Officials hope new signs will help control London's feral goldfish problem

Conservation officials have put up new signs in environmentally-sensitive areas around the city, instructing people not to release their unwanted goldfish into the wild, in the hopes of staving off an invasion before it happens. 

London already has two notable infestations. One is in Saunders Pond, just south of Parkwood Hospital. The second is in the east pond in the Coves where the fish can reportedly grow as large as a two-litre pop bottle. 

"They are invasive species," said Brandon Williamson, a land management technician with the Upper Thames River Conservation Authority whose responsibilities include invasive species within city ponds. "If people dump them into the environment, what happens is they reproduce really quickly."

Like the goldfish itself, stories of the species' legendary reproductive powers have reproduced themselves across the country, showing how the little fish can cause big problems. From taking over storm ponds in Alberta to a swimming hole in New Brunswick, feral goldfish reproduce in such large numbers and grow to such monstrous sizes, they've closed a lake in BC and are now believed to be the dominant species in Hamilton Harbour. 

Your mundane-looking pet fish is a monster in the making

"They can live in low oxygen water. They can live in murky water that other fish can't live in," Williamson said. "They'll eat fish eggs, they'll eat small younglings for fish, they'll also eat the vegetation our native species would feed on."

They also stir up mud, so that when they reach large enough numbers, the goldfish create clouds of sediment so thick it disturbs the nesting habits of native species. 

"They actually reduce the population of our native species in many different ways."

While peering at a brightly-coloured fish swimming around its ceramic castle, most goldfish owners wouldn't know that these pet fish are potential killers.

Most people think releasing goldfish into the wild isn't harmful

The new signs were installed in places like the Sifton Bog and the Coves, Williamson said, to tell anyone thinking they're doing the right thing by releasing Goldie into the wild, that their actions have the potential to upset their local ecosystem. 

"They're not aware of the harm," he said. "It's an awareness piece to let people know that not all fish species are welcome."

"Hopefully they'll see the sign and that will trigger why they can't do this." 

While predators such as Northern Pike or Great Blue Herons are known to snack on goldfish, Williamson said that often the populations aren't large enough to keep up with the fecund fish, which are known to spawn like mad. 

"It takes quite a heavy population of our native species to actually control them."

Once established, goldfish are 'really difficult' to remove

Without predators, they become the dominant species very quickly. Williamson said it happened at Redmond's Pond a decade ago after someone introduced goldfish to the dark waters at the bog's centre. Too acidic for native species, the fish quickly multiplied and took over. 

"The goldfish just thrived in that location," he said. "It's really difficult to remove them."

Williamson said to get rid of a goldfish infestation, conservation officials have to corral them into one section of the pond, where they can be netted by hand. There's also the electroshock option, which involves stunning the fish with an electrical discharge into the water and is heavily regulated by environmental legislation. 

In certain cases, Williamson said, conservation officials can't do anything. 

"When the water is too deep it becomes an issue and there may not be much you can do, really."