When University of Wisconsin-Milwaukee professor John Jannssen found a round goby in Calumet Harbor in 1994, he feared the worst. The small fish with an almost endearing wide-eyed stare reproduces prolifically and eats voraciously. Jannssen and other scientists expected it to gobble up the eggs of prized sport fish and hog the mussels and snails that other fish would normally eat.
Today millions of round gobies are thriving in the waters of Lake Michigan off Chicago and throughout much of the Great Lakes. But as is often the case with invasive species, their ecological impact has not been devastating, but complicated–even beneficial in some cases.
Like the majority of the 185 known invasive species in the Great Lakes, the round goby was transported in the ballast water of ocean-going ships that came down the St. Lawrence Seaway into the Great Lakes. It was first seen in the St. Clair River between Lake Huron and Lake Erie in 1990. Jannssen was the first to find one in Lake Michigan.
Now round gobies provide an abundant food source for valuable sport fish including native smallmouth bass and non-native, stocked species like brown trout. Cormorants, migratory water birds with massive populations across the Great Lakes, love round gobies so much that they have spared sport fish to gorge on them.
In the food chain’s opposite direction, round gobies eat harmful zebra mussels and quagga mussels, invasive species also introduced through ballast water. The mussels decimate the plankton food base and cost billions of dollars to remove from water-intake structures. Round gobies are among their few predators.
“When the round gobies arrived, lunch was already waiting for them,” said Linda Campbell, an associate biology professor at Queens University in Ontario.
But the news isn’t all good. Round gobies spread deadly botulism, picking it up from mussels and passing it on to ducks that eat them. Chicago-area fishermen who cast their lines from the shore are often infuriated by catching mounds of round gobies.
The gobies also defend their spawning habitat so aggressively that native small fish including sculpin can’t find a place to reproduce, and gobies eat the eggs and larva of other fish.
Jannssen described a delicate ecological dance between round gobies and smallmouth bass. If a smallmouth moves away from its nest, perhaps after being caught by a fisherman, round gobies will swarm in and devour its eggs. But once surviving eggs hatch, the young bass will begin a lifetime of feasting on gobies.
“Round gobies are a good example of how an invasive species can have a foot in two worlds,” said Joel Brammeier, president of the Alliance for the Great Lakes, who spoke at an international conference on invasive species at the University of Chicago this month.
“They come into a place like the Great Lakes, and take on another invader like zebra mussels, and ingratiate themselves to the native fish who consume them,” he said. “But the danger with invasive species is their unpredictability. Once they’re in there’s no telling how they will act. And once they are on the menu you can’t ever take them off.”
A Great Lakes Invader is Doing Some Good, Round Gobies
