American shad clog fish ladders and cause problems for salmon at the Lower Granite Dam

While salmon and steelhead trout swim up the river, they now face another challenge in the fish ladders: a non-native fish is competing for space: the American shad.

At the Lower Granite Dam on the Lower Snake River in southeastern Washington, American herrings are frolicking in the fish ladder. A few years ago, the population of American herring in the Columbia and Snake Rivers exploded into a mass phenomenon.

“They were introduced to the Sacramento Valley in the late 1800s and discovered the Columbia River,” said Elizabeth Holdren, senior biologist with the U.S. Army Corps of Engineers. “Just like ours, it's beautiful and they loved it. So they settled here.”

Currently, the American shad is causing the biggest problems at the region's fish ladders, she said.

“They kind of relax. They don't necessarily completely exit our ladder and then spawn. They die in the ladder,” Holdren said.

Related: Impacts of herring and other species on salmon in the Pacific Northwest

The dead fish clog screens and dam up the water, which overflows into the parking lots below the ladder at Lower Granite.

“It's not like the fish are swimming down there that much, but it could be a real problem,” she said.

In July, 900 to 1,000 herring swam into the ladders each week, caught in fish traps that were supposed to help salmon, said Darren Ogden, a biologist with the National Oceanic and Atmospheric Association.

“They are using up our expensive anesthetics,” Ogden said.

Related: Impacts of non-native fishes on salmon and steelhead in the Pacific Northwest

This anesthetic helps biologists quickly collect data and transport salmon.

According to the Washington Department of Fish and Wildlife, American herring can also cause oxygen problems for fish in ladders. It can make counting salmon and steelhead trout difficult.

It's not all bad news, though. The agency said the herring may be eaten by birds of prey and pinnipeds, which could act as a buffer for the salmon.

The Army Corps is working on methods to control the herring above Lower Granite, Holdren said, but there is no concrete solution yet.