Whirling disease in the Crowsnest River: an emerging threat to wild salmonids in Alberta

Abstract

We provide the first documented case of whirling disease (WD) impacts to wild, self-sustaining rainbow trout (RNTR, Oncorynchus mykiss) populations in Canada. Myxobolus cerebralis (Mc), the causative agent of WD, was first confirmed in Alberta in 2016. However, evidence of disease in local fish populations was unknown. Using a weight-of-evidence approach, we examined multiple parasite life cycle stages in the Crowsnest River, Alberta. Percentage of infected Tubifex tubifex worms actively shedding triactinomyxons (TAMs) exceeded known thresholds of Mc establishment and TAM densities instream exceeded thresholds known to cause ≥90% declines in RNTR populations. Mc was detected at 5 of 6 study sites in water, fish, and worms. Disease severity was highest in the lower watershed where 100% of sentinel fish tested positive for Mc 7 to 14 days post-exposure; up to 85% of wild fingerling RNTR showed clinical signs of disease and yearling trout were largely absent from the river suggesting reduced survival. Our findings indicate conditions necessary for outbreak of WD exist in Alberta, highlighting the need to consider this disease as an emerging threat to wild salmonid populations.