Beneath the murky surface of Hamilton Harbour thousands of giant goldfish are teeming, tearing up vegetation and threatening native species.

But now, thanks to a first of its kind study, researchers have a new weapon in the battle to keep the harbour from becoming a giant goldfish bowl.

It's called acoustic telemetry. About a dozen of the fish were sedated and fitted with sound-emitting tags about the size of an AA battery, allowing scientists with Fisheries and Oceans Canada to track where they move.

"There's only one other study of goldfish in a fresh water system and that's in a river, so there are no other telemetry studies on goldfish in freshwater lakes," explained aquatic research biologist Christine Boston.

Warming waters could lead goldfish to other Great Lakes

Goldfish are considered an invasive species around the globe, but Boston says there's not a lot of information about how they're behaving in North America.

Workers sedate and tag a giant goldfish in the Hamilton harbour. (Fisheries and Oceans Canada)

And, with climate change warming water temperatures, there's a possibility the former pets that are taking over the harbour could expand their territory and become a much bigger threat.

They just get too big. There's nothing really that could eat them. - Christine Boston, aquatic researcher

"What we find might be helpful for the future and the possible expansion of the species into other areas of the Great Lakes," said Boston.

It might be hard to believe, but many of the bulky, pumpkin-coloured behemoths pulled from Cootes Paradise and other area wetlands most likely began their lives in fishbowls or backyard ponds.

"Lots of people have goldfish as pets and don't always get rid of them the right way," said Jennifer Bowman, an aquatic ecologist with the Royal Botanical Gardens (RBG) in Burlington. "That's how they got into these areas and they've been able to survive."

Millions of eggs and an explosion of goldfish

Not only survive, but thrive.

In 2018, staff at Hamilton's fishway —designed to keep carp out of the marsh— pulled out 1,690 goldfish.

Three years ago, that number was closer to 2,500 large goldfish, along with about two million young.

"They're one of the most dominant fish in the fish community," said Boston. "They're in the top 10 for most abundant that we have."

Goldfish can live in low-oxygen water and spawn multiple times in a season, two advantages native species down't have. (Royal Botanical Gardens)

That explosion of goldfish has only happened in the past decade or so.

The early 1990s saw just a few of the invaders, but in 2012 low water levels allowed researchers to pull out about 8,000 carp, allowing aquatic vegetation to grow like crazy, according to Bowman.

With the carp gone and plenty of plants to scatter their eggs in, millions of baby goldfish were born.

A mature female goldfish can lay up to 100,000 eggs, said Bowman, and they're capable of spawning multiple times in a season.

"When there's a million babies produced, even if not all of them survive that's still a lot of fishing growing up."

Too big to eat with scales like 'armour'

Add in the fact that goldfish like warm water and are tolerant of the low levels of oxygen that exist in the harbour and they've got a lot of advantages native species like bass, pike and perch do not.

Then there's the way they feed.

It's raising alarm because so much effort has been put into Hamilton Harbour. We're trying to restore the fish community … and to see this happening is really frustrating. - Christine Boston, aquatic researcher

"They're very destructive. They uproot sediment looking for food," explained Boston.

"There are all kinds of problems with water quality and contaminates so there's not a lot of food for a native fish and it's just another mouth to feed out there and one we don't want to feed."

Large goldfish not only wreak havoc on the habitat while eating, but their size also keeps them off the menu for many predators.

A mature member of the species averages 20-25 cm in length, said Boston. Big enough that there are easier fish in the lake to make a meal out of.

"They're so big and deep-bodied and their scales are like armour when they get like that," she explained. "They just get too big. There's nothing really that could eat them."

Many of the goldfish in Hamilton Harbour started life as pets or in backyard ponds. (Fisheries and Oceans Canada)

Goldfish aren't the only target of the fisheries Canada study. Teams have tagged 13 species and are closely following a subset of eight, including predators like walleye and pike, along with problem fish like goldfish and carp.

Unlike carp, whose name has become a buzzword in the battle against invasive species and are the subject of millions in preventative funding, goldfish have received little attention. But as their impact continues to grow they're climbing up the ladder of concern.

"If they increasing in number we're going to have to start doing something more substantial," said Bowman, pointing out studies of carp have shown they travel to the Hamilton area from places as far off as Toronto and Niagara, so different jurisdictions each have to do their part.

"It really shows you everyone is going to have to work together if we want to solve the problems because the fish are moving around."

A 'frustrating' fight

The fisheries teams are continuing to track species and hopeful their new tool will make a difference.

Multiple organizations, including the Ministry of Natural Resources, RBG are working to combat invasive species too, but Boston said watching the rise of goldfish has made it clear how tough the fight will be.

"It's raising alarm because so much effort has been put into Hamilton Harbour. We're trying to restore the fish community … and to see this happening is really frustrating."