In the 1960's Parks Canada stocked a number of lakes in Banff National Park with sport fish.

Decades later those fish took over, wiping out the population of the native westslope cutthroat trout.

It's to the point where cutthroat only exist in about five per cent of the park.

Now parks staff is working on a project to try and boost the cutthroat population.

Hidden Lake, just north of the Lake Louise ski resort, is full of the unwanted sport fish and Parks Canada staff are using nets to catch eastern brook trout.

No one's sure how the brook trout made their way here, but now it's time for them to go. Parks Canada says the species turned out to be extremely dominant.

It may take a few years to catch all the brook trout, but it has to be done before the native westslope cutthroat can be reintroduced to the environment.

Rainbow trout and yellowstone cutthroat were also introduced in the park in the 1960's.  Those species interbred with the native fish creating a less vibrant hybrid.

"These hybrid fish don't make as many offspring," says Shelley Humphries of Parks Canada. "That opened up the door, we think, for the brook trout to come in and take over."

In addition to the nets, anglers are fishing for the brook trout. Downstream, on the upper corral creek, parks staff will be electrofishing up to a natural barrier.

"This is going to become basically a lake and four kilometres of stream that is going to be a refuge," says Humphries. "We hope that the westslope cutthroat will be safe forever up here."

Below the falls there are some brook trout but many genetically pure cutthroat. It's these fish that will be moved to re-populate hidden lake.

Parks Canada says while it only took a short time to introduce brook trout to this aquatic system, it will take years to remove the species.

Once all the brook trout have been removed, Parks Canada aquatic specialists will use fire fighting equipment to move the westslope cutthroat up to hidden lake. They'll use baskets filled with water hung under helicopters to make the trip quick and less stressful for the fish.

The brook trout that are culled from hidden lake and the upper corral creek will be frozen and donated to a "Birds of Prey" sanctuary that rehabilitates ospreys and hawks.