River Basin.

Not everyone agrees, however, that grizzly bears should be reintroduced into their former ranges. Arguments for reintroduction include a belief that the top predator should be included in the wilderness ecosystem to keep populations of other animals in check. The bears were there before humans and should be allowed to reoccupy some of their former range. Arguments against reintroduction include concerns about the safety of livestock and humans, as well as concerns about restrictions on land use that may accompany a reintroduction.

Divide the class into two groups, one that supports reintroducing grizzly bears and one that does not. Have each group brainstorm a list of facts and statements that support their arguments. Hold a class debate or invite other classes to observe and then vote on the decision. Analyze the vote in a class discussion. What persuaded the students to vote as they did? As an extension, students could debate the reintroduction of other animals, such as gray wolves and black-footed ferrets (western United States), red wolves (eastern United States), or the California condor (southwestern United States).

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