Lurking in the depths of a quiet St. Albert stormwater pond are schools of goldfish, threatening to out-compete native species if they spread.

Someone set free a pair of pet fish about four years ago. Now, the infested water glimmers gold with thousands of Asian goldfish.

For the past three years, the City of St. Albert has tried to eradicate the invasive species at Edgewater Pond on the city's northern edge.

In 2015, staff drained water so the pond would freeze completely over winter. The fish survived.

In 2016, they electro-shocked the water and then scooped the stunned fish out of the pond. Again, the fish survived.

This year, after goldfish were spotted at Ted Hole Pond downstream, the city decided to pump chemicals designed to kill fish into both ponds.

"The battle goes on," said Leah Kongsrude, St. Albert's director of environment. "I think of zombie movies when I think about [how] we froze the storm pond right to the bottom and they survived through that, when we tried to electro-fish 'em it didn't do anything.

"They're very resilient, very tough and our native fish species wouldn't have a chance if they got out there."

Kongsrude inspected Edgewater Pond on Tuesday morning as staff circled the shoreline in hazmat suits.

The province donated the chemicals, which target animals with gills. Goldfish are the only gilled species in the pond, Kongsrude said.

Before spraying the water, staff blocked all entries and exits to Edgewater Pond so the chemicals won't seep into nearby water supplies.

For the rest of the week, city staff will remove dead fish by skimming the pond with nets. More chemicals will be sprayed Thursday.

'They can grow into monster fish'

The city must eradicate the fish to prevent survivors from repopulating the pond, Kongsrude said.

Rogue fish can grow to be 12 inches long. Without natural predators, they can quickly dominate Canadian waterways.

"Because the species that's here now is extremely competitive with any native species, if it got into our Sturgeon River there's a risk that the native fish that are in there would not live," Kongsrude said.

"These fish would out-compete them and we would have lower biodiversity and our natural ecosystem would be permanently affected."

The province is urging pet owners not to dump dead or living fish into open water with its Don't Let it Loose campaign.

"The message is, don't let any of your fish or aquatic animals loose into the wild," Kongsrude said. 

"They may look cute in your fish bowl when you buy them but once you let them out into the natural world, they can grow into monster fish."

Goldfish

More than 1,000 goldfish have infested two stormwater ponds in St. Albert. (Zoe Todd/CBC)