Round goby, small but troublesome exotic fish in the Midwest. Photo by Ellen Marsden.
The latest exotic species to invade the Great Lakes is a diminutive fish from the Black and Caspian seas area. The round goby, which grows to only 10 inches long, was discovered in 1990 in the St. Clair River, which borders the U.S. and Canada near Detroit. For three years they remained within a few miles of their point of introduction, but in January 1993, several gobies were caught by anglers in the Grand Calumet River in Indiana. By 1994 they were found in Calumet and Hammond harbors, IL, South Haven, MI, and Cleveland, OH; and in 1995 gobies were found on the north side of Chicago and in Duluth, MN. This rapid spread was undoubtedly facilitated by the same vector that brought gobies to North America -- ballast water transfer from ocean-going ships in the Great Lakes. The presence of gobies in the Calumet River means that they have direct access to the Illinois and Mississippi river drainages, and thus to a large geographic range.
The reaction of scientists and fisheries managers to the round goby has been different than has been the case with other recent invaders. When zebra mussels were found in Lake St. Clair in 1988, they were immediately recognized as an ecological threat because of their centuries-long history of disrupting native communities and fouling intake pipes as they spread across western Europe. The appearance of European ruffe in Lake Superior near Duluth in 1987 raised similar alarms because ruffe had invaded Loch Lomond in Scotland a decade earlier and nearly wiped out the local populations of yellow perch. In contrast, when the round goby appeared in the St. Clair River, no one paid much attention. Round gobies had not previously spread beyond their native range, so they had no history of causing problems. This has been an unfortunate case of assuming innocence until guilt is proven -- an unwise strategy where exotic species are involved. Many exotic species that have become established in North America (and the rest of the world) have caused significant ecological disruption (consider cockroaches, starlings, Norway rats, dandelions, and the Medfly).
Round gobies are relatively small, benthic, soft-bodied fishes that are easily confused with native sculpin species. Two characteristics may be used to distinguish them from the natives: first, their pelvic fins are fused to form a disk. This pelvic disk is characteristic of the goby family. Second, their bodies are covered with fine, mail-like scales, in contrast to the local native sculpins, which are naked or sparsely covered with prickles. Round gobies also have a distinctive black spot on the spinous (front) dorsal fin, but this character is not diagnostic for gobies because many sculpins have a similar spot in the same location.
Round goby, small but troublesome exotic fish in the Midwest. Photo by Ellen Marsden.
Although there are a few euryhaline (able to tolerate a wide range of saltwater concentrations) marine gobies that are often found in coastal streams, the round goby is the first freshwater goby to proliferate in North America; the tubenose goby, which appeared in the St. Clair River at the same time as the round goby, has not spread widely. Round gobies are more fecund, more aggressive, and have lateral line systems that are more sensitive in still water than those of the native sculpins. Round gobies are natural predators of zebra mussels in their native eastern European range, and a substantial proportion of their diet in the Great Lakes is composed of zebra mussels; sculpins eat few zebra mussels. These features of the goby suggest that round gobies probably will be able to invade many regions of the Great Lakes, and may displace native sculpins by outcompeting them for shared resources of food and habitat.
Another feature that gobies share with sculpins is their ability to deeply penetrate the interstitial spaces in cobble substrates. For sculpins, this behavior makes them an effective predator of lake trout eggs. Lake trout were extirpated in Lake Michigan by the 1950s and are currently the focus of a massive stocking effort with the aim of population rehabilitation. The trout spawn in fall over cobble reefs and their eggs settle deeply into interstitial spaces, where they are mostly protected from storm-generated surges and from predators--except sculpins. Stocked lake trout are just beginning to spawn, in low numbers, in Lake Michigan (see INHS Reports 324); the addition of a new, highly abundant egg predator could mean a significant setback for their rehabilitation.
Gobies have not yet spread into areas where lake trout spawn, though they are within 15 miles of an important lake trout spawning site. However, in our laboratory experiments, which simulated conditions on a lake trout spawning reef, gobies readily retrieved and consumed eggs that had settled into cobble substrates. They also ate newly hatched fry under the same conditions. Gobies as small as 56mm, which are likely only one year old, could break and eat lake trout eggs. The largest gobies tested, which were 100-120mm long, ate over three eggs per day on average. By comparison, sculpins tested under similar conditions ate an average of two eggs per day, as did crayfish. Estimates of sculpin densities in the Great Lakes vary from 1 to 30 per square meter; densities of adult gobies near Calumet Harbor in Lake Michigan vary from 1 to 20 per square meter, and juveniles are present on sandy substrates at 8 to 133 per square meter.
Clearly, gobies have both the potential population densities and the appetite to pose a serious threat to lake trout reproductive success. Whether this threat will be realized depends on how rapidly gobies spread to lake trout spawning areas, and their preference for alternate food sources. Native fish species, including adult lake trout, are likely to prey upon gobies, although the gobies' sensitive lateral line system makes them highly effective at detecting and avoiding predators. For example, sculpins can be easily collected by scuba divers with a dip net, whereas gobies larger than 27mm are extremely difficult to catch. Gobies are only too easy to catch with a rod and reel, however; anglers in the St. Clair River and Calumet Harbor have been frustrated by the gobies' propensity to steal bait.
Like the other exotic species that have invaded the Great Lakes in recent decades, gobies have become a permanent part of the ecosystem; they are too numerous and too widespread to control. A few of their potential impacts are predictable: competition with sculpins, and predation on lake trout eggs and fry and other benthic organisms. Whether they will become a significant nuisance like the ruffe or alewife remains to be seen, and this possibility will be the focus of research by Survey investigators over the next few years.
J. Ellen Marsden and Michael A. Chotkowski, Center for Aquatic Ecology
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