With Nesting Area Relocated, Caspian Terns Feast on Fewer Fish in the Columbia River Estuary

An effort to relocate a large population of Caspian Terns in the Columbia River estuary to reduce their predation on juvenile salmon and steelhead has been successful, a U.S. Army Corps of Engineers biologist told the Council this month in Portland.

While the relocation of nesting habitat from Rice Island, upstream from Astoria, to East Sand Island, downstream of the city, a distance of about 10 miles, was successful, the number of birds using the island, peaking at about 6,000, remains slightly above the preferred limit in a federal management plan. Even though the East Sand Island nesting area is smaller than the area on Rice Island, Terns simply crowd onto the available space, nesting at a density not seen before.

In the late 1990s, the Tern population on Rice Island, which the Corps built with sand dredged from the shipping lane in the river, was considered the largest in the world. The birds laid their eggs and raised their chicks on the island’s broad beaches, foraging for salmon and steelhead smolts in the river.

“The vast majority of their diet was ESA-listed salmonids – 83 percent,” said Tim Dykstra, fish program manager for the Northwestern Division of the Corps, in Portland.

Using passive methods such as erecting snow fences and chasing the birds away before they had a chance to lay their eggs on the open sand, the Corps caused the Terns to look for an alternative nesting site, which the Corps provided downriver at East Sand Island. There, the mix of fish in the river includes fewer salmon and steelhead smolts and more ocean forage fish like anchovies and herring, and the percentage of ESA-listed species in the Tern diet subsequently dropped dramatically – to about 30 percent.

Also, the Corps hoped the smaller nesting area on East Sand Island would encourage Terns – at least some of them – to find alternative nesting sites away from the Columbia River estuary. Islands were built for Terns in south-central Oregon and near San Francisco Bay, and those sites in fact have attracted Terns.

Acreage, and not the number of birds using the acreage, has been the target for the Corps, Dykstra said. “We are constraining the amount of habitat available for Terns on East Sand and providing other locations for them to breed,” he said.

An environmental impact statement on managing Terns to reduce their predation on listed species of salmon and steelhead has a goal of 3,125 pairs of birds. In 2017 and again in 2018, the number was close to that limit, but still high. “They are now nesting at a higher density than we ever saw before,” he said.

This seems a bit incongruous, Council member Ted Ferrioli said. “There is an incongruity to managing to acres and not talking about numbers. It seems that if the acre was a strategy arrived at through negotiations, essentially the birds are smarter than the plan. We need to find a strategy to manage acres that also reduces avian predation. If that can’t be accomplished, there is no management.”

But the birds have responded, Dykstra said, with the 2017 bird count being close to the stipulated maximum in the EIS and the corresponding management plan that the Corps is helping to implement. “The hope is that we will continue to see a biological response,” he said.

Washington Council member Tom Karier said the problem with Terns has gone from “no management plan to where we are now, and I think we’ve made good progress.” Despite the progress, he said it seems “there is still room for fine-tuning.”

For now, the Corps will continue to manage the acre of habitat on East Sand Island – by law, the Corps can’t further reduce its size -- as well as the alternative nesting sites. That includes collecting Passive Integrated Transponder tags from the island – tags that had been inserted in juvenile salmon and steelhead, then deposited in the sand after the fish were digested by the birds. Managing the birds is the responsibility of the U.S. Fish and Wildlife Service, Dykstra said. Designating a target population size for the East Sand Tern colony is part of the current effort to develop an environmental impact statement for the federal Columbia River Power System to protect ESA-listed salmon and steelhead.

Dykstra said the Corps also is working to reduce predation on juvenile salmon and steelhead in the estuary by Double-crested Cormorants. Terns are protected by federal law, but Cormorants aren’t, and so the control effort included oiling nests – more than 6,000 in 2017 -- and killing birds – between 5,500 and 5,600 in 2017. Plus, Bald Eagles raided the colony that year and drove most of the birds away.

“The Cormorant issue has not gone away, but there have been some positive developments,” he said.

The Corps also is working to reduce predation on juvenile salmon and steelhead in the Mid-Columbia region near the Tri Cities, Washington, where the three Tern colonies are smaller than in the estuary (400-500 birds), but still are believed to consume a high percentage of fish, particularly upper Columbia steelhead.