LAKE HURON—The waters surrounding Manitoulin Island are sounding urgent alarms. Lake Huron faces a relentless squeeze from invasive species, rising temperatures and the unraveling of ecological balance that has supported these waters for millennia. Among the newest—and perhaps most troubling—signs came on July 14, when a recreational angler reported a dying grass carp floating in Baie du Doré, just a short drive north of Tiverton in Bruce County, close to the Bruce nuclear generating plant.
The Department of Fisheries and Oceans Canada (DFO) swiftly collected the fish, confirming it was a triploid (sterile) female grass carp—the third such carp found in Canadian waters of Lake Huron since surveillance began in 2013. DFO spokesperson Sam Di Lorenzo described it as one of the largest grass carp specimens processed by the agency, measuring 1,230 millimeters long, 780 millimeters in girth and weighing 26 kilograms.
Since 2012, the DFO’s Invasive Carp Program has processed 34 grass carp caught in Canadian Great Lakes waters. The two previous specimens in Lake Huron were caught near Sarnia in 2017 and 2018—both triploid and sterile as well.
Grass carp belong to a family of four invasive Asian carp species threatening the Great Lakes—the others being bighead, black and silver carp. Introduced to the southern United States in the late 1960s and 1970s as biological controls in aquaculture, these fish have since expanded aggressively.
Among them, grass carp are considered the most imminent threat to the Great Lakes. Researchers have found evidence of natural reproduction in US tributaries to Lake Erie, including the Sandusky and Maumee rivers, though no established populations have yet been confirmed in Ontario’s Great Lakes. The threat, however, remains real and intensifying.
Grass carp feast primarily on aquatic vegetation—up to 40 percent of their body weight daily. They digest about half and expel the rest, which pollutes the water and promotes algal blooms. Just 10 adult grass carp per hectare can reduce wetland vegetation by half, a devastating blow to habitats critical for fish spawning and birds that rely on aquatic plants. If these carp establish in Lake Huron, they could imperil 33 fish species and 18 bird species, profoundly disrupting the ecosystem and the livelihoods tied to it.
This latest discovery arrives amid already dire conditions. Alewife populations have surged unchecked since the sea lamprey—a parasitic invasive—devastated rainbow trout, one of alewife’s few natural predators. The result: an overabundance of alewife upsetting Lake Huron’s food web.
Not far to the south, Lake Erie, the Great Lakes’ shallowest, has long battled algae blooms and warming waters worsened by industrial runoff. The winter of 2023–24 was nearly ice-free, robbing aquatic species of the cooling rest they rely on. This spring, mass die-offs of temperature-sensitive fish were grimly visible: alewife washed up along Lake Erie’s shores; gizzard shad, vital forage fish, littered beaches on Lake Huron, including those beloved by Manitoulin Island’s communities.
By early July, some Lake Erie waters warmed to 30 degrees Celsius—bathwater heat that stokes algae blooms and suffocates fish.
Lake Huron’s fragile ecosystem now faces the double threat of these warming waters and invasive species pushing native populations closer to collapse. The confirmed presence of grass carp DNA and the recent large specimen in Baie du Doré marks a critical turning point.
DFO’s Invasive Carp Program is monitoring more than 30 high-risk tributaries and wetlands along Canada’s Great Lakes coastlines, including several rivers flowing into Lake Huron. Their surveillance, informed by scientific habitat modeling, seeks early detection and rapid response to invasive carp threats.
Beyond monitoring, outreach efforts educate the public on invasive species impacts and identification, urging anyone who catches a grass carp to take photos, note the location, and report immediately to DFO or the Invading Species Hotline (1-800-563-7711). If unable to report right away, anglers are advised not to release the fish back into the water.
For Manitoulin Island, these developments are far more than abstract ecological facts. The lake is life—sustaining fish, wildlife, plants and people whose cultures and economies are inseparable from these waters.
If the march of warming waters, unchecked invasive species and habitat loss continues, Lake Huron risks becoming a shadow of itself—murky, unbalanced and dominated by invaders.
The call is clear: vigilance, science-driven policy, community action. Because the future of Manitoulin Island depends on preserving the lake’s fragile balance before it slips beyond repair.
