Low Water, Too Many Lake Trout Threaten Flaming Gorge’s World-Class Kokanee Salmon Fishery

The Wyoming Game and Fish Department said there is hope for the kokanee salmon in Flaming Gorge but too many lake trout and low water levels are making it difficult for the prized fish to reach maturity.

John Thompson

June 27, 20236 min read

Wyoming Game and Fish says the kokanee salmon fishery at Flaming Gorge Reservoir is being threatened by low water levels and too many lake trout pups, which feed on kokanee.
Wyoming Game and Fish says the kokanee salmon fishery at Flaming Gorge Reservoir is being threatened by low water levels and too many lake trout pups, which feed on kokanee. (Getty Images)

Too many toothy predators and recent drawdowns that have lowered Flaming Gorge Reservoir's surface level are among the causes for the recent decline in the fishery, the Wyoming Game and Fish Department says.

Responding to Cowboy State Daily questions about the health of the fishery, Wyoming Game and Fish biologist John Walrath said in a written reply that a 2022 drawdown of the reservoir to provide water for users lower in the Colorado River drainage limited areas where kokanee salmon can spawn. This concentrated the salmon and made it easier for lake trout that eat kokanee to hunt them down.

Kokanee are a landlocked sockeye salmon prized for their rich, red flesh. Flaming Gorge has long been one of the top reservoirs in the intermountain region for anglers who pursue them.

However, kokanee also are the main menu item for another prized catch at Flaming Gorge — lake trout. And in recent years, the number of juvenile lake trout, called pups, has blown up. Pups are lake trout under 28 inches long and are believed to be the main predators of kokanee.

Wyoming Game and Fish research dating back to 1990 shows the number of pups is trending up while the number of lake trout 28 inches long and larger is trending down. Kokanee make up about 25% of the diet of small lake trout.

"If conservatively each small lake trout consumes one kokanee a month and there is a hypothetical population size of 150,000 small lake trout, then small lake trout would consume 1.8 million kokanee prior to the kokanee reaching a size that anglers can catch," according to Wyoming Game and Fish.

That makes Flaming Gorge a tough place to reach maturity for kokanee salmon. 

Reducing Lake Trout Numbers Can Preserve Fishery

Kokanee were first planted in Flaming Gorge in 1991. In about 2010, Wyoming, Utah and the Jones Hole National Fish Hatchery in Utah, set a goal to plant 1.65 million kokanee in Flaming Gorge annually. Most years they have reached that target, and some years have exceeded it. 

One of the questions posed to Wyoming Game and Fish relates to the long-term sustainability of planting kokanee in Flaming Gorge as a forage fish. 

The department's response was absolute.

"Yes," Walrath writes. "Stocking kokanee as both forage for trophy lake trout and as a highly sought after sport fish for anglers has worked for three decades and can continue to work as long as the number of small lake trout is reduced."

Catch And Keep The Lake Trout ‘Pups’ 

Flaming Gorge spans a state border and is managed by both Wyoming and the Utah Division of Wildlife Resources. Thinning the number of pups in the reservoir is a priority for both agencies.

The agencies have promoted the take of lake trout pups for nearly 20 years by teaching anglers how, when and where to catch pups, and by liberalizing the bag limit. An angler can keep 12 pups per day and have 24 in possession. Fully eliminating the limit on pups also is being considered. 

Suckers Are No Substitute 

Regarding other forage fish in the reservoir, Walrath writes there are white suckers and Utah chubs in the reservoir, but they are not as abundant as they were historically.

He said these two species are not used as food for lake trout regularly. When water temperatures warm in the spring, lake trout and kokanee head for the cooler, deeper water while suckers and chubs prefer warmer, shallower water.

Lake trout only eat chubs and suckers when their habitats overlap, according to the information provided. 

"This reduction in habitat overlap reduces their utility as prey," Walrath writes. "When the entire reservoir is cold, lake trout do opportunistically consume both species." 

Lake Trout Know How To Do Without 

Lake trout are somewhat picky eaters, have a natural resilience to starvation, and "are highly reliant on the availability of adult kokanee for forage," according to Wyoming Game and fish. They are also a long-lived species that don't cycle up and down the way kokanee sometimes do. 

Photographs of three large lake trout caught this spring out of Buckboard Marina were sent by Cowboy State Daily to Wyoming Game and Fish for evaluation. The fish were trophy size. One was measured at 44 inches long while another was 41 inches. But the fish lacked girth.

According to the Wyoming Game and Fish response, "When forage is limited over long periods of time, they (lake trout) will simply do without before occupying less desirable habitats where a different food source may be available." 

Building kokanee populations back and improving conditions for lake trout, gets back to the central theme, the number of small lake trout must be reduced. 

Lowering Reservoir Water Levels Wreaks Havoc On Kokanee

Lowering water levels in Flaming Gorge makes it harder for kokanee to survive.

Last August the Bureau of Reclamation dropped the level of Flaming Gorge by one million acre-feet to provide water for Lake Powell and Lake Mead. The drawdown continued through the fall and resulted in a 30-foot drop in the reservoir's surface level. 

When a reservoir is drawn down it leaves clean gravel exposed and forces kokanee to spawn in heavier sediments, Walrath writes. And increased sediments leads to smothered fish eggs. 

Kokanee lay eggs in Flaming Gorge in October and early November. Those eggs lay on the reservoir bottom until late April or early May when the fry (baby fish) emerge.

"When winter drawdowns occur, they can expose spawning areas and kill eggs before fry emerge, which can lead to decreased year-class strength," Wyoming Game and Fish said.

Further, the drawdown made it easier for lake trout to catch more kokanee because it reduced the reservoir's head space. 

"When Flaming Gorge Reservoir is full, there is an abundance of deep, cold water habitat for kokanee and lake trout. This allows for more spacial separation between kokanee and lake trout. As the reservoir was drawn down, over the last several years, the habitat available to predator and prey decreased. The increase in habitat overlap make the kokanee more susceptible to predation," according to the Wyoming Game and Fish.

On a brighter note with regard to Flaming Gorge’s fishery, Wyoming Game and Fish reports the number of burbot, an invasive predator fish illegally transported into Flaming Gorge, is declining. 

 "Burbot sampling conducted in fall 2022 was the lowest documented since sampling began in 2006," Walrath writes. "Crayfish numbers are rebounding as burbot numbers decline. The return of crayfish adds a critical forage component back to the fishery, especially for rainbow trout, cutthroat trout, smallmouth bass, and brown trout." 

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John Thompson

Features Reporter