Why is Utah poisoning all the fish in Navajo Lake?

Proliferating chub literally suck all the oxygen from the room and crowd out sport fish

(Tom Wharton | The Salt Lake Tribune) Utah's Navajo Lake, pictured here in 2016, is a popular fishing destination, but it has become overrun with chub, a non-sport fish native to Utah. Now state fisheries officials plan to poison all the fish in the lake this fall to create a chub-free trout fishery.

There are too many chub in Navajo Lake. This big minnow literally owns this popular fishing destination high on Utah’s Cedar Mountain in Kane County.

Now Utah wildlife officials plan to wipe out all the fish at Navajo and restock it with rainbow, brook and tiger trout and other species enjoyed by anglers.

Wait, the state is going to rid a lake of a native Utah fish to create a safe haven for nonnative sport fish? Isn’t that backward?

Not really, according to Richard Hepworth, fisheries manager for the Utah Division of Wildlife Resources’ southern region.

The Utah chub is certainly native to the Sevier River system, he said, but this species of minnow doesn’t really belong in reservoirs where they can quickly dominate an ecosystem to the detriment of other fish.

“By pure numbers and better reproductive capabilities, there gets to be so many of them, that in a place like Navajo, they outcompete for food,” Hepworth said. “But more importantly it’s about oxygen during the winter. All these chubs in here use up all the oxygen and then the trout all die.”

So in mid-October, DWR plans to mix 9,000 pounds of powdered rotenone into the 3-mile-long lake to achieve a concentration lethal to fish, but not toxic to other organisms.

“Trout are the preferred species to fish for at this lake, which is why we are working to restore Navajo Lake as a prize trout fishery,” Hepworth said. “With the low water levels this year due to drought, this treatment to reset the fishery would be much more cost-effective, so we thought it would be good timing.

The agency is holding an informational meeting Sept. 10 at 6 p.m. in the Duck Creek Village Fire Station.

“This project is not set in stone. We’re trying to get a lot of public feedback. Right now, most of it is positive,” Hepworth said. “But if we see some significant negativity toward the project, then we may have to step back and say, ‘OK, we’re going to have to put this off.’ We’re testing the waters.”

Unlike the Colorado River’s imperiled chub species, the Sevier’s are thriving. Anglers have long used baby Utah chub as bait, effectively planting them in reservoirs where they had never been before.

The problem with Utah chub is it reproduces quickly in lakes, becomes too big for many predatory fish to eat, and can survive a long time, upwards of 30 years. Some can reach 2 pounds in weight. Several Utah reservoirs are now overrun with chub, and none more than Navajo Lake, where chub now represent 90% of the fish biomass, according to DWR. Hepworth said that the proposed project would be the fourth—and hopefully last—time DWR tries to rid Navajo Lake of chub. The most recent rotenone application was in 1996.

“We’re going to go in there with the goal of eliminating them, knowing full well we probably won’t be able to do it,” Hepworth said. “I don’t know that I’m any better than my predecessors, but we’ll give it our best effort.”

DWR has successfully treated much larger lakes in the past, such as Panguitch and Strawberry, in 2005 and 1996, respectively, but past treatments on Navajo Lake have not stuck.

Rotenone is a naturally occurring chemical found in the roots of a South American legume. It is toxic to gill-breathing creatures at low concentrations, around 2 parts per million. Fisheries biologists regularly apply liquid rotenone in Western streams to rid them of rainbow and other nonnative trout with the goal of restoring native cutthroat trout populations. DWR conducts such treatments on tributaries to the Virgin River near Navajo Lake.

Applying rotenone in lakes is a much different operation, deploying a powdered form of the chemical with the consistency of flour. The material is applied from a boat tacking slowly back and forth across the lake, using an aspirator setup.

“We have these big pumps that pull water out of the lake, mix the powdered product and spray it back out,” Hepworth said. “A little bit of wind can move a lot of stuff around, so we’re paying very close attention to wave and wind action. When you put this powder out, it turns the water a light brown so you can actually see how it distributes.”

DWR prefers to control chub without chemicals. Introductions of sterile predatory sport fish, such as wipers and tiger muskie, have knocked down chub in Otter Creek, Newcastle and Minersville reservoirs, according to Hepworth, but such efforts haven’t worked at Navajo.

“We could get predators in there. They do really good that first summer,” Hepworth said. “They are eating the chubs but they don’t survive through the winter. So we start all over, every year with these 3- and 4-inch fish that never really reach a potential to eat chubs like they need to.”