Parks Canada will spend months trying to kill all the fish in Johnson Lake to keep a deadly parasite from spreading, specifically to Lake Minnewanka and Two Jack Lake.
All through May and June and again in the fall, Parks staff will be applying a high-voltage electric current to the lake. That stuns the fish so they can be scooped up in gill nets. Those that are still alive will be euthanized and incinerated or buried in a dry landfill.
The severe measure is the best option to prevent whirling disease, which causes spinal deformities and premature death in fish, from spreading to crucial westslope trout habitat, said Bill Hunt, manager of resource conservation with Banff National Park.
"We only have 10 core populations for westslope trout. What that means is they are genetically pure and in their natural water bodies. It's a very special situation and we have a responsibility to make sure these populations are protected."
The first-ever case of the contagious disease in Canada, named after the circular swimming patterns of infected fish, was confirmed in Johnson Lake in mid-August 2016. Access was restricted, fishing was banned and in late fall, Parks Canada officials began to look for ways to deal with the problem.
"We're not hoping to eradicate whirling disease. It's in the Bow River watershed and it's prevalent in the province. It's just not feasible," said Hunt.
But as Johnson Lake is part of the popular Lake Minnewanka loop, there is a concern anglers and boaters are moving between the lakes, carrying the infection with them.
"By removing the fish, we remove the risk."
Whirling disease has so far been confirmed in more than 30 locations by the Canadian Food Inspection Agency, which monitors diseases that are important to aquatic health or the Canadian economy.
Three locations in the Crowsnest River has been confirmed this year for the disease and a number of locations were confirmed in 2016, including the Bow River watershed, Oldman River basin, sections of the Elbow River, Spray River, Healy and Lott creeks.
The CFIA notes, however, that additional detections from ongoing sampling and testing don't mean the disease is spreading. It may have been present for several years and is just now being confirmed.
The parasite that causes whirling disease, Myxobolus cerebralis, is spread by contact between finfish and a freshwater worm. It can also be spread by birds or by people who transport infected fish or worms or contaminated equipment from one body of water to another.
The disease affects six species of fish found in Alberta: cutthroat trout, rainbow trout, mountain whitefish, brown trout, bull trout and brook trout.