The Round Goby: An Example of the "Perfect" Invader?
The round goby, Neogobius melanostomus is the most recent exotic fish (along with its cousin the tubenose goby, Proterorhinus marmoratus) to invade the Great Lakes. Introduced from their native range (the Black and Caspian seas) via ballast water, round gobies are small, benthic, soft-bodied fish that look similar to native sculpins, but can be identified by a black spot on their anterior dorsal fin and by fused pelvic fins in the form of a suctorial disc. First discovered in 1990 in the St. Clair River near Detroit, round gobies quickly spread and by 1995 had been reported in all five of the Great Lakes with population numbers reaching high densities in many areas in Lake Erie and Lake Michigan.
The Round Goby, Neogobius melanostomus.
Certain aspects of the round goby's biology provide us with an example of what the "perfect" invader is and why gobies may have been pre-adapted for live transport in ballast water of transoceanic ships. These same qualities will allow for their further expansion in North America, with the potential to threaten not only the Great Lakes aquatic communities but also their tributaries and other connected watersheds as well. The following adaptive features that gobies possess can be useful in determining what makes an invader successful and may predict who else could be a potential invader.
First, the round goby grows up to 10 inches and is a very aggressive, robust fish that is highly territorial and very competitive for food, shelter, and spawning areas. This competitive nature along with its larger size has already allowed it to displace smaller native benthic fish, such as the mottled sculpin (Cottus bairdi) and logperch (Percina caprodes), from some areas of the Great Lakes. Second, gobies can survive under a wide range of environmental conditions including fresh or salt water (40.5% saline), water with low dissolved oxygen and high levels of pollution, and also water from -1 to 33deg.C. Round gobies can also live in a wide range of habitat types, such as sandy or silty areas and macrophyte habitats, although they prefer rock, cobble, or riprap habitat. Third, gobies are voracious foragers with a very diverse diet comprised mostly of benthic invertebrates including zebra mussels, but may contain smaller fish and fish eggs. The use of zebra mussels in their diet provides gobies with a competitive advantage by giving them a food supply most native fish do not utilize. Round gobies also have a well-developed lateral line system that gives them the ability to feed at night. Finally, gobies spawn every 18-20 days and potentially up to six times during a breeding season. This reproductive pattern gives them an ecological advantage over native species which usually spawn only once.
Characteristics such as propagule pressure, suitability of habitat, and success in previous invasions can be valuable predictors for the success of a particular invasion. Round gobies have demonstrated that they possess these and other important characteristics, such as survivability in unfavorable conditions, adaptability to a new environment, territorial behavior, and other competitive advantages over native species that have allowed them to become an excellent invader. The question is, How far and fast will they spread and what impacts will they have?
Research by Survey scientists shows gobies can survive at least several weeks at temperatures as high as 33deg.C and optimal growth for round gobies occurs near 24deg.C. These results for temperature tolerance are very similar to those of zebra mussels, which are widely distributed and have spread as far south as New Orleans. In addition, with zebra mussels present, gobies are given a preferred food item with suitable habitat already present upon arrival to new areas. Based on this and temperature tolerance, if gobies are able to spread to the Illinois River from Lake Michigan, their range could potentially expand to the Mississippi River, thus giving them access to much of the interior of North America. It remains to be seen how fast gobies will spread, or if a proposed dispersal barrier in the Chicago Ship and Sanitary Canal will stop round gobies from spreading too far downstream in the Illinois River.
With optimal growth for round gobies occurring near 24deg.C, it is highly possible they will be more successful in streams and rivers, where their greatest impacts on native species could be felt. Since round gobies have already had negative impacts on sculpin and logperch populations in the Great Lakes, biologists fear that similar impacts may occur with darters should gobies continue to spread. Survey scientists have conducted competition experiments among round gobies and both greenside (Etheostoma blenniodes) and johnny darters (E. nigrum) in artificial streams and enclosures placed into small ponds. Research measuring growth of darters with gobies present or absent indicate trends toward negative impacts on darters by round gobies. Also, results provide evidence that gobies are equal or better competitors than are darters with fellow darters.
Because round gobies reach such high densities and are known to eat the eggs of other fish, researchers are also concerned about possible predation by gobies on eggs of nesting sunfishes during spawning periods. For this reason, experiments were also conducted at the Illinois Natural History Survey in which round gobies were added to large cattle tanks with spawning pumpkinseed (Lepomis gibbosus) and green sunfish (L. cyanellus). Sunfish nests were then filmed with video equipment to observe if round gobies would eat sunfish eggs or even attempt to do so. The video showed that round gobies will raid nests and successfully prey on sunfish eggs, sometimes even when the guarding male sunfish is present. It is not known from these results how significant an impact gobies could have on overall nesting success of sunfish; however, it appears that guarding the nest successfully comes at a high price energetically for the male sunfish.
There are many other ways in which round gobies could have major impacts on stream and river communities as predators, prey, or as competitors with native species. It is extremely important that more work be done in this area to help understand what impacts round gobies will have as their invasion continues to new areas. Since 1995, round gobies have been found in other areas of the Great Lakes, including a discovery as recently as this summer of large numbers of gobies living in Lake Superior near Superior, Wisconsin. They have also expanded their range to live much farther inland in both the Shiawassee and Flint rivers near Flint, Michigan. Most importantly, perhaps, gobies are now poised to invade the Illinois River system, having been found to exist at least 15 miles inland from Lake Michigan in the Cal-Sag Channel of Chicago. If the round goby and other aquatic invaders, such as the Eurasian ruffe, the sea lamprey, and the zebra mussel, that have similar qualities suited for invasion could be used as a model, we may be able to better predict what invaders will be successful or what other foreign animals could be potential candidates for future invasion, and possibly stop an invasion before it happens.
Cordell H. Manz, Center for Aquatic Ecology