Calgary hopes to freeze invasive fish species out of city ponds

A seemingly honourable goodbye to a pet is threatening Calgary waterways, as stormwater ponds and tributaries in the city are seeing more and more invasive species after people release their pet fish into them.

“The only way they get into these ponds is by people releasing them into the ponds,” said Corey Colbran, City of Calgary manager of wastewater and stormwater collection.

Quick to breed, without natural predators and even with some survival tricks up their scaled sleeves, Prussian carp and goldfish are the primary culprits Colbran and the city are trying to deal with.

One strategy the city is trying out is to drain a pair of stormwater ponds, one each in the Edgemont and Tuscany neighbourhoods. Colbran hopes low enough water levels will cause the invasive species to freeze.

He doesn’t know exactly how many of the foreign fish are in the two ponds, but “it’s enough for some of the little guys to plug the intakes of our pumps that are in there trying to keep the water levels down.”

Another option the city hasn’t pursued yet is the use of pesticides.

“We’re trying a couple of different strategies just to see how we can best eradicate these invasive species.”

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Both carp and goldfish will grow to as large as their environments allow.

One Ontario pond had a 10-pound goldfish pulled from it by Trout Unlimited Canada. And Colbran said Prussian carp can grow to the size of dinner plates.

The carp can be especially insidious as temperatures drop, burrowing into sediment at the bottom of the ponds.

Unfortunately, that means the city team won’t likely know how effective removing water for the winter months was until the spring.

But the burrowing brings another springtime problem, disrupting the next generation of fish indigenous in the area.

“That burrowing – as well as rooting around for food – destroys juvenile habitat, so rearing habitat for young fish,” Sylvia D’Amelio, fisheries biologist and CEO for Trout Unlimited Canada, told Global News.

“We do know clearly that invasive species can spread disease and infection — that’s been demonstrated in a lot of water bodies. We know that a lot of these species compete for resources,” she said, noting Prussian carp are a hardy species that breeds multiple times a year, in contrast to native species that only have annual spawning periods.

“We see this happening across the country where either animals are released into water bodies — that are released into stormwater management ponds — and then eventually work their way into natural water bodies and impact native fish and sport fisheries.”

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Calgary’s stormwater system – separate from the wastewater system – drains into the Bow River. And a period of heavy rain could flush the fish into the city’s largest river.

“I’ll mention brown trout – we do not want these species out competing our brown trout in the Bow River,” Colbran said.

D’Amelio commended the city’s pilot project to stop the spread of invasive species in Calgary.

“I think this is something that we should look at in all our watersheds and in all stormwater management ponds. But in addition to that, the idea that stormwater management ponds can at any point in time freely flow into water bodies is something we can also assess,” she said.

“I do believe if people knew the impact of the animals they were releasing that maybe we would see less animals being released and impacting our native waters.”

Colbran had some advice on the best ways to get rid of that pet guppy.

“If your pet fish dies, ideally we would want you to put it in the green bin for composting. Maybe in the garbage, but green bin would always be best,” he said.

“And certainly if you’ve got a live fish that you don’t want anymore, we’d be asking people to bring it back to the pet store or give it away to a friend who may want to fish, but not to be releasing it into the natural environment.”