Threatened Trout Spawn Naturally in Banff’s Hidden Lake for First Time

After more than a decade of work to restore a threatened native fish to a remote lake in Banff National Park, Parks Canada has recorded an important first.

Reintroduced westslope cutthroat trout have been observed spawning naturally in Hidden Lake for the first time since restoration efforts began, Parks Canada announced this week.

The alpine lake sits in the Skoki area behind the Lake Louise Ski Resort, on the north side of Richardson Ridge. It once supported a thriving population of westslope cutthroat trout, a native species whose Alberta population is now listed as threatened under the federal Species at Risk Act.

The trout observed spawning this year originated from eggs collected and incubated by Parks Canada in 2021 and 2022 before being reintroduced to Hidden Lake and its tributaries.

Natural spawning is an important milestone because it shows the reintroduced fish are able to reproduce in the restored habitat. Parks Canada says it demonstrates that Hidden Lake once again has the conditions needed for westslope cutthroat trout to complete their life cycle and represents an important step toward maintaining a self-sustaining population.

Getting to this point has taken years.

The problem dates back to the early 1900s, when mountain lakes in Banff National Park were stocked with non-native fish to create recreational fishing opportunities. Species including brook trout and rainbow trout became established, displacing native fish and, in some cases, creating the potential for hybridization with westslope cutthroat trout. Parks Canada no longer stocks lakes with non-native species, but says those historical introductions continue to affect aquatic ecosystems.

At Hidden Lake, the restoration effort began with removing non-native brook trout.

Between 2011 and 2017, aquatic staff used angling, gill netting and electrofishing in an attempt to eliminate the brook trout population. Those methods were unsuccessful.

Parks Canada changed approaches in 2018 and 2019, treating Hidden Lake with rotenone, a fish toxicant derived from plants that is used in fisheries restoration. The lake was treated once in each of those years.

Monitoring afterward, including environmental DNA sampling and physical surveys, indicated the non-native fish had been removed while insect and zooplankton populations remained healthy enough to support fish.

That cleared the way for westslope cutthroat trout to return.

In 2021, Parks Canada collected eggs and milt from a genetically pure population in Big Fish Lake in Banff National Park. The fertilized eggs were partially incubated before being placed in streamside incubators, allowing the young fish to complete their development in the water where they would live. That year, 311 fry were released into Hidden Creek, marking the first westslope cutthroat trout there in about 50 years.

The reintroduction continued as part of Parks Canada's broader effort to restore genetically pure populations of the species within their historic range.

By 2025, Parks Canada reported that population estimates at Hidden Lake indicated the restoration project was successfully complete. This year's observation adds another measure of success: the reintroduced fish are now spawning naturally.

Westslope cutthroat trout are particularly important to Parks Canada's restoration work because they are considered an indicator species. They depend on cold, clean water, meaning a healthy population can also provide information about the condition of the surrounding aquatic ecosystem.

Parks Canada says it will continue monitoring the Hidden Lake population to evaluate its long-term health and reproductive success.

For a restoration project that began with removing an introduced species more than a decade ago, the latest development marks a significant transition: westslope cutthroat trout are once again reproducing naturally in Hidden Lake.