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II. Background Information
- How Many Aquatic Nonindigenous Species Are There In The Great Lakes?
- How Are Nonindigenous Species Getting Into The Great Lakes?
- Where Are These Organisms (Species) Coming From?
- What Effect Are These Organisms Having On The Great Lakes?
1. How Many Aquatic Nonindigenous Species Are There In The Great Lakes?
 | | The record of nonindigenous aquatic species established in the Great Lakes in ten-year intervals since 1810. The rate of new invasions has been almost constant since about 1950 and the total is now at least 162. (Ricciardi, 2001). |
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The Great Lakes have a long history of invasion by nonindigenous aquatic species. 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.
Mills et al. (1993) conducted an extensive literature review and documented 139 nonindigenous aquatic species established in the Great Lakes as of 1990. Ricciardi (2001) updated that list via another literature review and revised the total to 162. Of the additional 23 species identified by Ricciardi, 12 appear to have entered since 1990.
The documented number of nonindigenous aquatic species in the Great Lakes 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 adequately sample the system. Most of the nonindigenous species identified to date are large or otherwise conspicuous (e.g., invertebrates and fish). Little attention has been paid to identification of nonindigenous bacteria, viruses, parasites, protozoans and microalgae. Indeed, since Ricciardi's update, a microsporidian new to North America, of the genus Heterosporis, has been reported in eastern Lake Ontario (Personal Communication, Jim Hoyle, OMNR, Picton, ON; Personal Communication, Dan Sutherland, University of Wisconsin). The Largemouth Bass Virus has extended its range into the basin, and its move into the Great Lakes appears inevitable (Personal Communication, John Hnath, MDNR, Matawan, MI). Finally, three species of Asian carp (bighead, silver, and black) are poised to move from the Mississippi River into the Great Lakes. These species, originally raised in farm ponds primarily in Arkansas for plankton and mollusk control in aquaculture, are now in the Mississippi River and are steadily moving northward (Rasmussen 2002). It is not known if the 9 April 2002 activation of an electric barrier on the Chicago Sanitary and Ship Canal will prevent Great Lakes entry.
The scientific community believes there may be many more nonindigenous species already in the lakes that have not been found or have been misidentified as native species. There are no basin-wide coordinated monitoring programs specifically targeting identification (early warning) of new nonindigenous species. Troublingly, the possibility now exists that establishment of new nonindigenous species in the Great Lakes is being facilitated by existing native and nonindigenous species resident in the system (Ricciardi 2001). For example, establishment of zebra mussels in the Great Lakes was followed shortly thereafter by arrival and successful colonization of nonindigenous amphipods and fishes (round and tube-nose gobies). Round gobies have now achieved pest status in the lower Great Lakes, where they live in abundance and steal bait from lines of recreational anglers, and there is concern that they may displace native fishes such as darters and sculpins.
The rate at which new AIS are colonizing the Great Lakes has not declined despite implementation of Canadian ballast water exchange guidelines in 1989, followed by mandatory ballast exchange requirements established in 1993 by the U.S. for ships entering the Great Lakes (see below; Ricciardi 2001). The best indicator of the extent to which humans are intentionally or accidentally introducing AIS to the Great Lakes is provided by Hebert and Cristescu (2002), who calculated that human-mediated dispersal of crustacean zooplankton now exceeds natural dispersal by up to 50,000 fold!
2. How Are Nonindigenous Species Getting Into The Great Lakes?
Mills et al. (1993) summarized the vectors by which AIS have been introduced to the Great Lakes, the primary being ships, unintentional releases, intentional releases, canals, and rail or highway.
Ships' ballast tanks were the major vector for nonindigenous species introductions to the Great Lakes during the 20th century (Mills et al. 1993; Ricciardi 2001). From 1959, when the St. Lawrence Seaway was opened, through 2000, 36 of 50 nonindigenous aquatic species established in the Great Lakes during that time period are attributed to ballast tank transport and discharge of untreated ballast water. New evidence suggests that the residual water and mud found in most "empty" ballast tanks may also be a source for some species invasions (see below). Hull fouling may also be a contributing factor, but is not believed to be a significant vector for new introductions to the Great Lakes. Most freshwater fouling organisms are not expected to survive the osmotic stress caused by prolonged exposure to saline waters during transoceanic voyages.
Unintentional releases 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. There is concern that baitfish use may help to spread the microsporidian Heterosporis into L. Michigan from inland lakes, to the detriment of perch, walleye, and pike fisheries. Likewise bass fishermen may introduce the Largemouth Bass Virus into the Great Lakes.
Canals, the historical vector of some significant exotic species such as sea lamprey and alewife, may once again be active. Three Asian carp species (silver, black, and bighead) are moving up the Mississippi River system and are within striking distance of the Great Lakes. The electric barrier now activated in the Chicago Sanitary Ship Canal is not thought, as presently designed and operated, to be a reliable deterrent to the movement of these fish into the Great Lakes. It was initially designed to deter movements of round goby and may require modifications to prevent movements of Asian carp and other invasive species between the Mississippi River and Great Lakes systems. Bighead carp have been reported from Lake Erie in recent years, though at very low numbers, indicating that the species may not be established yet (Personal Communication, Tim Johnson, OMNR, Wheatley, ON).
3. Where Are These Organisms (Species) Coming From?
Mills et al. (1993) identified the likely geographic source for each of the 139 species they identified. Ricciardi (2001) identified the native geographic region for the additional 23 species he identified. In the following chart we have combined the information from these two references, making the assumption that native geographic region of the species Ricciardi identified was also the source region for the invasion to the Great Lakes (this may not be true in all cases). An exception was made for Daphnia lumholtzi, a waterflea native to Africa, Australia and Asia, but known to have reached the Great Lakes in 1999 from an invaded reservoir in the southern U.S., probably via recreational boating activities.
By far, the majority of aquatic species invading the Great Lakes are native to Eurasia. Since the mid-1980s species native to the Ponto-Caspian basins (Black, Caspian, and Azov Seas) have been remarkably successful in establishing new populations in the Great Lakes. Of 15 new organisms since 1986, 11 are attributed to ballast tank discharges. Of these 11 ballast-implicated organisms, 8 are 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 biomass, abundance and productivity of food webs in the Great Lakes. These species are also moving or being exported to inland lakes and rivers in the U.S. and Canada, where they also have disruptive influences. It would be naive to suggest that 'the worst have arrived' to the Great Lakes, as many additional species, some with prominent histories of invasiveness, are currently spreading throughout Europe; if these species reach key low salinity ports (e.g., Rotterdam, Antwerp, etc.), they may be picked up by ships destined for North America.
 There are at least five invasion corridors from SE Europe that may allow species from the Black, Azov and Caspian Seas to move within Europe and later to the Great Lakes (MacIsaac et al. 2001). Genetic evidence has been used to link AIS in the Great Lakes to different source populations in Eurasia (e.g., Cristescu et al. 2001).
Coastal North Atlantic is the second most important known source for nonindigenous species (e.g., sea lamprey, alewife) in the Great Lakes, although it ranks far behind SE Europe as a source of new species. Examples of AIS from this region include sea lamprey, alewife and blueback herring. Importantly, this region is an historic source of AIS to the Great Lakes, but has recently been supplanted by the Ponto-Caspian region.
4. What Effect Are These Organisms Having On The Great Lakes?
There is accumulating and rather strong evidence supporting the notion that AIS are having dramatic and damaging impacts on the Great Lakes ecosystem. The most obvious change has been the remarkable improvement in water clarity, especially in Lake Erie, much to the delight of recreational users and lakeshore residents. However, such changes, although benefiting some users, can be seriously damaging to the Great Lakes ecosystem, resulting in loss of organisms and biodiversity, disruption of various food webs, and impacts on economically important fish species.
Losses of organisms from the Great Lakes are difficult to detect without a structured monitoring program, however, several species of native invertebrates have shown dramatic declines following the invasion of zebra and quagga mussels. One of the most significant of these has been the decline, and in some areas, depletion, of Diporeia. Diporeia is a deep-water macro-invertebrate that has been a dominant benthic organism since the formation of the Great Lakes at the end of the Ice Age. Populations of Diporeia have declined from 1,000s of animals per square meter to zero in many locations in lakes Michigan (Nalepa et al. 1998), Huron (Personal Communication, Tom Nalepa, NOAA, Ann Arbor, MI), Erie (Dermott and Kerec 1997), and Ontario (Dermott 2001; Lozano et al. 2001) since establishment of zebra and quagga mussels. Moreover, native clams and mussels have also dramatically declined in numbers in response to zebra mussels (Ahlstedt 1994; Nalepa 1994; Schloesser and Nalepa 1994). Evidence suggests that zebra mussels may be out-competing native bivalves for food and may be fouling their shells and causing additional stress to these native species, ultimately causing populations to decline and possibly causing extinction of some of the rarer species.
Aquatic invasive species (AIS) are also being felt throughout the entire food web through the process of food web disruption. AIS change the structure and function of the food web by causing shifts and reductions of important food web components (e.g., Diporeia) or by creating conditions that facilitate change. For example, food web disruption can be directly linked to declines in the body condition of lake whitefish, a commercially valuable fish species in the Great Lakes, following the decline in Diporeia (the primary food resource for lake whitefish). As a result, lake whitefish are becoming thinner and less marketable for the commercial fisheries. Moreover, declines in the popular yellow perch population in Lake Michigan followed the establishment of zebra mussels, but like the decline in Diporeia, a direct cause and effect relationship has not been established. Aquatic invasive species facilitation of ecosystem change also appears to be responsible for the increased frequency of toxic algal blooms (Microcystis) in the Great Lakes (Vanderploeg et al. 2001), where zebra mussels may be selectively rejecting bluegreen algae as food, while removing competing algae instead.
The accumulation of disruptions of the food web, caused by AIS, may also manifest itself in the ability of an ecosystem (i.e., the Great Lakes) to support the total fish community. For example, recent evidence suggests that zebra mussels have decreased the efficiency at which carbon and energy are transferred up the food web. The implication is that less energy is making its way up the food web to support the popular recreational fisheries (Personal Communication, Doran Mason, NOAA, Ann Arbor, MI). If the three species of Asian carp (see Item 1, above) become established in the Great Lakes, even more food web disruption is anticipated since bighead and silver carp are voracious feeders on plankton and black carp consume mollusks and crustaceans, and they range in size from 40 to over 100 lb. (Rasmussen 2002).
In summary, 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 appear to be affecting both the commercial and recreational resources for which the Great Lakes are best known.
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