John Brisco, a Quesnel city councillor and stand-up paddle boarder, first noticed the koi when paddling out on Dragon Lake.
"On a couple of occasions I saw schools with about 30 or 40 fish, about a pound," said Brisco. "We see lots of rainbows but we also see more goldfish than we've ever seen before."
Koi — which are sometimes mistaken for large goldfish — are an ornamental variety of carp commonly kept as pets. Brisco fears koi will choke out the trout population.
"I'm very concerned because it's a world-class rainbow fisheries and there are some very big rainbows in the lake," said Brisco.
Brisco brought his concerns to city council last week. He fears the lake will need to be stripped of all fish, and restocked from scratch, should the koi population continue to grow, and compete with trout.
"I would like us to see us do everything we can possibly do to mitigate and defer that from happening," said Brisco.
He considered a fishing derby, but that's not allowed because koi are not classified as game fish. Instead, Brisco wants the city to have an official policy of encouraging fishermen who accidentally catch koi, to avoid releasing them back into the lake.
Brisco also wants people to take better care of their pets.
"We really would like people to understand that when you're tired of your goldfish," said Brisco, "don't put them in a lake. Any lake."