Gill lice, a malady often fatal to fish under certain conditions, popped up in the kokanee salmon population at Elevenmile Reservoir in 2006 and now appears to have infested at least two other Colorado impoundments.
Anglers recently reported heavily infested kokanee at Green Mountain and Gross Reservoir.
Littleton angler Chris Thomas reported that Saturday he and a friend landed 10 Green Mountain kokanee, keeping nine.
“All nine had lice. A few were very bad,” Thomas said.
The parasite, which ultimately stifles the flow of oxygen to the victim, has no impact on human health.
“It doesn’t make fish inedible,” said Greg Gerlich, state aquatic manager.
But it can make a fishery very sick, particularly those built on a foundation of kokanee salmon, the species that appears most susceptible to the current outbreak.
The Elevenmile situation, revealed by the Colorado Division of Wildlife when its kokanee egg-taking operation turned sour last fall, severely depleted the three oldest age classes. Anglers noted a dramatic decline in catch rate, followed by matching sag in egg collection. DOW personnel collected approximately 300,000 eggs, about one-third of the expected volume.
Biologists believe the Elevenmile outbreak has run its course and that the current 1-year-old age group will proceed to maturity. While the precise cause can’t be known, officials point to exceptionally warm water conditions during 2006 as the likely contributor.
“It’s rare that the parasite has this effect by itself. Almost always, it works in concert with other factors such as poor environmental conditions,” said Peter Walker, DOW’s senior fish pathologist.
Walker said he became aware that gill lice were present in Colorado when he arrived a quarter century ago.
“I was aware of infestation in Green Mountain Reservoir and a lake in the upper Rio Grande drainage.”
Gerlich also noted that gill lice were found in trout in the North Fork of the South Platte River.
The parasite seems to be latent in many waters around the state, including DOW’s Poudre Rearing Unit. Lice have not been detected in any of the three hatcheries that process kokanee.
Gerlich said his agency is eager to collect information from anglers who find lice on fish they catch.
“It’s not something we’re trying to hide. We’re not going to become aware of it until we find some high population impact, such as at Elevenmile.”
But, as Walker ruefully notes, “It’s usually impossible to get rid of it.”
Walker said no treatment for the disease has yet been tested and made legal, but certain highly experimental research is underway.
