Predation and the Food Web

Predation and the Food Web

Salmon are part of a complex food web. They start as small fish in freshwater, where they eat insects and other tiny animals. As they grow, they eat larger animals, such as squid, baby crabs, and smaller fish, such as herring and anchovies.

Throughout their lives, salmon feed other animals as well such as larger fish, birds, seals and other marine mammals, humans, and orca whales. This is especially important on their journey back to their home rivers, where salmon feed many creatures, including bears, eagles, insects, young salmon, and even trees. This is one of the reasons that salmon are known as keystone species: they gather nutrients from the ocean and distribute them in rivers on their way home.

Salmon feed 138 other Washington and Oregon animals and plants throughout their lives.¹ While struggling to reach adulthood, they prey on animals smaller than themselves while dodging predators. Once at their spawning grounds, they die, delivering nutrients from the ocean to the upland and mountain habitats where they were born. Their bodies fertilize the stream and surrounding forests and feed wildlife great and small. Without salmon, these cold, shaded streams do not have as much food for the other fish and wildlife that rely on the stream.

People have changed the way that salmon interact with the land. Too many fish have been caught, preventing salmon from reaching their spawning grounds and delivering nutrients from the ocean. People also have created bottlenecks to migration, such as dams, that make salmon easy targets for predators. People have also changed the climate, which has made it more difficult for salmon to find nutritious food in streams and in the ocean.

PRESSURE: Decreased Food Availability

The food that salmon need is increasingly in short supply. Young salmon eat insects and other tiny animals. As they grow larger, they eat shrimp-like creatures called krill, squid, and forage fish such as herring and anchovies. These animals have declined because of fishing, habitat destruction, and poor ocean conditions. Forage fish habitat in Puget Sound has been damaged by shoreline armoring, such as bulkheads and rock walls along the shoreline, which reduces their spawning habitat.

Forage fish in Puget Sound spawn on sandy beaches and in eelgrass beds and kelp forests. All these habitats have been damaged by development. Shoreline armoring creates steeper beaches with larger gravels and cobble, rather than sand. Eelgrass beds are disturbed and destroyed by development and boat anchors. And kelp forests, once common in Puget Sound, have been decreasing for decades, with no obvious reason or solution.

Climate change makes these problems worse. For example, scientists have found that adult Chinook salmon diets have been changing because of changes in the ocean, and that they are eating more anchovies rather than a balanced mix of animals. This appears to cause premature death and illness because the simpler, anchovy-based diet does not provide enough thiamine (Vitamin B1). Similarly, juvenile salmon entering oceans during warm-water conditions often struggle to find enough nutritious food to grow and mature, reducing their chances of survival.

PRESSURE: Increased Predation

Marine Mammals

In the 1960s, the federal government became concerned about decreasing numbers of seals, sea lions, and other marine mammals and passed the Marine Mammal Protection Act, a law restricting their killing. The law proved successful, their numbers grew, and now they are eating lots of salmon. Between 1970 and 2015, seals and sea lions increased the amount of adult Chinook salmon they ate from 75 tons to 718 tons–double that of resident orcas and six times more than commercial and recreational fishing combined.²

Despite their name, sea lions are comfortable far from saltwater and have learned to take advantage of human infrastructure, such as dams and bridges, to improve their hunting odds. Stellar and California sea lions are common in late winter through summer throughout the Columbia River and its tributaries below Bonneville Dam, 146 river miles from the mouth of the Columbia River. Here, they prey on salmon, smelt, and sturgeon, often eating sizable proportions of returning salmon runs. Large groups of seals and sea lions spend most of the year in the Columbia River near its mouth at the Pacific Ocean.

Emerging science suggests that harbor seals eat many juvenile salmon in Puget Sound and Hood Canal, especially when other types of food, such as herring and anchovies, are scarce.³ Here, too, the predators have learned to focus on areas where salmon are forced close together, such as the Hood Canal Bridge, which appears to confuse young fish and forces them into narrow swimming paths as they migrate to the ocean, creating easy meals for harbor seals.

Birds and Fish

Changes in river conditions caused by dams have allowed fish and birds to eat more juvenile salmon. About 35 percent of the juvenile spring Chinook salmon from the upper Columbia River migrating to the ocean are eaten by birds, while fish eat millions more in the Columbia and Snake Rivers.⁴

PROGRESS AND PRIORITIES

Tribe and State salmon co-managers in Washington and Oregon have secured federal permits and started removing sea lions that habitually gather at bottlenecks for salmon such as Bonneville Dam, an action that should reduce the number of salmon eaten.

The U.S. Army Corps of Engineers manages many programs to reduce rates of predation by birds in the Columbia River estuary and the middle Columbia River. A bounty program that pays recreational anglers to capture pikeminnow has reduced significantly the number of salmon eaten. Meanwhile, the Washington Department of Fish and Wildlife has changed fishing regulations in many waters with salmon to encourage anglers to harvest walleye, smallmouth bass, and channel catfish, all of which are known to eat young salmon.

The Washington Legislature provided $10 million in 2024 to study and manage bird, marine mammal, and fish predation on salmon and steelhead across the state.

Researchers are working to identify bottlenecks in saltwater where juvenile salmon are being eaten in large numbers, such as the Hood Canal Bridge. Once these bottlenecks are identified, engineering solutions may make it faster for salmon to pass, making those areas less attractive to harbor seals and birds.

Salmon feed 138 other Washington and Oregon animals and plants throughout their lives.

Banner photograph of Cormorants by Fazlul Alam
Orca photograph by Candice Emmons, NOAA
Caspian tern and sea lion by Ingrid Taylor, NOAA

¹Cederholm, C. J., D. H. Johnson, R. E. Bilby, L.G. Dominguez, A. M. Garrett, W. H. Graeber, E. L. Greda, M. D. Kunze, B.G. Marcot, J. F. Palmisano, R. W. Plotnikoff, W. G. Pearcy, C. A. Simenstad, and P. C. Trotter. 2000. Pacific Salmon and Wildlife-Ecological Contexts, Relationships, and Implications for Management. Special Edition Technical Report, Prepared for D. H. Johnson and T. A. O’Neil (managing directors), Wildlife-Habitat Relationships in Oregon and Washington. Washington Department of Fish and Wildlife, Olympia, Washington.
²Chasco, B.E., Kaplan, I.C., Thomas, A.C. et al. (2017) Competing tradeoffs between increasing marine mammal predation and fisheries harvest of Chinook salmon. Scientific Reports 7, 15439. https://doi.org/10.1038/s41598-017-14984-8 Accessed June 24, 2020.
³Washington State Academy of Sciences. (2022). Pinniped Predation on Salmonids in the Washington Portions of the Salish Sea and Outer Coast. Seattle, WA: WSAS, 1-81.
⁴Northwest Power and Conservation Council, Predation section of website, https://www.nwcouncil.org/fish-and-wildlife/fw-topics/predation Accessed on November 9, 2020.