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

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 Tillbury, 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. 

"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."

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.

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.

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.