BANFF, ALTA. -- For decades beginning in the 1920s Parks Canada stocked water bodies in Banff National Park with various species of fish including eastern brook trout, lake trout, rainbow trout and Yellowstone cutthroat trout.

"We introduced a lot of fish here into lakes that were historically fish-less and then into lakes that actually had our native fish and we put non-native fish right on top of them," said Shelley Humphries, aquatic specialist with Banff, Yoho and Kootenay National Parks.

Native species like the cold water in the mountain parks and include bull trout, westslope cutthroat trout and Rocky Mountain whitefish.

Protection, conservation and restoration of species listed under the Species at Risk Act (SARA) is mandated in all Parks Canada management plans and species at-risk action plans.

Removing competition from non-native fish is the first step toward restoration and recovery of SARA-listed westslope cutthroat trout populations in Banff National Park.

Since 2000, Parks Canada has used gill nets, electro-fishing and angling to rid water bodies of non-native fish. But it's a long process.

"At Hidden Lake, we worked on it for six years and realized it wasn’t working," said Humphries. "After we changed our methods and we switched to the chemical method it took us one day to accomplish what we weren’t able to get done in six years."

That chemical method uses a fish toxicant called retenone. It's a naturally occurring compound that is derived from the roots of a tropical plant of the bean family and has been used by Indigenous people in South America to capture food fish for centuries.

"The rotenone is inside the water, it crosses over their gills, it enters into the blood and it goes into the mitochondria of the cells inside the fish and it interferes with their respiration," said Humphries.

In 2019, Parks Canada announced just over $2 million for Humphries and her team to work on five locations in Banff National Park to increase habitat for west slope cutthroat. This August, the team started at Helen Lake and successfully removed all the brook trout from it.

"What we’re doing right now is trying to go for really strategic locations where we can make a big difference for cutthroat or remove an active threat," said Humphries. "So next year we’re going to do Catherine Lake. It has yellowstone cutthroat trout (and) it’s a DNA problem for us."

Humphries says it’s unlikely all the non-native species will be removed from water bodies from the national park but she hopes, in time, the westslope cutthroat population will thrive once more here.