Yellowstone Fights Lake Trout With Nets, ‘Judas’ Fish And Pellets

Jul 27, 2019

Yellowstone National Park has been working for over a decade to protect one of the last genetically pure Yellowstone cutthroat trout populations in the Intermountain West. Its survival is the linchpin in a complex food web that includes animals as small as zooplankton and as large as grizzly bears. The Park is optimistic in its fight against the species’ biggest threat — non-native lake trout. 

On board the Northwesterner, a 46-foot-long contractor boat, Yellowstone Park’s invasive fish management team position themselves around a steel table near the bow.

“It gets really slick with blood and everything so just watch your footing as you move around,” says Captain Tom Short. 

Today they’re pulling in their gillnetting traps as part of an ongoing effort to drastically cut Yellowstone Lake’s non-native lake trout, which eat native Yellowstone cutthroat trout. 

“We’re seeing this year to year now where even though we’re putting out all this net and the same amount of effort or even more effort every year, we’re getting fewer and fewer lake trout caught in the nets," says Todd Koel, Yellowstone’s senior fisheries biologist. “Basically we’re crashing the population. It’s working, and we’re winning this war.”

Through rigorous gillnetting and millions of dollars, Park staff and contractors have managed to catch and kill over 3 million adult lake trout since 2012, reducing the population by 70 percent. Researchers are also following geo-tagged 'Judas' fish to spawning sites, which park staff will smother in the fall to knock out the next generation.

Koel says there are still probably about half-a-million adult lake trout in Yellowstone Lake, but he’s hopeful they can get it down to 100,000 in 3-4 years. 

Captain Tom Short turns on the lifter, which pulls the gillnet up and into the boat. Crew members untangle silver speckled lake trout from the net and slide them over to Mark Kundzins. 

He uses a small blade to slice through the trout’s swim bladder.

"It lets them rise and sink in the water column. We cut that so once they die, they sink and don’t float around,” says Kundzins.

He tosses the dying fish into a plastic bin and adds another mark to his tally. 

Nearby, crew member Cory Dick rolls a native cutthroat trout out of the net. The fish is a soft orangish-brown color with dark spots and a splash of fuschia across its jaw. It’s mouth and gills open and close as Dick gently picks it up.

“See this is one that I want to release," he says as he drops it over the side of the boat.

The cutthroat disappears into the dark water below. 

This is captain Tom Short’s seventh year, and he says he sees progress. The lake trout population is in decline, and fewer of them are reaching the large adult size.

“Every size class and every age class is going down pretty dramatically but definitely in the large mesh. It’s half of what it was two years ago,” says Short.

Cutthroats used to have free reign of the lake. When lake trout showed up in the 90s, the fish from the Great Lakes feasted on the Yellowstone cutthroat population, which started to plummet in the early 2000s. 

The ospreys and bears that used to binge on cutthroats migrating up streams to spawn each year couldn’t reach the lake trout that live in deeper water.

Now park biologists use that deeper water as a dumping site for the gillnetted lake trout. Captain Tom Short says they caught about 1,300 lake trout today.

Short says they’ve gotten a lot better at finding lake trout, thanks in part to so-called ‘Judas’ fish, which park staff can follow using to acoustic trackers. 

Yellowstone Fish Biologist Todd Koel used that data to identify 14 spawning sites where lake trout will lay their eggs in the fall. 

Koel says this year his team is amping up its efforts to kill the eggs with organic soy and wheat pellets, which turn doughy when wet and block oxygen from reaching the eggs. After a few days, the eggs are dead, and the material dissolves. 

Koel says egg suppression is still in the research and development stage, and Yellowstone and its partners are closely studying it, along with the dump site, to make sure solving one problem doesn’t accidentally cause another problem.

“You need to understand what other impacts you might be having. It’s not likely because the spawning sites are so small relative to the size of the lake that there’s going to be a big effect, but still we want to make sure we document that,” says Koel.

He says the lessons learned in Yellowstone could be applied in other places — like Glacier National Park and Lake Pend Oreille in Idaho — where there are efforts to keep lake trout populations in check. 

Koel says they will always have to manage lake trout in Yellowstone Lake. But if they can reduce the adult population, they can cut back on the gillnetting and divert resources to other restoration projects around the Park. 

“There is no technology out there — that we know of — that we can use to completely eliminate the lake trout from Yellowstone Lake,” says Koel. 

He says that’s the case with just about every invasive fish or aquatic species. The Park has boat inspection programs and tries to increase angler awareness to keep their boats and equipment clean.

“I mean, once they become established in a water body, they’re almost impossible to completely remove. That’s why the prevention of these introductions is so important,” Koel says.

The fish restoration program for Yellowstone Lake has an annual budget of $2 million. Private donations through the Yellowstone Forever Foundation provide $1 million, and the Park matches it with federal funding.

The Bozeman Daily Chronicle reported in June the Foundation is over $4 million in debt. Koel says he doesn’t see the funding for the fish restoration program changing.