A team on Friday found more than a dozen of the greedy predators while sampling sections of the Lower Rouge River in Inkster, said Sally Petrella of the nonprofit group Friends of the Rouge.
"There's huge potential for wiping out a lot of native fish," she said.
Gobies hang around prime spawning areas and gobble up eggs laid by trout, whitefish and other species important to the Great Lakes region's commercial and sport fishing industries.
The find comes as groups including Friends of the Rouge work with cities and businesses to improve fish habitats by reopening sections of the Rouge River that were blocked by dams.
The Rouge River flows into the Detroit River, where the round goby had an initial period of large growth, the group said, but over time their numbers have stayed steady. The Rouge River survey team, however, also found some native fish in the river that fare poorly in the face of round gobies.
It's too early to take aggressive action, Petrella said, such as fish poisons used in other parts of the Great Lakes system.
The voracious bottom-feeders, measuring up to 10 inches long, have spread rapidly since arriving in European freighters' ballast water in the 1990s. They help the ecosystem by eating zebra and quagga mussels, which are among the Great Lakes' most damaging invaders. But their appetite for other fishes' eggs offsets the good they do.
Gobies also spread botulism that has killed thousands of shorebirds.