Parasite behind fish-killing whirling disease found in B.C. lake

The B.C. government says it has detected more cases of a parasite that Parks Canada describes as potentially devastating to young trout and salmon.

The provincial Ministry of Water, Land and Resource Stewardship says three trout samples from the southern arm of Kootenay Lake have tested positive for Myxobolus cerebralis, the parasite that causes whirling disease.

The first case of the parasite in B.C. waters was found in September 2023 in Yoho National Park's Emerald Lake, which prompted the closure of nearby waterways as officials looked to stop the spread.

The province says it is not sure if the new cases in Kootenay Lake are linked to Yoho National Park, and biologists are testing more samples from this year in the ongoing investigation.

Kootenay Lake is one of the largest freshwater fisheries in the province and supports Gerrard rainbow trout and kokanee fisheries, which draw anglers to B.C. from across the continent, according to the province.

The microscopic parasite that causes the disease preys on young fish, causing them to swim in a whirling pattern, and often eventually killing them.

It has the potential to wipe out entire fish populations, causing lengthy closures that could devastate economies that rely on fishing and fish tourism.

In May, the Canadian Food Inspection Agency declared the Columbia River watershed as an infected area for whirling disease after first detecting Myxobolus cerebralis there in December 2023, imposing restrictions on movements of fish — dead or alive — to and from other waterways.

Parks Canada said in September that whirling disease could "decimate" as much as 90 per cent of young trout and salmon. It has closed all water bodies to watercraft and fishing in both the Yoho and Kootenay national parks until at least March 31, 2025. 

Whirling disease does not pose risks to humans, but officials say it may have spread to B.C. from Alberta through human contact.

There are no treatments for the disease.

The province says users of B.C. waterways should adopt the "clean, drain, dry" approach for all boats and equipment in contact with river and lake water to prevent spread, as well as refrain from transporting fish from one body of water to another.