Salmon's Predators
Enhanced Video Resource

This video segment from NATURE: “Salmon: Running the Gauntlet” describes efforts to protect salmon by killing and re-locating predators. Examples include paying bounty hunters to fish pikeminnow, trying to relocate terns and cormorants and using non-lethal bullets to scare away sea lions. The segment concludes with a look at how scientists are counting current numbers of salmon passing through Bonneville Dam, along the Oregon-Washington border.

Discussion Questions:

  • What do you think about the methods officials are using to protect salmon from pikeminnow and sea lions?
  • Do you think officials should build a new island for the terns and cormorants in order to keep them from eating salmon? If not, what actions (if any) do you think should be taken?
  • How are scientists keeping track of their progress in increasing the salmon population?

Background Essay:

In the past 200 years, the number of salmon in the Pacific Northwest has decreased dramatically due to a combination of natural and human-related factors. Many species of salmon in the Pacific Northwest are listed as threatened or endangered on the Endangered species list. In the Colorado River, the salmon population is approximately 3% of what it was in the early 1800s. Some factors that pose threats to salmon are rising stream and river temperatures, due to climate change; pollution, which is damaging and sometimes deadly to salmon and their prey; overharvesting (through commercial fishing, etc.); dams on the rivers, which obstruct passage to spawning areas; and natural predators. Four species that provide serious threats to Pacific Northwest salmon are: double-crested cormorants, Caspian terns, sea lions and pikeminnow.

  • Pikeminnow: Scientists believe fish hatcheries (which produce, raise and release salmon into the wild) and dams have directly contributed to increases in pikeminnow populations. Hatcheries release large quantities of juvenile salmon into the water at once. Salmon often get disoriented after passing through the fish ladders at the dams and fall victim to pikeminnow, which wait below the dam and feed on the salmon as they pass through.  In 1991, the northern pikeminnow management program began, which pays bounty hunters to fish for pikeminnow, in order to protect endangered salmon species. As a result of the program, more than 3 million pikeminnow have been removed from the Columbia and Snake Rivers and the amount of juvenile salmon and steelhead consumed by pikeminnow has decreased by 50%. Note: A bounty involves the payment of money to encourage people to remove a species that has been identified as a danger to others. This stems from a long tradition of paying people to remove species that pose a threat. Some species that have been targeted by bounty hunters include: coyotes, crows, foxes, gophers, rats, rattlesnakes, skunks and wolves.
  • Caspian Terns and Double-Crested Cormorants: Cormorants and terns on East Sand Island at the mouth of the Columbia River eat about 20 million young salmon a year. The island, owned by the US Army Corps of Engineers, houses the largest known nesting colonies of double-crested and Caspian terns in the world (approximately 13,600 breeding pairs of cormorants and 8300 breeding pairs of Caspian terns in 2010). The US Army Corps of Engineers is currently planning to relocate half the tern population to other sites in Oregon and California by 2015.
  • Sea Lions: Sea lions eat thousands of salmon and steelhead populations each year, while the fish are waiting to enter the ladders at Bonneville Dam on the Columbia River. In order to protect salmon and steelhead populations, officials first authorized the use of non-lethal means to scare away the sea lions, as depicted in the “Extreme Measures” video resource. Despite these efforts, however, the problem persisted and, in March 2008, the federal government gave fish and wildlife officials in Washington, Oregon and Idaho the permission to kill up to 85 sea lions each year below Bonneville Dam in order to control the population.  In addition to killing the sea lions, states try to relocate sea lions to aquariums and zoos, when possible. This decision to kill one species to protect another has been challenged by the Humane Society and other animal advocacy groups, who state that the sea lions are not as big a threat to salmon as other factors, such as overfishing by humans.

For more information, go to:

General information about Salmon’s Predators:

Terns and Cormorants:

Sea Lions:


National Science Education Standards:

Grades 9-12:

Content Standard C: Life Science
Fundamental concepts and principles that underlie this standard include:
  • The Interdependence of Organisms
    • Organisms both cooperate and compete in ecosystems. The interrelationships and interdependencies of these organisms may generate ecosystems that are stable for hundreds or thousands of years.
    • Living organisms have the capacity to produce populations of infinite size, but environments and resources are finite. This fundamental tension has profound effects on the interactions between organisms.
    • Human beings live within the world’s ecosystems. Increasingly, humans modify ecosystems as a result of population growth, technology, and consumption. Human destruction of habitats through direct harvesting, pollution, atmospheric changes, and other factors is threatening current global stability, and if not addressed, ecosystems will be irreversibly affected.
  • The Behavior Of Organisms
    • Organisms have behavioral responses to internal changes and to external stimuli. Responses to external stimuli can result from interactions with the organism’s own species and others, as well as environmental changes; these responses either can be innate or learned. The broad patterns of behavior exhibited by animals have evolved to ensure reproductive success. Animals often live in unpredictable environments, and so their behavior must be flexible enough to deal with uncertainty and change. Plants also respond to stimuli.

Content Standard F: Science in Personal and Social Perspectives and Technology
Fundamental concepts and principles that underlie this standard include:

  • Science and Technology in Local, National, and Global Challenges
    • Humans have a major effect on other species. For example, the influence of humans on other organisms occurs through land use—which decreases space available to other species—and pollution—which changes the chemical composition of air, soil, and water.

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