Hamilton Harbour has a new public enemy No. 1.

It used to be the common carp - the bloated, invasive, bottom-feeder that has been churning up sediments, destroying plant growth and pushing out native fish species for nearly a century.

But more than two decades of efforts to keep the non-native fish from spawning grounds in Cootes Paradise and Grindstone Creek have led to a dramatic 75 per cent decline in numbers.

Yet as the carp declined, goldfish have increased in recent years. Descendants of fish discarded from aquariums or washed out of artificial ponds have multiplied like crazy, moving into areas of the harbour vacated by carp.

But here is the kicker: scientists are discovering that goldfish - which are actually a small member of the carp family - are even more destructive than the common carp and causing new problems.

•They not only cloud up waters like carp when they forage for food on the bottom, but they are more likely to eat fragile vegetation that harbour scientists are hoping to replenish.

•They are not as fussy about spawning areas as the carp and so are better able to reproduce.

• Unlike carp, sexually mature goldfish are small enough to pass through the grates of the Desjardins Canal carp barrier into Cootes Paradise and through grates of carp barriers in Grindstone Creek.

"Goldfish uproot plants like carp do," says Christine Boston, a harbour researcher from Fisheries and Oceans Canada. "But they also eat them as well."

Both fish are omnivores but carp much prefer insects, crustaceans and benthic worms and are less likely to chow down on plants.

Boston and other federal scientists have been tracking goldfish populations in Hamilton Harbour for decades. They found there was a small population in the late 1980s and 1990s that largely disappeared until 2006. Since then there have been steady increases, with numbers growing exponentially since 2016 and overtaking the carp population.

Scientists are not sure why that happened but believe the goldfish took advantage of habitat that opened up when the carp declined.

"We managed to get the carp out of the main spawning areas of the harbour and there are less sewage problems that have improved water quality," says Tys Theijsmeijer, the RBG's head of natural lands. "It set the stage for a contest to decide who can survive the remaining challenges of the waters best. And the goldfish is so far the winner."

It means goldfish are setting back efforts to restore Cootes Paradise by putting stress on plant growth.

"We can grow plants but unfortunately we now have someone who wants to eat them. That is an interesting challenge," says Theijsmeijer.

Boston has been outfitting goldfish with telemetry equipment to monitor their movements. Scientists especially want to keep track of where they are spawning beyond Cootes and Grindstone Creek.

But Theijsmeijer says the only really effective way to deal with the influx in goldfish is to improve the water quality of the harbour to a point that native species have the oxygenated and clear waters they desire. Once that happens, they will push out their non-native invaders.

Hamilton Harbour has chronic dissolved oxygen issues, mostly traceable to too many nutrients getting into the bay from sewage treatment plants and urban runoff. Dissolved oxygen - which comes from surrounding air and photosynthesis in plants - is necessary for fish to breathe, but the amount required varies depending on the species.

Non-native species such as carp and goldfish require less oxygen than native species such as walleye or bass. So as long as oxygen levels are low, non-native species will have the advantage.

Scheduled improvements to the Woodward Avenue Wastewater Treatment Plant should help the situation. The city is exploring options to upgrade the Dundas Wastewater Treatment Plant, which would also help water quality once the work is done.

Goldfish

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. The goldfish (Carassius auratus) is a freshwater fish native to Asia and a member of the carp family.

. Even though there are hundreds of different types of goldfish, all of them descend from the same carp species.

. Goldfish are omnivores.

. The carp was domesticated in China more than a thousand years ago for ornamental purposes.

. The average lifespan of a pet goldfish in an aquarium is five to 10 years. In the wild, they can live as long as 25 years and grow very large, sometimes reaching 40 cm in length or more.

. Goldfish should not be released into the wild. Not only is there a problem of them reproducing and pushing out native species of fish in waterways, but there are concerns about goldfish introducing diseases to native fish.

mmcneil@thespec.com

905-526-4687 | @Markatthespec

Goldfish

•The goldfish (Carassius auratus) is a freshwater fish native to Asia and a member of the carp family.

•Even though there are hundreds of different types of goldfish, all of them descend from the same carp species.

•Goldfish are omnivores.

•The carp was domesticated in China more than a thousand years ago for ornamental purposes.

•The average lifespan of a pet goldfish in an aquarium is five to 10 years. In the wild, they can live as long as 25 years and grow very large, sometimes reaching 40 cms in length or more.

•Goldfish should not be released into the wild. Not only is there a problem of them reproducing and pushing out native species of fish in waterways, but there are concerns about goldfish introducing diseases to native fish.



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