One of the most horrifying invasive species in Ontario is about to begin its annual breeding frenzy: the parasitic, vampire-like sea lamprey.

These slippery serpentine creatures native to northern oceans have been wreaking havoc on the Great Lakes for almost 200 years, likely introduced through ships travelling the Erie Canal.

The jawless eel-like species has established itself well in this period by preying on local fish in an absolutely revolting fashion.

Sea lamprey attach themselves to prey with suction-cup-like mouthparts lined with horrifying concentric rings of teeth that act as sawblades to help them gradually drain victims of blood.

Its victims' blood stays flowing through the slow and painful ordeal with the help of an anticoagulant, hemolytic and cytolytic chemical secreted in lamprey saliva called lamphredin.

rl stine literally predicted this pic.twitter.com/gU2y4KMqZt

— Horror4Kids (@horror4kids) July 16, 2023

While they've been present in Toronto waters since back when the city was known as the Town of York, lamprey have been able to penetrate much further inland since the early 20th century, when the Welland Canal gave all aquatic species unobstructed access to the upper Great Lakes.

Established sea lamprey populations typically explode in the spring and early summer months during their breeding season.

Sightings of sea lamprey in Ontario lakes and waterways can be reported using social network biodiversity app, iNaturalist, which shows spikes in encounters from April to June that offer some insight into the seasonal spread of sea lamprey.

Every year, the Toronto and Region Conservation Foundation (TRCA) works to curtail the spread of this invasive species, rounding them up in the hundreds and dispatching them.

It’s #InvasiveSpeciesAwarenessWeek
We support invasive removal projects on both land and water! Sea Lamprey are invasive fish that feed on bodily fluids of other fish. In 2021, TRCA removed 953 Lamprey benefitting ecosystems locally and the Great Lakes! https://t.co/nAuagUgzjF pic.twitter.com/fkTjWRClkB

— Toronto and Region Conservation Foundation (@TRCFDN) February 28, 2022

The Rouge River at the east edge of Toronto is another hotspot for sea lamprey. The Great Lakes Fishery Commission just concluded a treatment of the river this spring, killing off thousands of lamprey larvae.

The Rouge River, in the GTA in Ontario, will be treated for sea lamprey larvae between 4/28/24 to 5/1/24. @FishOceansCAN is working hard to protect the many important fish species of the watershed and Lake Ontario. This treatment will target 3,167 larvae. https://t.co/FJ9Ww9uhF4 pic.twitter.com/fid7wCCUFz

— GLFC (@LampreyControl) April 28, 2024

Still, despite boatloads full of these predators being removed from the province annually and many more killed off as larvae, local anglers know all too well the damage being caused by these invaders.

In 2022, angler Dustin Pearl caught a massive pike while fishing from the dock wall of a downtown Toronto park. Many commenters suggested that the large fish had endured multiple lamprey encounters based on several circular scars visible.

A massive pike caught in the midst of downtown Toronto - 📹 Dustin John Pearl https://t.co/LwSikYbRXy #Toronto #Fishing pic.twitter.com/MsXGfou1HK

— blogTO (@blogTO) March 23, 2022

So, next time you decide you want to take a swim in the lake to cool off on a hot day, think about things like lampreys — but also predatory water fleas, aquatic zombie plants, self-replicating crayfish and other invasive freshwater horrors you may not have known about.