Parasitic Infestation in West and Dowdy Lakes Impacting Fish Populations

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

Colorado Parks and Wildlife officials at its aquatic animal health laboratory have identified an anchor worm infestation as the cause of dead and dying fish at West and Dowdy lakes in western Larimer County.

Anchor worm is a parasite that can cause severe inflammation resulting in open bleeding sores on fish. Adult females are permanently fixed to a fish, resulting in the observed sores. It can lead to stress, osmotic imbalance, and to secondary infection. The free-swimming immature stages of the anchor worm can feed on gill tissue, causing considerable damage that can impact respiration and osmoregulation, in addition to being an irritant.

Anchor worms are not new to both West Lake and neighboring Dowdy Lake. They can become problematic to the fish populations every several years according to CPW Area Wildlife Manager Jason Surface.

“We want anglers to be aware of this situation in these two lakes,” 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.”

Anglers should look for the parasites green, red, or white in color attached to the body or fins of the fish and in the gills. CPW officials noticed them on both trout and suckers. All remaining stockings at West Lake have been canceled for the season.

“It is reasonable to conclude such an infestation of anchor worm alone is capable of causing the observed morbidity and mortality,” said John Drennan, State Fish Pathologist for CPW.

CPW is working through its process to determine the best options to halt any spread of the anchor worms and how to best recover the fisheries.

Crowded conditions favor the transmission of the parasite, and those two lakes have high densities of fish. The fishery at West lake consists of brook trout, brown trout, rainbow trout, and suckers while Dowdy Lake has the same species composition in addition to cutbows.

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

Dowdy and West Lake are stocked annually with both catchable (~10 inches) and sub-catchable (5-7inch fish). The lakes, sitting above 8,000 feet in Red Feather Lakes, receive high angling pressure year-round.

Dowdy Lake State Wildlife Area has a campground and waterfowl hunting is also permitted in-season. The West Lake State Wildlife Area also has a campground, but recreational activities are limited to angling and camping. Boating is prohibited at West Lake SWA except for float tubes or craft propelled by hand, wind, or electric motor.



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