This voracious U.S. catfish species is now in Ontario, possibly due to climate change

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Scientists have found evidence the flathead catfish — a species from the southern U.S. that’s known for its huge size and monstrous appetite for fish — has established itself in the Thames River in southwestern Ontario.

Over a six-year period, researchers netted 11 catfish from the lower Thames River near Tilbury, a 2019 research paper said. It also noted “the capture of two juveniles (total lengths 78 mm and 82 mm), the first records of juveniles in Canada, is a strong indication that reproduction has occurred.” 

The species has since been spotted by an angler in London, Ont. Last October, the angler posted the image of the olive green flathead catfish she caught, using a worm to lure it, to iNaturalist, a crowd-sourced biology website used by amateur naturalists and scientists alike.

The flathead catfish is no stranger to the Great Lakes basin, but, until recently, had stayed on the American side off Lake Erie for about a century. The recent discovery of flathead catfish in Canada is a sign they’re flourishing in such large numbers south of the border that they’ve started to move north to avoid overpopulation, researchers say. 

Catfish known for their size, big appetite

“I think that very likely climate change is part of it,” said Nicholas Mandrak, a University of Toronto professor of biology who studies invasive fish species. 

“As our waters warm, they become a more appropriate temperature for these southern catfishes to survive in Canada.”

WATCH | Toronto biology prof on issues with U.S. flathead catfish ending up in Ontario:

‘Elsewhere in North America they’ve become highly invasive’

University of Toronto biology professor Nicholas Mandrak explains the potential dangers posed by the flathead catfish to Ontario’s Thames River, a species native to the southern United States.

Mandrak said flathead catfish can be a problem because they’re big — some specimens have weighed as much as 110 pounds — and have a monstrous appetite for meat.

In the case of the Thames River — which begins near Woodstock and meanders through rolling southwestern Ontario farm country, through London and Chatham, before it empties into Lake St. Clair near Tilbury — that could spell trouble for the 25 at-risk fish and mussel species already in those waters.

The river is vulnerable because of pollution from urban and rural areas, and introducing a species as voracious as the flathead catfish could further weaken the waterway’s ecology. 

Flathead catfish “get very large and they eat a lot of fishes, so they’re likely to have a substantial negative impact if we do not do anything to control their populations,” Mandrak said. 

What conservation authorities are doing, if anything, isn’t clear.

CBC News contacted both the Upper Thames River Conservation Authority and Lower Thames River Conservation Authority. Both agencies said staff biologists were unavailable for an interview Thursday and Friday. But this story will be updated if comment is provided.

Flatheads have spread rapidly since 1940s

South of the border, the fish have spread rapidly almost everywhere they’ve been introduced since the early 1940s, according to the Smithsonian Environmental Research Center.

Two men lift a dead catfish to weighed in a laboratory
Nicholas Mandrak (left) and another researcher weigh a flathead catfish, one of 11 caught in the lower Thames River and dissected at a University of Toronto lab. Mandrak said one flathead had 15 gizzard shad in its stomach, each about 30cm long. (Edina Illyes/University of Toronto)

From its original home in the Gulf of Mexico basin, the species has spread throughout the U.S. South. It has been spotted in New England, the Potomac River, on the West Coast into Oregon and Washington and, until recently, had spread as far north as the southern coast of Lake Erie in Ohio. 

In certain states, such as North Carolina, there is evidence the species has had an impact on other species, such as sunfish and bullhead catfish, creating an imbalance in local ecosystems. 

Mandrak said his research team found a startling number of fish in the bellies of the 11 catfish from the Thames River after they were dissected. 

“Any one fish is going to be eating kilograms and kilograms of fish every day,” he said, noting the team found up to 15 gizzard shad about 30 centimetres in length in a single stomach. 

“We can tell by dissecting the fishes they are going to have a substantial impact.” 

The popularity of catfish noodling

In the U.S., flathead catfish have become a favourite of anglers because of their size and aggression. Anglers also say they have tasty flesh. 

There’s also the offbeat cultural phenomenon called catfish noodling, where “noodlers” reach into underwater holes and, using their bare hands or sometimes feet, wrestle out catfish weighing up to 100 pounds each.

In many U.S. states, notably in the south, thousands of people take part in this sport of sorts, which is controversial among the freshwater fishing community

Still, there’s also no shortage of noodlers posting their adventures on YouTube.

While some may find it entertaining to watch someone trying to grab a giant fish, Mandrak stressed the species shouldn’t go unchecked, adding it’s “not an excuse to let them establish here.”

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