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Y’stone uses dead lake trout to kill more lake trout

October 26, 2016 GMT

With geyser basins, broad mountain faces and lodgepole pine stands as a backdrop, picture commercial fisherman afloat on pristine Yellowstone Lake grinding up thousands of salmonid carcasses, then depositing the chunks of rotting flesh on eggs below to kill yet more fish.

It’s a scene, undoubtedly, that on its face is incongruent with Yellowstone, the world’s first national park and a landscape where natural processes are supposed to rule the day, unencumbered by human interference.

But using lake trout carcasses to kill their spawn has been perhaps the most promising technique to date used on Yellowstone Lake to kill the picivorous exotic char before they grow into swimming organisms that take a toll on native cutthroat trout. If the method proves viable on a large scale, it could be a key tool in the fight to help cutthroat stage a comeback and rebound to their historical numbers. In turn, park managers hope to restore a multi-level trophic cascade that could, in theory, result in grizzly bears eating fewer elk calves.

Yellowstone Fisheries Supervisor Todd Koel came up with the idea to use lake trout carcasses to kill their own spawn — a method that’s never been tried before.

“We were just trying to create an extremely negative environment for lake trout embryos to survive, and one idea was to use carcasses,” Koel said. “They caught 350,000-plus this year, and we dump them back into the lake anyway.”

So far, in the experimental phase, the tactic has worked.

Near Yellowstone Lake’s Carrington Island, a dozen 9-square-meter cobble plots were implanted with lake trout spawn this fall and then smothered in hundreds of corpses of net-caught lake trout. Half the fish-strewn plots were covered with tarps. The Park Service, Montana State University and the U.S. Geological Survey-Montana Cooperative Fishery Research Unit are collaborators on the experiment, which has since been disassembled because all the mackinaw eggs died.

“We were killing the embryos at up to a 100 percent rate inside of three weeks,” Koel said. “It far exceeded our expectations.”

How does it work?

Exactly why smothering lake trout spawning beds with the carcasses of their brethren works to stymie development is not entirely clear.

“I don’t know the mechanism,” Koel said. “Obviously when fish decay there’s a lot of bacteria growth and fungus and a lack of oxygen. I’m not sure of the exact mechanism, and to be honest I’m not sure it really matters.”

Nevertheless, a team of Montana State University researchers has been working in the laboratory trying to figure why the spawn die. Montana Cooperative Fishery Research Unit assistant leader Chris Guy said the early hypothesis was inadequate dissolved oxygen, but the fisheries scientists have since moved away from that theory.

“They repeated some studies and it looks like it may be fungal growth,” Guy said. “That’s actually better news for us.

“We tried killing embryos just with a tarp, and we couldn’t get the dissolved oxygen to go down enough,” he said. “Now we put the lake trout carcasses on and we get 100 percent mortality in about three weeks and that, to me, corroborates fungus.”

A host of other spawning bed techniques Yellowstone fisheries scientists and partners have experimented with in the big lake so far have inspired less optimism. The tarp method alone failed, and using an electric apparatus to fry spawn was also found to be ineffective, Koel said.

“We did a four-year study on the electrofishing grid,” Koel said, “and it turns our water quality is so pure it’s very, very poor at killing eggs.

“On the surface it kills them all,” he said, “but 20 centimeters down, which isn’t very far, there really was much less success in killing the eggs.”

Another technique being tried is using a suction dredge system to pull lake trout eggs out of the substrate. In the laboratory, fisheries scientists also experimented with using rotenone, an organic fish poison, to kill spawn.

So far no techniques have been deployed on a large scale in a way that meaningfully dampened the number of lake trout fry that are squirming out of their eggs into Yellowstone Lake.

Though the carcass test is just a year old, it’s already being eyed as a broader means of hitting lake trout before fry emerge from their spawning beds. Apart from the experimental plot, Koel and his colleagues also dumped “thousands” of ground-up lake trout on top of spawning sites this fall in 50 to 60 feet of water.

“We had divers go down and see how well we did, and it looked like it’s working really well,” Koel said.

“This is all new to us,” he said, “and we’re trying to develop the best way.”

Concurrent with the experimental lake trout egg killing techniques, a separate research effort using telemetry equipment is aimed at pinpointing where exactly lake trout head to spawn. Unlike cutthroat, which run upstream in spring to procreate, mackinaw are purely lake spawners, and can do so in deep water.

Places to hide

Northern Rocky Mountain Science Center biologist Bob Gresswell is at the center of the effort, which relies on lake trout being implanted with GPS transmitters. Some 180 fish, currently, are swimming around broadcasting their locations. In five years of research Gresswell has learned that 136-square-mile Yellowstone Lake — easily Wyoming’s largest body of water — has lots of suitable nooks and crannies where the nonnative trout deposits its eggs.

“What we’re learning,” Gresswell said, “is that we’re seeing some very site-specific places and it’s much more dispersed than I originally anticipated.

“Certainly we’re coming close to knowing more where the general spawning areas are, but the specifics within those sites is a little trickier,” he said. “That’s the $64,000 question: Where are those spots? And can we develop some kind of technique that maybe is useful?”

While those questions are being answered, contracted Yellowstone Lake fishing boats will continue intensive gill netting, which is known to drive down laker populations.

With one more week in the netting season to go, the fleet had caught and killed 350,000 mackinaw — the highest number ever, by far, Koel said. The catch rate was also about stagnant, a sign that the population overall had not declined in the last year.

“Again, here we went through a whole year, and my goodness there’s still a lot of lake trout out there and there probably will be next year, too,” Koel said. “All the scientists say this can’t continue forever, so we’re going to continue to not let it up.”

There were fewer small lake trout caught this year and fewer big trout. Koel said the predominence of medium-size fish suggests netting is having a “huge effect.”

They’ll keep it up

Gresswell, likewise, was not discouraged by the continued high catch rates.

“You have to recognize that nobody’s ever tried this on this scale anywhere, ever,” Gresswell said. “I thought it would collapse two or three years ago ... but it just takes longer than we believe sometimes. If we doubled or tripled the effort, would it collapse sooner? Absolutely. But we don’t have that kind of money.”

Trout Unlimited has been a major contributor to the multimillion-dollar cutthroat restoration effort in Yellowstone Lake, and the Park Service also foots a hefty annual bill.

The vision, in the long run, is to one day wean off of expensive netting and rely equally on killing lake trout in their spawning beds before they can reach adulthood. Although the carcass numbers in Yellowstone Lake will inevitably dwindle, the techniques debuted this fall suggest that the net-caught fish corpses may be used for years.

“I feel like this is finally a method that they could start deploying,” Guy said. “Is it going to kill every lake trout in the lake? The answer is no. This is not the silver bullet to eradicate lake trout, but it has potential to really lower natural recruitment and I think it’s worthwhile testing on a larger scale.”