An angler spotted something weird floating in Lake Huron in July.
It turned out to be an enormous grass carp, one of the most invasive fish the Department of Fisheries and Oceans Canada has ever dealt with.
What's happening?
According to the Brampton Guardian, the carp measured 1,230 millimeters long, had a girth of 780 millimeters, and weighed 26 kilograms.
It was a sterile female, so it can't reproduce — but even one of these fish can cause serious problems for native species.
"This was one of the largest grass carp specimens DFO has processed," DFO spokesperson Sam Di Lorenzo said.
The DFO said the finding in Lake Huron is the third grass carp found in Canadian waters since 2013.
Commercial fishers near Sarnia caught the other two in 2017 and 2018. The good news was that both fish were sterile. Since 2012, the DFO's Invasive Carp Program has collected 34 grass carp from the Canadian side of the Great Lakes.
These fish are part of a group that includes black, bighead, and silver carp, which were introduced to the southern United States decades ago to control aquatic weeds. However, the grass carp's huge appetite later proved to be a problem.
Why are grass carp a problem?
The Invasive Species Centre noted that grass carp can eat 40% of their body weight in vegetation every day. They digest half and expel the rest, clouding water and encouraging algal blooms. Just 10 per hectare can wipe out half of a wetland's plant life.
That loss hurts people, too — fewer aquatic plants mean fewer nursery areas for fish, and murkier water for swimming and paddling.
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Grass carp have already started reproducing naturally in U.S. rivers feeding into Lake Erie. According to the Invasive Species Centre, if they spread further, 33 fish species and 18 bird species could be at risk.
Similar invasions elsewhere show the cost. In Australia, a Mary River cod was found choking on a tilapia. Snakehead fish overgrowth required dam control in Maryland. Meanwhile, sea lampreys have haunted Great Lakes fisheries for decades.
What's being done about grass carp?
DFO runs annual early detection surveys across over 30 high-risk Canadian waterways. The program also educates the public about how to spot and report sightings. Extra surveillance near Baie du Doré in Lake Huron is planned through 2025.
If you catch one, don't release it. Take a photo, note the location, and report it by calling 1‑800‑563‑7711, emailing [email protected], or submitting a tip on eddmaps.org. Releasing non-native fish is illegal under federal law.
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