Calgary·Video

Okotoks battles giant goldfish menace with pesticide rotenone

An Alberta town is beating back an invasive giant goldfish population with pesticides to protect nearby waterways from the potentially dangerous koi.

Officials ask locals not to flush or dump unwanted fish in public waterways

Why the giant goldfish in Okotoks pose a threat to the surrounding ecosystem

CBC News Calgary

5 years ago
0:38
The city will apply rotenone, a plant-based chemical that targets fish, to the Drake Landing and Crystal Ridge storm ponds later this summer. 0:38

An Alberta town is beating back an invasive giant goldfish population with pesticide to protect nearby waterways from the potentially dangerous koi. 

Hundreds of wild goldfish, which measure up to 25 centimetres long, have overrun two storm water ponds in Okotoks.

Officials worry they could spill over into the Sheep and Bow rivers and their tributaries, where they could out-eat and out-live native fish while spreading disease among plants. 

"They have no natural predators, and they can grow really large and get out into the natural ecosystem and cause all sorts of problems," said Bridget Couban, landscape inspector with Okotoks's parks department. 

The fish are prolific breeders, laying eggs up to three times a year. 

They're also extremely tolerant of the low-oxygen environments of storm ponds and can overwinter in the mud, Couban said.

Koi, also known as Russian carp, are 'very elusive' and adept at hiding from predators and people, says Couban. (John Lodder/Flickr)

The city first became aware of the problem in 2015 when residents phoned in to say children were fishing for goldfish in the Drake Landing and Crystal Ridge storm ponds. 

While the city has no way of knowing how the koi got there, they suspect people had either flushed their store-bought pets down the toilet or dumped them in the storm waterways.

"They probably had the best of intentions thinking it would be a great thing for their fish, but it's a terrible thing for the natural environment and surrounding ecosystems," Couban said.

In partnership with the provincial government, the city has arranged for the plant-based chemical rotenone, which specifically targets fish, to be applied to the ponds in late August and September. 

"We are going to treat them, because we know they're there, even though they're quite elusive."

The city is also asking residents not to dump unwanted pets in public waterways through the provincewide "Don't Let It Loose" education campaign. 

With files from Mario De Ciccio

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