By Sharon Oosthoek

Fisheries biologists have unexpectedly discovered round gobies in the Thames, Sydenham, Ausable and Grand rivers and are now sounding the alarm over how this invasive fish may affect endangered species.

The Great Lakes tributaries, Canada’s most diverse aquatic ecosystem, were long thought to be immune to such an invasion because, since each ecological niche was taken up, invaders could not gain a foothold.

But a team of scientists from the universities of Toronto and Guelph, and the Ontario Ministry of Natural Resources, stumbled across the round gobies in 2006 while checking on the health of various tributaries in southwestern Ontario. “We didn’t anticipate finding round gobies. That wasn’t the goal of the work,” says Mark Poos, a PhD student in biology at the University of Toronto.

Poos is the lead author of a recent study showing that gobies could harm up to 89 percent of fish species and 17 percent of mussel species in the tributaries. Round gobies push other fish, such as the threatened eastern sand darter, out of their spawning beds and will in some cases eat their eggs, thereby reducing their populations. Gobies threaten mussels more indirectly, by decreasing the number of fish on which young mussels rely.

Young mussels, such as the endangered snuffbox mussel, attach themselves to the gills of at-risk fish. Without the help of their hosts, mussels could not move from one river system to another and their geographical ranges would shrink, so a single catastrophe could wipe out an entire species of mussel.

Round gobies were first discovered in the St. Clair River in 1990, and probably arrived through the dumping of ballast water from ocean-going ships. But until recently, gobies had not found their way into the Great Lakes tributaries. “It truly is a new invasion,” says Poos. “The question is why. That’s the one thing I don’t think we have a handle on.”

There are some educated guesses, however. Perhaps the goby population has now grown large enough to push its way into the tributaries. Or maybe there are two genetic types of gobies – one of which is more invasive than the other – and it is the hyperinvasive type that has now made its way into the Great Lakes system.

Whatever the cause, scientists such as Poos hope to influence people who fish in the Great Lakes tributaries not to transfer bait – which could include gobies – caught in one area to another.

“We’re not entirely sure what’s going to happen,” says Poos. “These fish and mussel species are already getting hit by other things – habitat alteration and turbidity. So when a new species comes in that can compete with them, I think it poses a serious problem.”