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Invasive Species

Introduction

Lake Simcoe has experienced a steady influx of non-native aquatic species over the last century, many of which have reshaped fish communities, food webs, and fisheries. The introductions range from large-bodied fish to tiny predators and mussels, and their combined effects have altered habitat structure, prey availability, and predator–prey dynamics in both nearshore and offshore areas.

Timeline of key invasive fish and associated aquatic invaders

1896 — Common carp (Cyprinus carpio). One of the earliest fish introductions that increased turbidity and disrupted aquatic plants.
1961–1962 — Curly‑leaf pondweed and Rainbow smelt (Osmerus mordax). Early mid‑20th century changes in prey base and vegetation patterns followed these arrivals.
1987 — Black crappie (Pomoxis nigromaculatus). A warm‑water fish that expanded the community of panfish in the lake.
Early 1990s — Zebra mussel (Dreissena polymorpha). A benthic invader that substantially increased water clarity and altered benthic–pelagic energy transfer.
1993 — Spiny water flea (Bythotrephes longimanus). A zooplankton predator that reduced native zooplankton and changed forage availability for small fish.
2004–2006 — Quagga mussel, Rusty crayfish, Scud, Round goby (Neogobius melanostomus). A wave of invaders that further altered benthic habitats, prey fields, and competitive interactions for juvenile and benthic‑feeding fish.
These documented dates and arrivals are summarized in regional invasive‑species guides and local fisheries summaries for Lake Simcoe.

Ecological effects on fish communities

The combined effect of invasive fish and invertebrates has been multi‑faceted. Habitat modification by common carp and rusty crayfish reduced submerged vegetation that native juvenile fish rely on for refuge and feeding. Food‑web restructuring followed zebra and quagga mussel establishment because increased water clarity favored plankton shifts and altered the distribution and abundance of prey available to planktivorous and age‑0 fishes. Predation pressure from newcomers such as rainbow smelt and spiny water flea lowered populations of small native forage species and juvenile fish, producing cascading effects up to piscivores and sport fish species. Competition and behavioural disruption from benthic invaders like round goby have displaced native benthic fishes and preyed on eggs and invertebrates that historically supported native species, changing recruitment patterns for multiple fish taxa.

Consequences for fisheries and species at risk

Sport and commercial fisheries have felt the impacts through shifts in target species abundance, altered growth rates, and changes in angler catch composition. Native coldwater species remain particularly vulnerable because warm‑water invaders and altered food webs reduce thermal refuges and the prey base required for successful recruitment. Species at risk that depend on specific habitat or prey types face increased stress from habitat loss, heightened competition, and higher juvenile mortality linked to invasive predators and habitat‑altering invaders.

Management, monitoring, and outlook

Management has emphasized prevention, public outreach, and targeted monitoring to detect and slow new introductions and limit spread within the watershed. Ongoing actions include boat‑inspector programs, angler education about bait and equipment decontamination, coordinated monitoring of fish and invertebrate communities, and research into control options where feasible. The outlook depends on sustained prevention and adaptive management because established invaders are difficult to eradicate and can produce long‑term ecosystem shifts that require decades to stabilize.

Conclusion

From the arrival of common carp in the late 19th century to the wave of benthic and planktonic invaders in the 1990s and 2000s, Lake Simcoe’s fish community has been repeatedly reshaped by non‑native species. Those invasions have altered habitat, retooled food webs, and challenged both native biodiversity and fisheries. Continued monitoring, strong prevention measures, and informed management remain essential to reduce further damage and to support the resilience of Lake Simcoe’s fisheries and at‑risk species.

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.

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