Walleye were first brought to the Columbia River nearly 60 years ago. Since then, the toothy fish have preyed on all sorts of aquatic species, including salmon, steelhead and lamprey.
Over the past decade, walleye numbers have dramatically increased in the Snake River. They’ve been spotted upstream of Lower Granite Dam – the most upstream of the four lower Snake River dams.
On Wednesday, the Northwest Power and Conservation Council discussed the growing concerns caused by this walleye boom at its Fish and Wildlife committee meeting.
The council has monitored other predator concerns, including other invasive fish, birds and penipeds.
“This will be our first chance to really, so to speak, sink our teeth with (the) walleye (discussion),” said Patty O’Toole, the Fish and Wildlife division director.
A recent uptick
While walleye are native to the Great Lakes and Mississippi River basins, researchers, anglers and fish managers have noted an uptick in walleye numbers, particularly in the Snake River.
Walleye have recently expanded throughout their current range. In Washington and Idaho, they’ve been found in multiple rivers, including the Walla Walla, Tucannon and Clearwater. They’re also in the Snake River to Hells Canyon Dam and the Salmon River to Riggins, Idaho.
The Nez Perce Tribe uses gillnets to monitor for walleye. The size of gillnets they use — and the timing — are planned so they don’t accidentally catch salmon or steelhead. In one specialized gillnet on the Tucannon River, the tribe caught 56 walleye.
Those walleye had salmon in their bellies. And the salmon had PIT tags from all of the tribe’s hatcheries, said Jason Vogel, the fisheries research division director for the Nez Perce Tribe
“So that was really an eye-opener for us,” Vogel said. “First off, there are a lot of walleye down there, and second, the prey items that they have represent (salmon) from all over the place."
Biologists say that could equate to hundreds of salmon eaten by just those walleye.
The changing food web
Another new study analyzed what walleye on the Snake River were eating. The study found Snake River walleye could actually be helped by other invasive fish.
That’s because invasive fish are likely altering the food web. The phenomenon is called the invasional meltdown hypothesis.
To study the hypothesis, researchers looked both at the food in walleye’s bellies. The study also analyzed the chemical signatures of the walleye tissue and compared that to the chemical signatures of what the walleye ate.
“ It’s like your parents probably told you when you were younger, ‘You are what you eat,’” said Jonny Armstrong, who is with Oregon State University. “If a predator is eating a certain prey item, the isotopes in their tissue will reflect that.”
Both what was in their bellies and what was in their tissue confirmed walleye are eating salmon in the spring. When that migration is over, walleye mostly eat invasive American shad. Year-round, they snack on invasive Siberian prawn.
“ You can think of (Siberian prawn) as kind of like potato chips,” Armstrong said. “ They're just this year-round snack that walleye always have.”
Essentially, the shad and prawn aren’t providing a buffer for salmon.
Instead, “They’re providing this unique foraging opportunity at a different time of year. And populations of walleye are likely supported by other invasive species,” Armstrong said.
Anecdotally, he compared the booming walleye numbers on the Snake River to parts of the Columbia River — where there aren’t American shad or Siberian prawn.
“We're not seeing exponential population growth in walleye,” he said.
Possible paths forward
Marika Dobos, who is with Idaho Fish and Game, said a group from around the Northwest is working to better understand how to conserve salmon without allowing walleye to take over.
“ These fish are incredibly resilient and very difficult to manage once they get established in areas where we don't want them,” she said.
At the same time, she said, many anglers enjoy catching walleye.
Dobos says that makes solutions complicated.
Washington, Oregon and Idaho encourage anglers to keep or kill walleye, in an effort to help reduce their numbers.
“Go fishing. No limits,” said Andrew Murdoch, with the Washington Department of Fish and Wildlife. “Unless we start getting serious, there are going to be walleye in the Columbia mainstem for a long time.”
Read the audio transcript here.
