Billions of invasive round goby are in the Great Lakes. UTSC researchers want to stop them from spreading inland
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They may be small, but invasive round goby can have a significant impact on local aquatic ecosystems (photos by Don Campbell)
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Credit: Don Campbell/University of Toronto
For decades, the round goby has been one of the Great Lakes’ most destructive invasive species. Scientists estimate its population numbers in the billions, making their eradication unrealistic.
But new U of T Scarborough research suggests the fight against the invasive fish is far from over.
In a new study published in the Journal of Great Lakes Research, postdoc Piatã Marques and Professor Nick Mandrak suggest governments and conservation agencies should rethink how they manage round goby as the fish expands beyond the Great Lakes into rivers, streams, lakes and other inland waterways.
“The Great Lakes are essentially lost when it comes to round goby,” says Mandrak, a renowned invasive species expert.
“They’re established throughout the lakes, they number in the billions, and we’re never going to fully remove them from those systems. Now we’re seeing them move into connected and isolated waters where we still have a chance to slow or even stop their spread.”
Their research shows the fish spreads more slowly through smaller waterways, creating an opportunity to contain or eliminate local populations before they become a problem.
Originally native to the Black and Caspian seas, round goby arrived in the Great Lakes in the early 1990s in the ballasts of cargo ships. Since then, they have spread throughout all five Great Lakes where they compete aggressively with native fishes for food and habitat, consume fish eggs and young, and alter aquatic food webs.
Despite measuring about the length of a smartphone, round goby have an outsized impact on freshwater ecosystems. Their aggressive behaviour, voracious appetite and ability to thrive in a wide range of conditions have helped them multiply into the billions.
Marques says the invasion has now entered a new phase.
“Over roughly the last 15 years, we’ve started seeing round goby moving upstream into inland waters, including rivers, streams and ponds,” he says. “These freshwater systems contain some of Ontario’s richest biodiversity, including endangered species, so this is an area of concern.”
Among the species potentially at risk is the redside dace, an endangered fish that inhabits streams across the GTA. Researchers worry round goby could outcompete this and other native fishes for food and further disrupt inland freshwater ecosystems.
Using more than two decades of GTA river monitoring data from local conservation agencies, Marques and Mandrak found round goby advances only a fraction of a kilometre to about two kilometres per year, providing a window for intervention before populations explode.
Rivers and streams are smaller and more accessible than the Great Lakes, making targeted management more practical. They suggest a combination of trapping, electrofishing, physical barriers and rapid-response programs could reduce local populations and, in some cases, eliminate them.
Marques says understanding how the fish adapts to these new environments is key to developing effective control strategies. Research in the Mandrak lab is looking at what round goby eat, how they compete with native species, whether urbanization helps them move upstream and why they’re able to successfully colonize new freshwater systems.
The team is also testing environmental DNA (eDNA), which detects fish from generic material in water samples, allowing scientists to identify invasions even when the fish are difficult to capture.
“They’re very flexible in terms of the conditions they can tolerate,” says Marques. “They can feed on many food sources, and we think they’re changing how they reproduce and survive as they move upstream. We’re trying to understand exactly which characteristics allow them to persist in these new environments.”
Marques says the goal is to give resource managers practical guidance on which methods work best and how much effort is needed to reduce their populations.
The team is also studying whether urban development is inadvertently helping the invasion. Road salt and other pollutants increase dissolved ions in streams, potentially creating conditions that make it easier for round goby, which are native to brackish waters in Eurasia, to move farther inland.
For Mandrak, the research highlights an important shift in how invasive species should be managed.
There is still an opportunity to protect these inland ecosystems,” he says. “If we act before round goby becomes established in rivers and streams, we have a better chance of protecting native biodiversity. If we wait until the invasion reaches the same scale we’re seeing in the Great Lakes, it will be too late.”
Students in Mandrak's lab, including postdoc Piatã Marques (centre) have found populations of round goby living in Toogood Pond Park in Markham.
Credit
Don Campbell/University of Toronto
Professor Mandrak is a renowned invasive species expert. He says while round goby might never be eliminated from the Great Lakes, there is still hope in protecting smaller inland bodies of water.
Research in the Mandrak lab is looking at what round goby eat, how they compete with native species, whether urbanization helps them move upstream and why they’re able to successfully colonize new freshwater systems. The ultimate goal is to help control the spread of the fish.
Credit
Don Campbell/University of Toronto
Journal
Journal of Great Lakes Research
Article Title
Urgent need for reassessing mitigation actions as long-term, well-established, invasive species secondarily spread into new habitats
Article Publication Date
12-Aug-2026
