The first known Canadian case of whirling disease in fish has been found in Johnson Lake in Banff National Park.
Parks Canada said access is being restricted to the lake just east of the Banff townsite to prevent spread of the disease, which is not harmful to humans or other mammals, but can have a significant impact on some fish populations.
Fishing and recreational activities have been banned.
Whirling disease affects trout and salmon, and can cause infected fish to swim in a whirling pattern and die prematurely.
The disease is caused by the microscopic parasite Myxobolus cerebralis, which can be transmitted to other water bodies through gear and equipment used for swimming, paddling, boating and fishing.
Parks Canada conservation manager Bill Hunt said the disease is extremely difficult to contain, as the parasites are microscopic.
"It's several times smaller than the diameter of a human hair. And so these spores can persist in the mud for up to several decades," Hunt told CBC News.
He said it's important people don't move fish between bodies of water, and that entrails are disposed of properly.
Other ways to prevent the spread of the disease is to clean, drain and dry boats and equipment, he said.
The Canadian Food Inspection Agency said this the first time it has confirmed the disease in Canada. The agency said it is working closely with Parks Canada and Alberta Environment and Parks (AEP) to ensure a co-ordinated response.
Fish from other bodies of water in Banff National Park are being tested and AEP is sampling fish downstream from the park.
Suspected cases should be reported to 1-855-336-BOAT.