Invasive Species

The introduction of aquatic invasive species into Lake Simcoe began over a century ago with the introduction of the Common Carp. Since then, numerous unwanted species have found their way into the lake and in their own way have altered the aquatic ecosystem. Here is a list of the known invasive species and their dates of introduction into Lake Simcoe.  

- Common carp (Cyprinus carpo) – 1896 

- Rainbow smelt (Osmerus mordax) – 1962 

- Eurasian watermilfoil (Myriophillum spicatum)  – 1984 

- Curly-leaf pondweed (Potamogeton crispus)– 1961-1984 

- Black crappie (Pomoxis nigromaculatus) – 1987 

- Zebra mussel (Dreissena polymorpha) – early 1990s 

- Spiny water flea (Bythotrephes longimanus) – 1993 

- Bluegill (Lepomis macrochirus) – 2000 

- Quagga mussel (Dreissena bugensis) – 2004 

- Rusty crayfish (Oronectes rusticus) - 2004  

- Scud(Echinogammarus ischnus) – 2005

-Round goby (Neogobius melanostomus) – 2006 


Round Goby

Round gobies are fast breeders, aggressive fish, and feed on the eggs of native species. They are bottom dwellers and will feed on the insects and plant life other species rely on.

The round goby came to the Great Lakes in the ballast water of ships from Eastern Europe in the late 1980's first appearing in the St. Clair River. The goby is believed to have been accidentally introduced into Lake Simcoe by anglers, through the reuse of exposed bait. Bait transport is a common pathway for the introduction of invasive species between watersheds.

Rainbow Smelt 

Rainbow Smelt invaded Lake Simcoe in 1962. Smelt compete directly with native fish for food. Smelt will even eat other fish in their early or larval life stages. These larval fish or fry can include lake trout, whitefish, walleye and cisco, a forage species native to Lake Simcoe. There is some evidence that the rate of mercury accumulation in top level fish consumers accelerates when they switch to a diet of smelt. Rainbow smelt are also rich in thiaminase, an enzyme that destroys thiamin, which is necessary for development of fish embryos; as lake trout and other sport fish consume the smelt, their ability to successfully reproduce diminishes because of thiamin-deficiency.

Zebra Mussel

Zebra mussels invaded Lake Simcoe in 1994.It is believed that due to their rapid, widespread establishment and high filter feeding capacity, they have likely impacted water quality in Lake Simcoe in numerous ways.These include increased water clarity, reduced algae and lower alkalinity and calcium concentrations. Zebra mussels are also thought to affect phosphorus cycling in lakes by converting particulate-bound phosphorus to more reactive forms and by increasing concentrations near the shore. Research on the zebra mussel continues.


The zebra mussel originated in the Black Sea and Caspian Sea area and is thought to have been brought to this continent in the ballast of foreign freighters. Zebra mussels get their name from a striped pattern which is commonly seen on their shells, though not all shells bear this pattern. They are usually about the size of a fingernail, but can grow to a maximum length of nearly 2 in (5.1 cm). An adult female zebra mussel may produce between 30,000 and one million eggs per year.

Dreissinid Mussels 

In Lake Simcoe, dreissenid filtering has increased water clarity and increased Phosphorus in the nearshore. Recent surveys have determined that macrophyte biomass has increased in the shallow areas of the lake. The increased oxygen concentrations in the deeper areas of Lake Simcoe suggest that like other lakes in the region, off shore waters have lower algal biomass post-dreissenid invasion.



Rusty Crayfish


Rusty crayfish, a Ponto-Caspian european invader arrived in Lake Simcoe in 2004. It is estimated that rusty crayfish can consume twice as much food as native species of the same size due to a higher metabolic rate. Rusty crayfish have a ravenous appetite for aquatic plants; they can degrade aquatic plant beds to the detriment of the aquatic invertebrates and juvenile fish that depend on these areas for habitat. It has been said that what the rusty crayfish does to aquatic plants is the equivalent of clear-cutting forests. By consuming large quantities of benthic invertebrates, fish eggs and young fish, these crayfish could also compete with juvenile game fish and forage fish species for food.

Not only does the rusty crayfish out-compete native crayfish species for food, they also chase them out of the best daytime hiding locations. This makes native populations vulnerable to being eaten by birds and fish. Many fish prefer native crayfish because they have a softer shell compared to the rusty crayfish and a decrease in the their numbers could limit a food source for fish. The rusty crayfish is also more aggressive and under fish attack will not swim away like the native crayfish, but will hold its claws up in a defensive manner.

Common Carp

The Common carp invaded Lake Simcoe in 1896. Common carp have predominantly vegetarian diets but will also feed on aquatic invertebrates. Their feeding activity has severe impacts on wetland habitats because they suck up sediments and organisms from the bottom, uproot and destroy vegetation and muddy the water.