The round goby has long been one of the most destructive invasive species in the Great Lakes, with scientists estimating its population in the billions.
While eradicating the bottom-dwelling fish - which first arrived in the early 1990s in the ballasts of cargo ships - is all but impossible, University of Toronto researchers say the fight is far from over.
In a new study, postdoctoral researcher Piatã Marques and Nick Mandrak, a professor in U of T Scarborough's department of biological sciences, argue that governments and conservation agencies should rethink how they manage the round goby as the species 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. "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, published in the Journal of Great Lakes Research , shows round goby spread 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 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," 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 Greater Toronto Area. 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 river monitoring data from local conservation agencies, Marques and Mandrak found round goby advance 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, say the researchers. They suggest a combination of trapping, electrofishing - a survey method that involves using a mild electrical current to temporarily stun fish so they can be collected and monitored - physical barriers and rapid-response programs could reduce local populations and, in some cases, eliminate them.
Marques says understanding how the fish adapt 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, which detects fish from genetic 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, 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."
