Billions of Invasive Round Goby Are in the Great Lakes — Researchers Aim to Stop Their Inland Spread

Researchers at the University of Toronto Scarborough are urging governments and conservation agencies to rethink how they manage the invasive round goby as the fish, whose population in the Great Lakes numbers in the billions, spreads into rivers, streams and other inland waterways. The study, led by postdoctoral researcher Piatã Marques and Professor Nick Mandrak, was published in the Journal of Great Lakes Research.

"The Great Lakes are essentially lost when it comes to round goby," said Mandrak. "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."

Round goby are native to the Black and Caspian seas and arrived in the Great Lakes in the early 1990s in cargo ships' ballast water. They compete aggressively with native fish for food and habitat, eat fish eggs and young, and alter aquatic food webs. Over the past 15 years they have also begun moving upstream into rivers, streams and ponds, including Toogood Pond Park in Markham, threatening species such as the endangered redside dace.

Using more than two decades of monitoring data from Greater Toronto Area conservation agencies, Marques and Mandrak found round goby advance only a fraction of a kilometer to about two kilometers a year in rivers and streams, far slower than in the open lakes. That pace creates a window for intervention through trapping, electrofishing, physical barriers and rapid-response programs before local populations explode.

The team is also testing environmental DNA (eDNA) analysis, which detects fish from genetic material in water samples, to identify invasions even when the fish are hard to capture. They are separately examining whether road salt and other pollutants, which increase dissolved ions in streams, are helping the goby — native to brackish waters — move farther inland.