Koi caught at Boundary Dam reservoir

Boundary Dam is stocked with large-mouthed bass and other fish, but not koi like this brute. Officials think pet owners are slipping them in when they get too big for the backyard pond. (Courtesy Landon Polk)

Some mighty big fish are being landed at the Boundary Dam Reservoir near Estevan, Sask. and people are expressing surprise.

They're koi fish, a species that is neither native to Saskatchewan, nor among the species that the reservoir is stocked with.

Recently, Landon Polk caught himself a 30-pound (13.6-kilogram) koi and then, a little later, one that weighed 20 pounds.

"I thought it was pretty weird," said Polk, who had been looking for a large-mouthed bass before shooting the koi with a bow and arrow. "People's goldfish swimming around here?"

Conservation officers told him they suspect the fish had been a pet that got too big for a backyard pond before being placed into the reservoir.

Jennifer Merkowsky, a fisheries biologist with the Environment Ministry, said there's a warm-water outflow from the dam that's likely kept the big fish alive over the winter.

Koi are bottom-feeders and can stir up sediment, something that's not good for other fish in the reservoir, she said.

The government is telling people not to put invasive species like koi into provincial waterways, she said.