Whirling disease parasite spreads to Athabasca watershed

A microscopic parasite that devastates young fish populations has been confirmed in the McLeod River, triggering a federal order to designate the entire Athabasca watershed in central Alberta as a contamination zone.

The Canadian Food Inspection Agency declared the Upper Athabasca and McLeod sub-watersheds as official infection areas for whirling disease, which attacks the cartilage and nervous system of juvenile fish before their skeleton hardens. The order means strict regulatory controls now apply to moving fish and sediment across the 159,000-square-kilometre area — roughly 24 per cent of Alberta's landmass.

The designation is significant because the Athabasca sits adjacent to the Bow, Oldman, Red Deer, and North Saskatchewan River watersheds, all already declared infected since whirling disease was first detected in Alberta in 2016.

Affected fish often swim in tail-chasing "whirling" motions, making them unable to feed and leaving them vulnerable to predators. In Colorado during the 1990s, the parasite wiped out 90 per cent of trout populations. University of Alberta professor Patrick Hanington warned that Alberta is now entering the window where population-level impacts might begin to manifest — roughly 10 years after the disease's first detection.

The discovery is particularly concerning for rainbow trout listed as endangered both federally and provincially. Parks Canada has already restricted recreational watercraft in several key lakes across Banff, Yoho and Kootenay national parks to prevent spread.