The Great Lakes basin is the aquatic gateway to the heartland of North America and a hot spot for aquatic species introductions to major interior sections of the U.S. and Canada. Nonindigenous species pose one of the most serious and difficult threats to the Great Lakes today.

How Many Aquatic Nonindigenous Species are there in the Great Lakes?

The earliest record of an aquatic species invasion in the Great Lakes is the sea lamprey, first entering the Great Lakes from the Atlantic Ocean via the Erie Canal during the 1820s. Extensive literature reviews have documented 162 aquatic nonindigenous aquatic species established in the Great Lakes as of 2000, representing fishes, invertebrates, aquatic plants, algae, and pathogens. This number is best interpreted as a minimum. Identification depends on our ability to find, recognize, verify, and document new species, which is, in turn, dependent on our ability to sample the system, and how often.

How are Nonindigenous Species Getting into the Great Lakes?

Ships' ballast tanks were the major vector for nonindigenous species introductions to the Great Lakes during the latter half of the 20th century. From 1959, when the St. Lawrence Seaway opened, through 2000, 36 of 50 nonindigenous aquatic species established in the Great Lakes are attributed to ballast tank transport and discharge of untreated ballast water. New evidence suggests that the residual water and mud found in many "empty" ballast tanks (i.e., "no-ballast-on-board" or "NOBOB" tanks) is also a potential source for some species invasions. Hull fouling is not thought to be a significant vector for new introductions to the Great Lakes, because most freshwater fouling organisms would not survive prolonged exposure to salt water during transoceanic voyages. Unintentional releases have made the second highest contribution of nonindigenous aquatic species to the Great Lakes. This includes escape from aquaculture sites and the aquarium and bait trades. Canals connecting watersheds and river systems were responsible for some significant historical introductions of exotic species such as sea lamprey and alewife. Presently (Sept 2002), the Chicago Sanitary and Ship Canal, which connects the Mississippi River system with Lake Michigan, is being watched with great concern as a probable route for three Asian carp species (silver, black, and bighead) moving up the Mississippi River system. These species are considered a major threat to the biological integrity of the Great Lakes.

By far, the majority of aquatic species that have invaded the Great Lakes are native to Eurasia. More recently, species native to the Ponto-Caspian basins (Black, Caspian, and Azov Seas) of eastern Europe have been remarkably successful in the Great Lakes. Of 11 ballast-implicated invasions since 1986, 8 are by Ponto-Caspian species. Prominent among Ponto-Caspian invaders are zebra mussels, quagga mussels, round gobies, fishhook waterfleas and Echinogammarus amphipods. These Ponto-Caspian taxa now constitute a very significant component of the biomass and productivity of Great Lakes' food webs. Coastal North Atlantic is the second largest known source for nonindigenous species in the Great Lakes, although it ranks far behind Eurasia. Examples of AIS from this region include sea lamprey, alewife, and blueback herring.

What Effect are these Organisms Having on the Great Lakes?

There have been major negative impacts on the Great Lakes ecosystem that appear to be directly, and indirectly, linked to the establishment of AIS. These impacts have appeared at every level of the food web and are affecting both the commercial and recreational resources for which the Great Lakes are best known. Examples include:

• Loss of the native lake trout population in the early 1900s attributed to the sea lamprey.
• The remarkable improvement in water clarity, especially in Lake Erie, representing rapid removal of the "pelagic" (i.e., water column) food base by zebra mussels.
• The disappearance from Lakes Michigan, Huron, Erie, and Ontario of a small shrimp-like invertebrate called Diporeia (pronounced "Die poor eye ah") which has been in the Great Lakes since their formation at the end of the Ice Age.
• Native clam and mussel populations dramatically declined due to zebra mussels.
• Poor body condition of lake whitefish, a commercially valuable fish in the Great Lakes, probably due to the decline in Diporeia (the primary food for lake whitefish).
• Declines in the yellow perch population in Lake Michigan appear to be associated with one or more invasive species, and increased frequency of toxic algal blooms (Microcystis) in the Great Lakes, apparently associated with zebra mussels.
• Clogging of fishing nets by the fishhook waterflea increases costs for commercial fishermen. If any of the Asian carp (see front page) become established in the Great Lakes, even more food web disruptions are anticipated since bighead and silver carp are voracious feeders on plankton and black carp consume mollusks and crustaceans.