Invasive weed continues to spread

Christine Peterson, Casper Star-Tribune via Wyoming News Exchange
Posted 8/23/21

Tom Durst noticed the weed years ago in a place called Walleye Bay in Pathfinder Reservoir. It was green and leafy and made it hard for him to fish for the walleye the bay was named after.

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Invasive weed continues to spread


CASPER — Tom Durst noticed the weed years ago in a place called Walleye Bay in Pathfinder Reservoir. It was green and leafy and made it hard for him to fish for the walleye the bay was named after. 

But it wasn’t until recently that he noticed curly-leaf pondweed farther north in Pathfinder, on a little strip of submerged sandbar he always loved to fish. And now he can’t. 

“It stops you. It stops you from fishing shorelines. You can’t jig,” said Durst, a longtime Casper angler and devoted walleye fisherman. 

Worse yet, the weed continues to spread. Curly-leaf pondweed is yet another invasive weed that’s made its way into Wyoming. 

On the hierarchy of invasive species, it’s not as worrisome to fisheries managers as, say, zebra or quagga mussels, but it is a concern, said Matt Hahn, Wyoming Game and Fish’s Casper regional fisheries supervisor. 

And that concern is growing as the weed continues to spread. 

What happens when it’s discovered downstream in Alcova Reservoir, the Grey Reef section of the river, and ultimately Glendo Reservoir, is hard to know for sure. But the weed is one more reminder of how quickly and easily a destructive species can enter a water and once there, never be removed. 

“The stuff we’re doing to prevent the introduction of zebra mussels into the state are also effective against other species,” Hahn said. “But it’s important people in the state, our local boaters, understand the risks as well if they go boat on Pathfinder and go to Alcova the next day. They’re not worried about moving zebra mussels, but they could be hastening the spread of curly pondweed.”

Curly-leaf pondweed is, as its name suggests, an aquatic weed. It’s native to Eurasia, Africa and Australia, and likely made its way to the U.S. with the common carp, when carp were intentionally planted as a game fish in the 1880s, according to Minnesota’s Department of Natural Resources. It then spread on boats and as part of the aquarium plant trade. 

Both curly-leaf pondweed and Eurasian watermilfoil were once popular aquarium plants, and ended up in waterways when people dumped the vegetation down toilets or in local waters. 

It’s been in Keyhole Reservoir for years, and was first discovered in the Miracle Mile section of the North Platte River above Pathfinder Reservoir about a decade ago, said Eric Hansen, an aquatic invasive species specialist for Game and Fish. 

While Eurasian watermilfoil has not yet been discovered in Wyoming – though it has been located in some of our neighboring states – curly-leaf pondweed is here. 

It spreads easily and quickly, as with most invasive species. Boats transport the weed on their props and tucked away in hard-to-reach spots. Even a piece of a leaf is enough to start a new colony of the weed, Hansen said. 

Worst yet, the reproductive part of the plant, called turions, are about half an inch long, resemble pointy pinecones, and can stick to any fishing gear and can live two or three years in sediment. 

Once established, it grows in mats so thick and dense that it clogs boat motors and even trapped kayakers in Keyhole Reservoir. 

The best way currently to treat it is by mechanically removing plants, which may knock it back but won’t eliminate it for good, Hansen said.

“It creates a pain in the neck for fishing,” he said. “It alters the habitat, and if fish are in the curly pondweed you can’t get at them.” 

Hahn is careful when he talks about where the invasive pondweed currently exists. 

It hasn’t been discovered in Alcova Reservoir, yet, he said, emphasizing “yet” because it seems inevitable that it will one day appear in the lake. 

It could be a bigger problem in Alcova than it’s been in Pathfinder, largely because Alcova Reservoir’s water levels don’t drop as significantly as Pathfinder’s. Pathfinder can vary from so full water tumbles over the spillway, to so shallow some yawning arms are nothing more than spits of water. 

Because curly-leaf pondweed grows in 15 feet of water or less, it will likely dry during low water years, keeping it at least partially at bay. 

The area near Alcova’s marinas is likely most at risk since it’s shallower than the rest of the lake, Hahn said. Once it reaches the river below Alcova, the impact is anyone’s guess. The Grey Reef section of the North Platte River — one of the most popular trout fisheries in the region — already has a number of native plants, particularly in late summer. Researchers don’t yet know if curly-leaf pondweed will outcompete the native plants, simply replacing them, or add to the natural abundance and make fishing even more difficult. 

The impact in Glendo Reservoir is similarly unknown. If the weed ends up in smaller reservoirs like Goldeneye outside Casper or the Plains Lakes near Laramie, the impact could be far worse for fish themselves. 

Smaller lakes and reservoirs carry a higher risk of winter kill, which is when ice forms on the water, snow covers the top and sunlight can’t get through. 

Without sun, plants die, and once aquatic plants die and begin to decompose they use oxygen and release carbon dioxide. That means plants which once helped supply fish with critical oxygen are instead using it up, leading to a mass of floating fish come spring. 

The weed was recently discovered in Flaming Gorge Reservoir, and biologists are monitoring the population. 

But the best way to keep it at bay is to prevent its spread in the first place, Hansen said. Make sure all boats are drained, clean and dry, even moving between local waters. 

Anglers wading to fish should also be careful to clean, dry and inspect their waders and boots. A wading boot may be unlikely to transport a 15-foot long weed, but it could harbor bits of leaf, seeds, snails or mussels. 

“It’s a battle we’re fighting to see when and where it pops up,” Hansen said. “We can slow it down with the public’s help.”