Pacific Flyway and Riverine Predation

A NextGen Blog post by Johanna Mitra, Stony Brook University.

All Photos © Alison M. Jones, unless otherwise noted.

This is the latest post to our NWNL NEXTGEN BLOG series. Since 2007, NWNL has supported watershed education with college internships and blogging opportunities. This NWNL NEXTGEN BLOG series posts students’ essays; sponsors a forum for our student contributors; and invites upper-level students to propose work focused on watershed values, threats and solutions.

Johanna Mitra is currently an undergraduate student at Stony Brook University. She is majoring in ecosystems and human impact with a focus on wildlife conservation and minoring in geospatial science. This is part 3 in a blog series on migratory birds and the Flyways of the Americas. Part 1 is onThe Raritan River Basin and The Atlantic Flyway and part 2 is on The Mississippi Flyway and Flood Management Conflict.

In the previous two blogs we explored how freshwater resources across the United States play a crucial role in the life cycles of migratory birds. From the East Coast’s Atlantic Flyway, to the Midwest’s Mississippi and Central Flyways, and then west to the West Coast’s Pacific Flyway, rivers in each flyway offer birds a number of critical resources during the difficult migration season. The Pacific Flyway, spanning the entire western coasts of North and South America, is one of the most important flyways for waterbirds. Each year, it hosts millions of terns, sea ducks, gulls, geese and more – traveling between Alaska and the southern tip of South America.[mfn]Bird Life International[/mfn] For many of these birds, California’s Central Valley San Joaquin River Basin and the Pacific Northwest’s Columbia River Basin are important stopover point along the flyway, as its rivers, wetlands and estuaries create ideal nesting and breeding grounds.

The Columbia River Basin itself stretches across seven states in the northwestern United States and extends further north into the Canadian province of British Columbia. Its importance to the Pacific Northwest is undeniable – with much of the region’s power being supplied by the Columbia River’s 19 hydroelectric dams, and its 1,200 miles of river providing drinking water, recreational opportunities and important fisheries.[mfn]American Rivers[/mfn] However, the Columbia River Basin’s appeal to migratory birds using the Pacific Flyway has put its wild salmonid population in a rather precarious situation — as well as the people who depend on the river and its fish.

How are migratory birds affecting the Columbia River’s salmon populations?

The Columbia River Basin was once home to the most prolific wild salmon runs anywhere in the world.[mfn]American Rivers[/mfn] Gradually, overfishing and the construction of dams along the Columbia River reduced the number of salmon returning to spawn each year.[mfn]Runwal, Priyanka[/mfn]Further hampering their recovery in this river basin are colonies of fish-eating birds, who prey on the salmonids at a number of “foraging areas” along the river.[mfn]Harrison, John[/mfn] Researchers have found that Caspian terns are the main culprit. Every summer, these migratory birds fly from Central America to the Columbia River Basin along the Pacific Flyway to nest in the upper reaches of the Columbia and Snake Rivers.[mfn]Runwal, Priyanka[/mfn]

Caspian tern with salmonid smolt at the East Sand Island colony near the mouth of the Columbia River. Courtesy of Dan Roby, Oregon State University and Creative Commons.

For the populations of young steelhead and salmon who make their own migration to the Pacific Ocean during this time, colonies of nesting terns looking for food to raise their young pose a significant threat. Researchers have discovered that more than 50% of all mortalities of Upper Columbia steelhead smolts can be attributed to predation by Caspian terns and other fish-eating birds, such as double-crested cormorants and gulls.[mfn]Harrison, John[/mfn] The fewer salmonids that make it to the ocean, the fewer that return to spawn later on in their life cycles, raising concerns for conservationists monitoring their already endangered populations.

What does this mean for the Columbia River and Pacific Flyway?

The persistence of the salmon population of the Columbia River Basin is incredibly important to the region and its people. The Yakama, Warm Springs, Umatilla, Colville and Nez Perce tribes all rely on the return of adult salmon to the Columbia and Snake Rivers for the fishing season. Additionally, the salmonids themselves help to restore nutrients to the river ecosystem when they die.[mfn]Columbia River Inter-Tribal Fish Commission[/mfn]

Lower Elwha River salmon in its natural cycle of dying after spawning

Part of the difficulty in managing this conflict is due to the fact that all species involved are protected under the Endangered Species Act or the Migratory Bird Treaty Act.[mfn]Runwal, Priyanka[/mfn] Researchers have observed a 64% decrease in the number of breeding tern pairs in the Columbia River Basin since 2008, and also a drop in the number of Caspian terns using the Pacific Flyway overall.[mfn]Harrison, John[/mfn] This decline is partly due to the loss of nesting ground in the Columbia River Basin and elsewhere in the Pacific Flyway, as wildlife managers try to deter the birds from nesting in areas where salmon spawn.[mfn]Harrison, John[/mfn]

California’s role in the Pacific Flyway

The loss of viable nesting areas elsewhere in the Pacific Flyway, specifically in California’s Central Valley, is another cause for concern for conservationists. While a once thriving wetland habitat, today only 10% of the original Central Valley wetlands still exist, leaving the millions of waterbirds that ordinarily rely on it without a migratory stopover.[mfn]Arthur, Samantha[/mfn] The Central Valley’s recent 7-year drought has exacerbated wetland loss and also contributed significantly to the loss of breeding waterfowl in the Pacific Flyway.[mfn]Frost, Garrison[/mfn] As suitable habitat continues to shrink and birds seek out alternatives, it could put additional pressure on the Pacific Flyway’s already-fragile change of fresh-water ecosystems, including those further north in the already-mentioned Columbia River Basin.

California’s Central Valley

What can be done to resolve the conflicts?

In order to ensure that Caspian tern, wild salmonid and other interrelated and endangered Pacific Flyway populations are able to recover, conservationists are seeking various solutions, including encouraging terns to nest elsewhere in the flyway, such as “alternative nesting sites…in southeastern Oregon and near San Francisco Bay.”[mfn]Harrison, John[/mfn] The hope is that Caspian terns in these nesting sites will feed on other non-endangered fish species in those areas, like herring and sardines.[mfn]Runwal, Priyanka[/mfn]

Chinook hatchery salmon underwater

For migratory birds using flyways, places like the Columbia River Basin and California Central Valley are especially important for offering safety and resources that support them year after year. Certainly the loss of nesting sites can be detrimental to their populations. As well, the loss of local fish populations can be equally devastating to freshwater ecosystems and local economies. This conflict between salmonids and migratory birds is a prime example of how delicate freshwater ecosystems can be.

As a final note, we would like to share an excerpt from the newly published book, Pacific Flyway: Waterbird Migration from the Arctic to Tierra del Fuego by Audrey DeLella Benedict:

The magnificent wetland corridor for waterfowl that migrate through Washington, Oregon and California has been greatly altered by human activities.  Wildlife refuges and natural wetlands now exist mostly as islands within a grid of irrigated industrial agriculture.… The determined [migratory] birds remind us that given a carefully selected and appropriately managed array of favorable habitats, viable populations of migratory waterfowl can persist even in some of the most intensively human-altered landscapes.[mfn]Benedict, Audrey DeLella[/mfn]

Bird seen at Elizabeth Lake Wildlilfe Center, British Columbia


Benedict, Audrey Delella. Pacific Flyway: Waterbird Migration from the Arctic to Tierra del Fuego.  Ward,CO: Cloud Ridge Publishing, 2020, page 64.

“Columbia River.” American Rivers, n.d. Accessed on Jan 27, 2021 by JM.

Harrison, John. “Avian Predation: A River System Out Of Balance.” Northwest Power and Conservation Council, May 18, 2020. Accessed on Jan 28, 2021 by JM.

Harrison, John. “Fish-eating Birds Kill Half, Or More, Of Upper Columbia Steelhead Smolts, Research Shows.” Northwest Power and Conservation Council, Feb 19, 2020. Accessed on Jan 28, 2021 by JM.

“Pacific Americas Flyway Factsheet.” Bird Life International, n.d. Accessed on Jan 27, 2021 by JM.

Runwal, Priyanka. “A Silicon Valley Disruption for Birds That Gorge on Endangered Fish.” The New York Times, Nov 12, 2019. Accessed on Jan 28, 2021 by JM.

“Tribal Salmon Culture.” Columbia River Inter-Tribal Fish Commission, n.d. Accessed on Jan 28, 2021 by JM.

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