Understanding the Threat of Grass Carp: What You Should Know

Attention Canadian anglers and watercraft enthusiasts! Although there are no reproducing populations of Grass Carp in the Canadian waters of the Great Lakes, there have been individual captures, making them the most imminent threat. If you have an appreciation for the Great Lakes and the native fishes that call these waters home, be sure to keep reading to learn more about the threat of Grass Carp!

What are Grass Carp?

Bighead Carp, Black Carp, Silver Carp, and Grass Carp are four invasive species of carp currently threatening to invade the Canadian waters of the Great Lakes. These four species belong to the Cyprinidae family, meaning they are closely related to various species of carps and minnows. These fishes are all native to the rivers, reservoirs and lakes of China and southern Russia. They were first introduced into the Southern U.S. in the 1970s for use as biological control in aquaculture facilities based on their specific diets. Flooding events allowed the fish to escape these facilities and move beyond contained environments into open freshwater systems, such as the Mississippi River Basin. The spread of Grass Carp in the United States has also been a result from stocking them in ponds for aquatic vegetation control. Among the four species, Grass Carp is the greatest danger to the health of the Great Lakes.

Grass Carp have oblong shaped bodies, a flattened head, and moderately small eyes centred on the side of the head. Their scales are large and overlapping, with coloration ranging from blackish to olive-brown with brassy or silvery-white on the sides and belly and outlined by darker-coloured pigments on the back and sides. This gives them a cross-hatched/ checkered appearance. It can take up to 2 years for Grass Carp to reach sexual maturity, but sometimes over a decade to grow to their maximum size. Like other species in the Cyprinidae family, Grass Carp can grow quite large, reaching weights surpassing 80 pounds and lengths of 1.5 metres!

Ecological Impacts

In their invasive range, Grass Carp are particularly problematic due to their specific diets and aggressive eating behaviour. Grass Carp eat primarily aquatic plants, with a single individual capable of consuming up to 40% of their body weight in food daily. This could potentially range from 20-30 lbs. a day for a fully-grown Grass Carp! Due to this species' tendency to overeat, Grass Carp are frequently incapable of fully digesting all the plant material they consume. As a result, Grass Carp must expel undigested plant material back into the water, creating water pollution or murkiness and excess nutrients in the water, promoting the growth of toxic algal blooms.

Grass Carp can alter habitats, compete with native fishes for food and resources, and act as carriers for diseases or parasites that could spread to native fishes. Grass Carp grow very large very quickly, and it is unlikely that their populations would be controlled by native predators in the Great Lakes basin if they were to establish, as they would outgrow the gape (mouth) size of native predatory species. The eating habits of Grass Carp can disturb lake and river bottoms, leave native small/ juvenile fishes without sufficient protection from predators, and reduce possible spawning habitats for species that prefer to nest in aquatic vegetation. If Grass Carp ever became established in Canadian waters, high impacts are predicted for over 30 native freshwater fish species, including Walleye, Muskie, Northern Pike, Yellow Perch, Smallmouth Bass, and more!

Economic Impacts

Apart from ecological impacts, economic impacts are also predicted upon the arrival of Grass Carp in Canadian waters. The Great Lakes basin is an incredibly valuable resource to the Canadian economy through several ventures, including recreational fishing. The Canadian recreational fishing industry is worth roughly $556 million USD, and Grass Carp would impact the industry by causing a reduction in populations of favoured angling species. Anglers contribute a large amount of money to the economy via fishing licenses, purchasing fishing equipment, gear, and tourism. Reduced recreational fishing due to Grass Carp would affect other small businesses and livelihoods that depend on the growth of this sector, including bait and tackle shops

Are Grass Carp Already Established in Canada?

No. Despite a few rare captures of individual Grass Carp that have taken place within the Great Lakes in recent years, there are currently no established populations of Grass Carp in Canadian waters or evidence of Grass Carp reproduction/spawning. We want to keep it that way! Nevertheless, if proper prevention does not take place, it is predicted that Grass Carp will eventually establish self-sustaining populations in the Great Lakes. For an updated map of Canadian Grass Carp captures, visit our website

What can I do to Help?

The most important thing you can do as an angler is learn how to identify Grass Carp and report all sightings of suspected Grass Carp. If you think you have captured a Grass Carp or any other invasive aquatic species, DO NOT release it back into the water! In Ontario, it is illegal to import, possess, transport, or release Grass Carp unless they are dead and eviscerated (gutted). All invasive species, including Grass Carp, can be reported to the Invading Species Hotline at 1-800-563-7711, online via EDDMaps.org, or the EDDMapS App (available for Apple and Android).

Never dump live bait back into a water body! In Ontario, it is illegal to dump live bait within 30 metres of any water body, and it can be hard to determine what species might be in your bait bucket! Juvenile Grass Carp can be hard to identify and may look very similar to common baitfish species. Check out the Government of Canada Baitfish Primer, A guide to identifying and protecting Ontario’s baitfishes for more information.