Fierce invader steals nests from a native fish.

Since the round goby arrived in the Great Lakes more than a decade ago, the small but feisty fish has spread rapidly and has caused local extinctions of a native species. Researchers now have identified just how the gobies take over.

In those places where the pencil-length bottom-dwellers have proliferated, populations of the mottled sculpin, an important prey species for larger fish, have crashed. In a soon-to-be-published study, lake researchers show that the gobies win turf by appropriating sculpins' nesting sites. With its ability to spawn diminished, the days of this indigenous population are numbered, the researchers say.

Round gobies (Neogobius melanostomus) originating from around the Black Sea first reached Lake Huron and Lake Erie in 1990, probably by hitching rides across the Atlantic in ballast water of seagoing vessels. The Eurasian invader has since spread to all five Great Lakes (SN: 7/31/99, p. 68).

Gobies can thrive on a varied diet. They compete fearlessly for resources with each other and with the slightly smaller mottled sculpins (Cottus bairdi). It's not yet clear whether sculpin-eating predators can hunt gobies as efficiently.

For their study, David J. Jude of the University of Michigan in Ann Arbor and John Janssen of the University of Wisconsin-Milwaukee observed mottled sculpins in Calumet Harbor on southern Lake Michigan. The researchers began accruing data in 1994, shortly after other biologists first detected round gobies there. Over the next 4 years, the local sculpin population declined precipitously.

Donning scuba gear, the researchers monitored sculpin abundance and behavior during the animal's spawning cycle. As expected, they found that females released eggs and that males guarded nests. But once the gobies had taken hold in the area, the researchers encountered no young sculpin.

In a separate experiment using an artificial stream, the researchers found that male gobies consistently evict spawning sculpins from their nesting sites, eat virtually all sculpin eggs, and then begin to defend the nests as their own. In the upcoming fall JOURNAL OF GREAT LAKES RESEARCH, Jude and Janssen conclude that sculpins are unable to reproduce in goby-occupied waters.

Other forms of competition between the two species, including rivalry for shelter from predators and food may exacerbate the sculpins' decline, says Jude.

Although the findings could help lake managers cope better with the ongoing goby invasion, no easy solution is at hand. Janssen speculates about one possible sculpin-saving tactic: mass production of artificial sculpin nests shaped to deter gobies.

Other scientists hold little hope for sculpins in goby-invaded waters. "Where the two species co-occur ... it's likely that the goby will lead to the demise of the mottled sculpin," says Lynda D. Corkum, an aquatic ecologist at the University of Windsor in Ontario.

In response to the threat of round gobies and other invasive species traversing a series of canals and rivers that leads from Lake Michigan to the Mississippi River, the Army Corps of Engineers is building an electrical barrier designed to turn the would-be invaders back. Slated to begin operating this fall, however, it comes too late to fully contain the gobies. Some have already appeared downstream from the barrier.