Round Goby Invasion
The round goby (Neogobius melanostomus) was first discovered in the St. Clair River in 1990. Presumably introduced via ballast water from their native range (the Black and Caspian seas), round gobies have successfully colonized sites in all five of the Great Lakes and are moving into adjacent wetlands and tributaries.In Green Bay, round gobies have quickly spread from initial invasion sites at Escanaba and Sturgeon Bay to almost entirely throughout the bay (Figure 1). They are presently very abundant about 15 km north and south of Sturgeon Bay along the Door Peninsula. Gobies have been found between Escanaba and Oconto in limited densities.
Round gobies are small (< 10 inches in length), benthic, fish that look similar to native sculpins. Gobies are different in appearance from sculpins in that they have fused pelvic fins that form a suction disc on the ventral surface (see picture below). This disc likely is used as a brace against current in high flow events. Another distinguishing characteristic of the goby is that it has scales, unlike the sculpin.
This video shows round gobies in their prefered rocky habitat. Gobies glue each of their eggs to the ceiling of the cavity that they spawn in. Here we have flipped over a rock with attached goby eggs. The black male is attempting to defend its nest from other round gobies that are cannibalizing the eggs. Gobies spawn repeatedly, every 18-20 days, resulting in up to six batches of eggs per breeding season (Charlebois 1997). Round gobies are tolerant of a broad range of environmental conditions including low dissolved oxygen content, water temperatures of -1 to +30 ° C, and salinity (Moskal’kova 1996).
The behavior of the goby has also contributed to its success at colonization. The round goby is a territorial fish that competes aggressively for food, shelter, and spawning areas. This behavior has allowed it to displace native fish, such as the mottled sculpin (Cottus bairdi) (Janssen and Jude 2001) and logperch (Percina caprodes), from some areas of the Great Lakes.
Round gobies are bethic foragers with a diverse diet comprised mostly crustaceans, molluscs, smaller fish and fish eggs (Charlebois 1997). Unfortunately, where round gobies have become abundant they are also altering the invertebrate forage base. Crustaceans such as amphipods and isopods are considerably less abundant at sites with round gobies. This may have a negative impact on young perch and other benthic species which also utilize invertebrates for food. Below are graphs of isopod and amphopod densities at study sites with and without gobies in the Door Pennisula.
The use of zebra mussels in their diet provides gobies with an advantage by giving them a food supply most native fish do not utilize (Manz 1998). Round gobies larger than about 100 mm tend to feed mainly on zebra mussels and alter their size distribution (Djuricich and Janssen 2001). Zebra mussels have refuges from predation beneath rocks, in crevices, and alongside zebra mussels that are too large to be eaten. The rocks along the Door Penisula are mainly smooth flagstones, with little shelter for zebra mussels. Here there is a statistically detectable difference between round goby infested vs. round goby free sites, Several sites in the Door Penninsula were studied in the summer of 2002 and the graph below depicts zebra mussel densities in sites where gobies were abundant versus absent.
Round Gobies are known egg predators. This past spring we set up an underwater camera to see what happens when a smallmouth bass leaves its nest unattended. We pulled the bass of its nest by hook and line and this was the result….
It is almost certain that eggs in a nest with the defending male removed will not survive and it may be that, even if the male is released, there will be much damage to the nest before the male returns and the male may be too exhausted to effectively defend its nest from the horde of round gobies. That will require further study. The Green Bay anglers are very proactive in protecting their sport so, if there is a threat, we think the anglers will act responsibly.
Unfortunately, some anglers have damaged their sport by releasing round gobies. The two known instances are the Flint and Shiawassee Rivers, which are tributary to Saginaw Bay, Lake Huron. In stretches of of both streams, upstream of dams, serious round goby infestations were discovered in 1996. Those fisheries have declined. It is likely that anglers either used round gobies for bait on these streams, or attempted their own fishery management by stocking what the though would be good forage. Unfortunately, the natural and diverse forage has been seriously impacted