‘Mindblowing’ population of feral goldfish in Ottawa pond to be euthanized

The urban center of capital of canada is preparing to euthanize a school day of invasive carassius auratus living in a stormwater pool in the Central Park neighbourhood, though the size of the population remains in question.

In March, city staff removed about 5,000 dead goldfish "using nets in the water and by hand" from the pond in Celebration Park, according to a written statement.

The city now says it has found a new population of goldfish in the same pond. Based on "a rough visual assessment," the surviving school is believed to number somewhere between 500 and 1,000, the city said.

"The fact that we’ve had approximately 6,000 fish in this pond, in this year, is mind-blowing,” said Coun. Riley Brockington, who represents the area.

"It’s just a number that’s difficult for me to wrap my head around."

Over 1,000 goldfish live in this Ottawa pond. The city says that's a problem

But even that number may underestimate the scale of the invasion, according to Carleton biology professor Steven Cooke.

"These are the kinds of fish that are extremely fecund — fancy word for they make lots of babies," he said.

"The average female goldfish can generate 6,000 or so offspring a year, and so all you need is one boy, one girl, and you got an awful lot of offspring."

Cooke estimates the actual population could far exceed the city’s estimate and may already number in the millions, but he added most of the fish would have been spawned this year and would still be "tiny."

Native to eastern Asia, goldfish are considered an invasive species with established populations throughout Canada and the mainland United States. Hamilton Harbour, for example, is overrun with a particularly acute infestation.

In stormwater ponds, the fish encounter few predators. Left unchecked, they tend to proliferate, displacing native species and limiting aquatic plant growth by stirring up muck from the pond bed.

"They wind up smothering, basically dominating the waters in which they’re introduced," Cooke said. "They can be really quite devastating in these small systems."

It's not known how the goldfish ended up in the stormwater pond.

In a written statement, the city said it is required by the Ontario Ministry of Natural Resources and Forestry to euthanize all invasive fish captured in stormwater ponds.

"When staff remove these goldfish, they will continue to use nets to collect the fish from the pond," the statement reads. "The City will not drain the pond or use any chemical treatments as part of this work."

Cooke said netting could help decrease the population size, but he warns it’s not an effective method if the goal is to eradicate the population entirely.

"All you need to do is miss one male, one female, and then you’ve got a pair that are ready to start cranking out babies," he said.

The most effective option for a closed water system, Cooke said, would be to use a fish-specific pesticide to wipe out all fish living there. But authorities often lack the "social licence" required to take that approach, he added.

"The most important message from this is, let’s use this as a lesson and stop this from happening," he said, urging people not to release their pet goldfish into the wild.

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