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Fish in Larimer County lakes infested with parasitic worm

Colorado Parks and Wildlife warns the anchor worm can kill fish, but people can still eat the fish after removing the worms and cooking the fish.

LARIMER COUNTY, Colo — A parasitic worm is hitting the fish populations in two Larimer County lakes, causing fish to die. 

Colorado Parks and Wildlife (CPW) warns the parasitic infestation of the anchor worm is impacting fish in West and Dowdy lakes in Larimer County. 

The worm causes severe inflammation resulting in open, bleeding sores on fish. The infestation can cause stress and secondary infection. The worms also feed on the fish's gill tissue, impacting their breathing.

“We want anglers to be aware of this situation in these two lakes,” CPW area wildlife manager Jason Surface said. “These could be spread around if anglers are not aware of that potential, and they should consider decontaminating waders and other equipment when traveling from these waters to other locations. People can eat the fish after removing the worms and cooking the fish.”

Surface said the anchor worms are not a new problem in Larimer County lakes, and can become "problematic to the fish populations every several years."

CPW said people can still eat the affected fish after removing all the worms and cooking the fish.

Anglers should look for the green, red or white parasites attached to the body or fins of the fish and in the gills. CPW said they have been spotted on both trout and suckers.  

Credit: Colorado Parks and Wildlife
A fish with a heavy load of Lernaea sp. (anchor worm) attached to the body and fins.

> Video above shows planes helping Colorado Parks and Wildlife stock fish in alpine lakes.

The remaining fish stockings at West Lake are cancelled for the season as CPW works to find options to stop the spread of the anchor worms and help the fisheries recover.  

“Slow-moving water also favors infestations, and those places have minimal flow,” John Drennan, state fish pathologist for CPW, said. “If you get rid of the anchor worm, you don’t have any problem unless it gets introduced again."

The West Lake fishery consists of brook trout, brown trout, rainbow trout and suckers, while Dowdy Lake has the same species composition in addition to cutbows.