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3 Aquatic Invasive Species in the Great Lakes This chapter provides an overview of aquatic invasive species (AIS) in the Laurentian Great Lakes to set the context for the commit- teeâs examination of ways to eliminate further introductions of AIS by vessels transiting the St. Lawrence Seaway. After a brief histor- ical overview, invasion vectors and pathways are discussed, with emphasis on the ballast water vector, which has played a predom- inant role in ship-mediated AIS introductions into the Great Lakes since the opening of the modern seaway in 1959. The impacts of AIS are considered, with particular reference to the zebra mussel, one of the high-proï¬le invaders. Other AIS that have attracted less study, and in many cases less public attention, are also discussed. Historical trends in AIS introductions are important in assessing the effectiveness of measures taken to prevent such introductions, and the challenges in interpreting observed trends are examined. Brief remarks about future AIS introductions are made, and the chapter concludes with a summary of key points pertaining to the committeeâs task. HISTORICAL OVERVIEW The Laurentian Great Lakes have an extensive history of human- mediated introductions of AIS, both intentional and unintentional, beginning almost 200 years ago. Precise numbers of introduc- tions are hard to determine, given the challenges of finding and 43
44 Great Lakes Shipping, Trade, and Aquatic Invasive Species identifying some new species and verifying their nonnative status, as well as uncertainties associated with sampling. As discussed later, confusion may also result over the inclusion or exclusion of micro- scopic organisms such as viruses, bacteria, parasites, or protozoans (Drake et al. 2007). The number of reported AIS established in the Great Lakes has, however, increased substantially from the early 19th century to modern times, with current estimates ranging from a low of 136 to more than 180 species of nonnative algae, ï¬sh, in- vertebrates, and plants (U.S. Geological Survey n.d.; Ricciardi 2006). During the period of human-mediated biological invasions, a number of transitions have occurred with respect to both the types of AIS that have established and the mechanisms by which they en- tered the Great Lakes. Fish and plants were the most common in- vaders before the 20th century, with most introductions resulting from human releases (Mills et al. 1993). Algae and invertebrates be- came more common invaders after transoceanic shipping converted to use of liquid ballast around 1900. Shipping appears to have be- come the dominant means by which AIS have been transported into the Great Lakes during much of the 20th century, and notably since the opening of the St. Lawrence Seaway in 1959. The origins are less certain for invasive plants that now dominate coastal wetlands in highly urbanized areas, such as Lake Michiganâs lower Green Bay. INVASION VECTORS AND PATHWAYS In invasion biology, a vector is deï¬ned as the physical means or agent by which a species is transported. There are multiple vectors by which AIS gain access to the Great Lakes, including commercial shipping, recreational boating, angling or bait ï¬shing, aquaculture, commercial and home aquaria, water gardens, canals, and rivers. For the purposes of this report, an invasion pathway is deï¬ned as the geographic path over which a species is transported from its origin (donor area) to its destination (target area). An analysis commissioned by the committee examined pathways of introduction for AIS reported as being established in the Great Lakes since the
Aquatic Invasive Species in the Great Lakes 45 opening of the St. Lawrence Seaway in 1959 (Kelly 2007). Informa- tion on the most probable geographic sources of species and the vector assignments showed Eurasia to be the dominant source, accounting for 67 percent of established AIS, followed by North America with 14 percent; 15 percent had unknown or widespread origins, and Australasia and Africa each contributed a single species. Committeeâs Charge The committee was asked to identify and explore options for elim- inating âfurther introductions of nonindigenous aquatic species into the Great Lakes by vessels transiting the St. Lawrence Seaway.â Thus, its charge was limited to one speciï¬c invasion route into the Great Lakes (the St. Lawrence Seaway) and addressed only non- indigenous aquatic species (i.e., AIS) and not terrestrial invasive species. Most AIS that have drawn attention in recent decades in- habit benthic or pelagic areas of the Great Lakes, although the sys- tem has been invaded by and remains vulnerable to invasions by wetland species such as hybrid cattail (Typha x glauca). In addi- tion, the committeeâs charge was restricted to one group of vectors by which AIS enter the Great Lakes, namely, those associated with shipping (vessels). There are many dispersal vectors for the transport of organisms on board ships, but the most important is widely acknowledged to be ballast water, which is needed for vessels to operate safely (NRC 1996). As discussed in Chapter 4, the mechanisms by which AIS enter the Great Lakes in shipsâ ballast water have been extensively investigated. Hull fouling is another vector by which vessels can transport AIS, although this vector is thought to have played a minor role compared with ballast water in introducing AIS into the Great Lakes (see later). Importance of the Ballast Water Vector To help set the context for its work, the committee commissioned the aforementioned analysis of AIS reported as being established in the Great Lakes since the opening of the St. Lawrence Seaway in
46 Great Lakes Shipping, Trade, and Aquatic Invasive Species 1959 (Kelly 2007). By means of a series of criteria to determine in- troduced status, 59 species were identiï¬ed as nonindigenous, with an additional 21 species (mainly algae) classiï¬ed as cryptogenic (not clearly introduced or native). Shipsâ ballast water was the leading vector for approximately 55 percent of species identiï¬ed as nonindigenous.1 Deliberate releases, unauthorized introductions, range extensions, hull fouling, and recreational boating were all of lesser importance, each representing less than 10 percent of species. A clear vector could not be established for 11 percent of the conï¬rmed AIS. Other less conservative analyses suggest that shipsâ ballast water may account for as much as 65 or 70 percent of the total documented inventory of AIS in the Great Lakes (Ricciardi 2006; Holeck et al. 2004). In summary, the evidence points to shipsâ ballast water as the leading vector for recorded introductions of AIS into the Great Lakes since the opening of the seaway, with other vectors collectively accounting for the remaining 30 to 45 per- cent of introductions. Hence, eliminating further AIS introductions by vessels transiting the seaway, as specified in the committeeâs charge, would be expected to have an important impact on the AIS problem. However, data on the relative importance of different invasion vectors suggest that this action would not prevent all further AIS introductions into the Great Lakes. Hull Fouling Vector Ballast water, as well as residual ballast water and sediment, has been well characterized for transoceanic vessels entering the Great Lakes.2 In contrast, there is a dearth of information with regard to the risk of AIS introductions posed by hull fouling on such vessels. Marine-to-marine introductions on hulls of ships have occurred for hundreds of years, and studies conducted in marine 1 The analysis showed Europe to be the source of 94 percent of ballast-mediated invasions, includ- ing 15 species from western and central Europe, 12 from the Ponto-Caspian region, and three from the Baltic Sea. 2 As discussed in Chapter 4, transoceanic vessels (i.e., those engaged in international trade) are the majorâbut not the onlyâsource of ship-vectored AIS introductions into the Great Lakes.
Aquatic Invasive Species in the Great Lakes 47 and estuarine habitats have demonstrated that hull fouling is as, or more, important than ballast water to the introduction of non- indigenous species (see, for example, Gollasch 2002). However, ecologists have perceived the Great Lakes as less at risk of intro- ductions via the hull fouling vector owing to the required transfer of fouling species from a freshwater course (the vesselâs port of ori- gin), across an inhospitable saline medium (the Atlantic Ocean), to a freshwater destination (the Great Lakes). To date, only one study has explored the role of the hull fouling vector for the Great Lakes. Drake and Lodge (2007) examined a single ship that required dry-docking on Lake Ontario for emergency repairs. This vessel had just been purchased âas isâ after extended stays in Algeria and Chile before entering the Great Lakes and was not, therefore, rep- resentative of Great Lakes vessels in general. A large number of species (29) were found encrusted on a small surface area of the vessel hull, suggesting that the total number of hull fouling species could be as many as 200. However, further analyses of the species found on the hull determined that only eight are not presently re- ported in the Great Lakes and, of these, only two appear capable of survival in freshwater.3 Additional examinations of exterior surfaces (e.g., hulls and rudders) of vessels entering the Great Lakes will be needed to help determine whether hull fouling is an important mech- anism for AIS introductions. Absent a solid body of evidence indi- cating the importance of the hull fouling vector, the committeeâs analysis of options for preventing further introductions of AIS by ves- sels transiting the St. Lawrence Seaway focused on ballast water as the predominant vector associated with shipping into the Great Lakes. IMPACTS OF AIS Judgments about the impacts of AIS are frequently anecdotal and qualitative, and the impacts of many species have not been studied explicitly. However, available data demonstrate clearly that some 3 Personal communication from Sarah Bailey, Fisheries and Oceans Canada, to Hugh MacIsaac, committee member, January 2008.
48 Great Lakes Shipping, Trade, and Aquatic Invasive Species AIS have had severe economic or ecological inï¬uences in the Great Lakes. The most problematic invasive species include alewife, com- mon carp, Eurasian ruffe, Eurasian water milfoil, purple loosestrife, zebra and quagga mussels, rainbow smelt, round goby, rusty cray- ï¬sh, sea lamprey, and spiny and ï¬shhook waterï¬eas (U.S. Geologi- cal Survey n.d.). These species alone have contributed to extirpations and some extinctions of native taxa, along with severe alterations in local food webs. In addition, invasive plants in Great Lakes coastal wetlands, such as Typha and Phalaris arundinacea, have resulted in the exclusion of many native species and declines in plant species richness and evenness of distribution. The following section summarizes the impacts of one of the Great Lakesâ highest-proï¬le invadersâthe zebra mussel. Other AIS that have attracted less study are then discussed, with particu- lar reference to plants and to viruses and bacteria. Zebra Mussel The zebra mussel (Dreissena polymorpha) has been one of the most successful Great Lakes invaders. Most estimates of the costs of ship- vectored introductions of AIS into the Great Lakes have focused on this species and on the costs of cleaning infrastructure depen- dent on raw lake water, such as power generation plants, public and private drinking water plants, industrial facilities, navigation lock and dam structures, marinas, and golf courses. One expert re- ported to the committee that the total cost of zebra mussel cleanup during the period 1989â2004 has been tentatively estimated at $1 billion to $1.5 billion.4 Other reports put the estimate as high as $5 billion for cleanup since the discovery of the zebra mussel in 1988 (Lovell and Stone 2005). In addition to its economic impacts, the zebra mussel has had both direct and indirect effects on the Great Lakes ecosystem. Zebra mussels are efï¬cient ï¬lter feeders, competing directly with indige- 4 Zebra Mussel Economic Impacts, 1989â2006. Presentation to the committee by Chuck OâNeill, Cornell University/New York Sea Grant, May 24, 2006.
Aquatic Invasive Species in the Great Lakes 49 nous mussel species for food and space. Zebra mussel numbers, particularly in Lake Erie, have reached such levels that concentra- tions of particulate matter in the lake (much of it phytoplankton, also the basis of the pelagic food web) have fallen below levels pre- dicted by phosphorus availability. Redirection of algal biomass to benthic from pelagic food webs has implications for both rates of nutrient recycling and the efï¬ciency with which energy makes its way up the food web to support the ï¬shery. The âbenthiï¬cationâ of the Great Lakes by dreissenid mussels portends greater energy and resources for many benthic species of animals and plants and less for planktonic species (Hecky et al. 2004). The invasion of the Great Lakes by the zebra mussel, and by the quagga mussel, has also had indirect effects. Reductions in sus- pended particles have increased water clarity, allowing deeper penetrations of light into the water column. This, in turn, fosters growth of nuisance, attached algae (Cladophora glomerata) along shorelines. Rooted aquatic vegetation has increased dramatically in shallow areas of the lakes. These changes are facilitating numer- ous benthic invertebrates, including the rapid spread of a âPonto- Caspianâ associate of the zebra mussel, the benthic amphipod Echinogammarus ischnus (Vanderploeg et al. 2002). In addition, because the biomass of zebra mussel populations is orders of mag- nitude higher than that of native mussel populations, they have an important impact on contaminant dynamics in invaded lakes. As suspension feeders, zebra mussels bio-accumulate metals and or- ganic contaminants, and the transfer of these contaminants up the food chain to waterfowl and ï¬sh that eat mussels has the potential to change contaminant cycling in the system signiï¬cantly (see, for example, Mazak et al. 1997). The zebra mussel was almost certainly introduced into the Great Lakes in freshwater ballast discharged by vessels engaged in inter- national trade (see Chapter 4). Since its discovery in Lake St. Clair in 1988, the zebra mussel has spread throughout the Great Lakes, into hundreds of small inland lakes and rivers in most of the states and provinces bordering the Great Lakes, and into the Mississippi River, and it can now be found all the way down to New Orleans.
50 Great Lakes Shipping, Trade, and Aquatic Invasive Species In January 2008, the species was found in San Justo Reservoir in San Benito County, California. This rapid expansion has highlighted concerns about the spread of invaders as a result of the inter- connectivity of waters through canals, boat trafï¬c, and recreational practices. Less Studied AIS While invaders such as the zebra mussel are unlikely to go unno- ticed because of their abundance, size, and readily observable and widespread impacts, the same cannot be said of all AIS. For exam- ple, analyses of beach sand revealed introduced species of benthic copepods and testate rhizopods, species that are both inconspicu- ous and low impact (Horvath et al. 2001; Nicholls and MacIsaac 2004). Moreover, interest in and study of AIS have not historically been even across taxa, as illustrated by the following discussion of plants and of viruses and bacteria. Plants Gollasch et al. (2007) report that the ï¬rst biological study to infer ballast water as a vector for nonindigenous species introductions concerned phytoplankton.5 In the 100 years since that ï¬rst study, however, research on shipping as a vector for AIS has shifted sub- stantially toward animals. Where data on plants are available, the primary emphasis has been on macrophytes, with some attention to benthic algal species, such as the widely publicized spread of the macroalgae Caulerpa taxifolia in the Mediterranean, rather than microscopic pelagic phytoplankton. In the Great Lakes, the intro- duced diatom Thallossiosira baltica evaded detection from 1988 until 1994, even though it was a dominant member of the phytoplankton community (Edlund et al. 2000). Those reports that do consider phytoplankton are principally focused on marine-to-marine trans- fers. There is general agreement that ballast water, or ballast tank 5 Publishing in the inaugural volume of the journal Plankton in 1908, C. H. Ostenfeld reported a mass occurrence of the Asian phytoplankton species Odontella sinensis in the North Sea, which he believed originated in either the Red Sea or the Indian Ocean (www.jncc.gov.uk/page-1663).
Aquatic Invasive Species in the Great Lakes 51 sediment, could act as a vector for introduction of phytoplankton, and the appearance of novel algal species in the Great Lakes has been documented (see, for example, Mills et al. 1993), but it is un- clear whether these taxa arrived via ballast water or on ship hulls. Data on the potential ecological signiï¬cance of exotic algae for the Great Lakes are lacking, although studies on marine systems sug- gest that they could pose threats to aquaculture, aquatic food webs, and human health. Thus, while the impacts of high-proï¬le AIS in the Great Lakes are well documented, the impacts of relatively in- conspicuous AIS, particularly plants, have not attracted the same degree of attention. Viruses and Bacteria A further area that has attracted relatively little attention until re- cently is invasions by aquatic microorganisms, such as viruses and bacteria. These microorganisms are orders of magnitude more abundant in aquatic systems than macroorganisms such as ï¬sh and crustaceans, indicating that large numbers could enter shipsâ ballast tanks during normal operations (Dobbs and Rogerson 2005). Mean abundances of 8.3 Ã 108 bacteria per liter and 7.4 Ã 109 viruslike particles per liter have been reported in the ballast water of vessels entering the Chesapeake Bay from foreign ports (Ruiz et al. 2000). The invasion biology of aquatic microorganisms is not nearly as well studied as that of vertebrates and macroinvertebrates (Drake et al. 2007), perhaps in part because of the sophisticated micro- biological investigation techniques required. Moreover, some re- searchers have questioned whether free-living microorganisms should be designated as invasive species at all, arguing that these species are found everywhere and that their distribution is not spe- ciï¬c to certain geographical areas (Finlay 2002).6, 7 Other researchers 6 This argument applies only to free-living microorganisms. In the case of a virus attached to a host, for example, if the host has a biogeography (i.e., has a distribution that is speciï¬c to certain geo- graphical areas), the virus will have one too. 7 In discussing bacteria, for example, Fenchel notes that âevery habitat will contain a pool of bacte- rial species that do not thrive locally, but may grow if the environment becomes more favorableâ (2003, 925).
52 Great Lakes Shipping, Trade, and Aquatic Invasive Species dispute the argument that âeverything is everywhereâ and point to a large body of work supporting the idea that free-living micro- bial taxa exhibit biogeographic patterns (see, for example, Martiny et al. 2006).8 The debate about the invasive nature of microorganisms is di- rectly relevant to efforts aimed at preventing further âintroduc- tionsâ of aquatic microorganisms, such as the virus responsible for the ï¬sh disease viral hemorrhagic septicemia (VHS) (see Box 3-1), into the Great Lakes. In light of this ongoing debate and the nu- merous unknowns and uncertainties with regard to the appear- ance of VHS in ï¬sh populations in the Great Lakes, it remains unclear whether shipsâ ballast water played any role in the recent VHS outbreak. However, in view of the natural abundance and widespread distribution of aquatic microorganisms, the commit- tee sees merit in the approach taken by the International Maritime Organization, which differentiates microorganisms from other AIS in its proposed ballast water performance standard (IMO 2004). Some of the issues related to this differentiation are discussed in a recent paper by Dobbs and Rogerson (2005).9 EXAMINING TRENDS IN INVASION HISTORY Curves showing the cumulative number of AIS in an ecosystem as a function of time (cumulative invasion curves) are often used to illustrate trends in invasion history and sometimes to assess the 8 These authors do not consider the question of whether the distribution of viruses is speciï¬c to cer- tain geographic areas because âtheir biology adds further complications and, in most cases, far less is known about their distribution than that of other microorganismsâ (p. 103). 9 The results of an analysis commissioned by the committee suggest that eliminating viruses from shipsâ ballast water may be a daunting task because of their small size and their possible abundance in shipsâ ballast tanks (Kelly and Kazumi 2007). These authors note, however, that the VHS virus in a free, non-host-associated form can be inactivated by a variety of means. To control its spread in aquaculture facilities, for example, fish eggs are usually treated with iodophors and hatchery waters with ultraviolet irradiation. If the VHS virus is carried on an infected ï¬sh host rather than being in an unassociated form, physical treatment technologies such as screens and media ï¬lters could be used to remove both ï¬sh and virus from ballast water.
Aquatic Invasive Species in the Great Lakes 53 BOX 3-1 Viral Hemorrhagic Septicemia VHS is a disease that can cause ï¬sh to hemorrhage, resulting in rapid mortality. Classiï¬ed as a reportable disease by the World Organization for Animal Health because of its high mortality and severe economic consequences (U.S. Department of Agriculture 2006), the virus responsible for VHS poses no known health threat to humans. How long the VHS virus has been present in the Great Lakes ecosystem is unknown. VHS was ï¬rst observed in the Great Lakes in 2005, when it caused ï¬sh die-offs in Lake Ontario and Lake St. Clair, but the virus may have entered the Great Lakes much earlier, perhaps in 2002 or 2003. In 2006 the disease was detected in an increasing number of ï¬sh in Lake Erie, and in 2007 it was conï¬rmed in Lake Huron. Although the exact pathway of VHS infection is at present unknown, restrictions have been imposed on the movement of live ï¬sh in an effort to prevent (or limit) the spread of the disease. Fish die-offs, together with these restrictions, have had serious implications for the Great Lakes commercial and sport ï¬shing industries. Several strains of the VHS virus are known to affect freshwater and marine ï¬sh around the world. In North America, strains are found in the marine and estuarine waters of the Paciï¬c and Atlantic Oceans. The VHS virus observed on ï¬sh in the Great Lakes is a North American strain and is most closely related to that detected in marine ï¬sh within waters of the Atlantic and eastern Gulf of St. Lawrence (Fisheries and Oceans Canada n.d.). As noted in the text, there is an ongoing debate about the âinvasiveâ status of aquatic microorganisms, including viruses. If the VHS virus in the Great Lakes is indeed an AIS, vectors of introduction could include migrating ï¬sh (Elsayed et al. 2006), commercial aquaculture operations, and the ballast water of vessels entering the Great Lakes from marine waters in eastern North Americaa (see, for example, (continued)
54 Great Lakes Shipping, Trade, and Aquatic Invasive Species BOX 3-1 (continued) Whelan 2007). As yet, none of these alternative vectors has been conï¬rmed or excluded from further consideration. Despite these uncertainties, some organizations have already taken precautionary measures aimed at preventing or slowing the possible spread of the virus by shipsâ ballast water. In March 2007, the Lake Carriersâ Association introduced voluntary ballast water manage- ment measures to combat the spread of VHS by U.S.-ï¬agged lakers operating within the Great Lakes, and in September 2007, the U.S. National Park Service closed Lake Superior waters within the boundaries of Isle Royale National Park to the release of untreated ballast water (Lake Carriersâ Association 2007; Lake Superior Magazine 2007). a In the case of the ballast water vector, the virus could have entered the Great Lakes either suspended in ballast water or on an infected ï¬sh transported in ballast water. effectiveness of measures aimed at preventing invasions. The com- mittee, however, chose not to characterize the progressive invasion of the Great Lakes by AIS by using such a curve because of two in- herent sources of uncertainty: (a) time lags, which vary by taxa and possibly also by decade of study, and (b) the level of investigator interest. Time Lags The time scale on typical invasion curves does not reflect the time of entry of an AIS into the Great Lakes. Instead, the date assigned is the reported date of identification of the species. There is an unknown time lag between the date of entry and the date of iden- tification, the duration of which depends on a variety of factors. A time lag could result from the need to build populations suf- ï¬ciently large to be detected, assuming adequate sampling. Under this scenario, lags depend on the nature and intrinsic life history
Aquatic Invasive Species in the Great Lakes 55 of the species. Microbes, for example, are far less likely to be re- ported than larger organisms unless, like VHS, their impacts are relatively widespread and readily observable. Lags also depend on extrinsic factors, such as initial propagule pressure,10 extant tem- perature, or facilitation of new invaders by preoccurring ones (see, for example, Vanderploeg et al. 2002). Alternatively, a small population could persist undetected at a relatively low level for a considerable period of time until the emergence of a superbly adapted local genetic variant results in the establishment of a de- tectable population.11 The nature of the time lag for any particular species inï¬uences the pattern of cumulative numbers of invasions over time. Wonham and Carlton (2005) demonstrated that the cumulative number of invasions in the northeast Paciï¬c Ocean increased at linear, qua- dratic, and exponential rates depending on taxa, invasive path- ways, and spatial scales. Consequently, these authors argue against conclusive statements with regard to invasion rate that are based on reported discovery rates. Investigator Interest A second important cause of uncertainty in estimating the arrival time of diverse species resides in the number of expert investiga- tors engaged in both searching for and identifying the broad spec- trum of potential AIS. The total effort engaged in AIS identiï¬cation includes lay observers, such as anglers, boaters, and others partic- ipating in recreational pursuits, as well as trained professionals. It is the latter, however, who possess the knowledge and experience to distinguish invaders from native speciesâa challenging task when species may be distinguishable only by subtle differences or, 10 Propagule pressure, also referred to as âintroduction effect,â depends on the number of intro- duction events, the number of propagules introduced per event, and the condition of the propa- gules upon release. 11 See, for example, Lee 2002, in which the author identiï¬es a reasonable number of citations sup- porting the role of genetic attributes in invasion success.
56 Great Lakes Shipping, Trade, and Aquatic Invasive Species in some cases, only at a genetic level. The number of such profes- sional specialists actively engaged in looking for and identifying in- vasive species is likely to be variable and limited, with resulting variability in the rate of discovery of new AIS. Global interest in invasive species in general over the past few decades has grown (Ricciardi and MacIsaac 2008), and it is safe to assume a parallel growth in interest in and study of AIS in the Great Lakes region. However, this interest may not be even across taxa, which may explain why many more introduced invertebrate species than microbial species have been described in the Great Lakes. As noted earlier, the committee recognizes clearly that the num- ber of AIS in the Great Lakes has increased over time and that the number of such species introduced by shipsâ ballast water is be- tween 55 and 70 percent of AIS introduced since the opening of the seaway. However, it has not attempted to draw on a cumulative in- vasion curve to interpret the dynamics of the invasive species biol- ogy or draw inferences about policy options. The invasion curve is too uncertain a tool to support such conjecture and is not needed to formulate strong policies at this time. FUTURE AIS INTRODUCTIONS The vectors and pathways by which AIS enter the Great Lakes are continuously changing. In the case of the shipping vector through the seaway, for example, changing trade patterns and routes may provide opportunities for AIS from new donor areas to enter the Great Lakes or open up alternative pathways for AIS from known donor areas. Thus, Kelly (2007) reports that the 1992 opening of the Main Canal connecting the Danube with the Rhine system provided a new westward colonization pathway for AIS from the Black Sea basin. Changes in the target area (the Great Lakes), such as those associated with climate change, may also result in changes in the pattern of AIS introductions. The nature of changes that could affect future AIS introductions into the Great Lakes is often difï¬cult to anticipate, which adds to
Aquatic Invasive Species in the Great Lakes 57 the challenge of eliminating further AIS introductions. However, some progress in identifying the most likely species and sites for further introductions has been made. The following paragraphs provide a brief overview of approaches to identifying âhot species, hot spots, and hot momentsâ for future AIS introductions and note some possible effects of climate change on AIS introductions into the Great Lakes. Hot Species, Hot Spots, and Hot Moments A paper commissioned by the committee in support of its work notes that there have been a number of recent efforts to identify new AIS most likely to establish populations in the Great Lakes (âhot speciesâ), as well as locations that may be particularly sus- ceptible to such AIS introductions (âhot spotsâ) (Vander Zanden 2007 and references therein). The paper also discusses the concept of âhot moments,â or windows of opportunity, when introduc- tions are most likely to occur. Efforts to identify hot species have used risk assessment meth- ods, together with information on invasion corridors,12 vectors, patterns of invasion, environmental match between donor and recipient regions, and other factors likely to affect the probability of successful introduction. Although such efforts are widely acknowl- edged to have limitations, they have had at least one notable suc- cess. Hemimysis anomala was identified as having a high risk of introduction into the Great Lakes several years before its discovery in Lakes Michigan and Ontario in 2006 (Ricciardi and Rasmussen 1998; Grigorovich, Colautti, et al. 2003). The concept of hot spots appears to have attracted less attention than that of hot species. However, areas with high concentrations of new AIS, as well as invasion âcold spots,â have been reported (Grigorovich, Korniushin, et al. 2003), indicating that some loca- tions within the Great Lakes ecosystem are seemingly more sus- ceptible to invasion than others. 12 An invasion corridor is a physical conduit over or through which a series of species moves en route from a common origin to a common destination.
58 Great Lakes Shipping, Trade, and Aquatic Invasive Species Possible Effects of Climate Change Biological responses to climate change are complex and difï¬cult to predict. In addition to changes in the occurrence and distribution of current resident species in the Great Lakes, changes are expected in the survival rates of new AIS. As a result of the higher water tem- peratures arising from climate change, northward movements of warm water species and the extirpation of cooler water species are anticipated. Warmer water temperatures may also speed rates of growth and development, increasing the possibility that minimum viable populations of AIS will establish before the onset of un- favorable winter conditions (see, for example, Millerd 2007; Great Lakes Water Quality Board 2003; Kling et al. 2003). CONCLUDING REMARKS Shipsâ ballast water has historically been the major vector for AIS introductions into the Great Lakes since the opening of the seaway in 1959, and during the ensuing period of almost 50 years it has ac- counted for 55 to 70 percent of reported AIS introductions from all sources. Current evidence indicates that the hull fouling vector has played a very minor role in introducing AIS into the freshwater ecosystem of the Great Lakes. Eliminating AIS introductions by vessels transiting the seaway would have an important impact on the AIS problem in the Great Lakes but would not eliminate all further AIS introductions. A number of high-proï¬le AIS introductions, such as that of the zebra mussel, have had major economic and ecological impacts on the Great Lakes region and have been extensively studied. Other AIS introductions are less readily observable or less widespread and are often less well characterized. The interpretation of historical trends in AIS introductions is challenging because of the variable time lags between the intro- duction and reporting of a new AIS and the variable levels of inves- tigator interest, both over time and across taxa. Thus, linking
Aquatic Invasive Species in the Great Lakes 59 observed trends to measures aimed at preventing further introduc- tions is complicated. In seeking to eliminate further ship-vectored AIS introductions through the seaway, as required by the com- mitteeâs task, the continuously changing nature of both invasion pathways and the Great Lakes ecosystem must be recognized. Thus, adaptive elimination strategies will likely be needed to respond to the ever-changing threat of AIS introductions. REFERENCES Abbreviations IMO International Maritime Organization NRC National Research Council Dobbs, F. C., and A. Rogerson. 2005. Ridding Shipsâ Ballast Water of Microorganisms. Environmental Science and Technology, Vol. 39, June 15, pp. 259â264. Drake, J. M., and D. M. Lodge. 2007. Hull Fouling Is a Risk Factor for Intercontinental Species Exchange in Aquatic Ecosystems. Aquatic Invasions, Vol. 2, No. 2, pp. 127â137. Drake, L. A., M. A. Doblin, and F. C. Dobbs. 2007. Potential Microbial Bioinvasions via Shipsâ Ballast Water, Sediment and Biofilm. Marine Pollution Bulletin, Vol. 55, pp. 333â341. Edlund, M. B., C. M. Taylor, C. L. Schelske, and E. F. Stoermer. 2000. Thalassiosira baltica (Grunow) Ostenfeld (Bacillariophyta), a New Exotic Species in the Great Lakes. Canadian Journal of Fisheries and Aquatic Sciences, Vol. 57, pp. 610â615. Elsayed, E., M. Faisal, M. Thomas, G. Whelan, W. Batts, and J. Winton. 2006. Isolation of Viral Haemorrhagic Septicaemia Virus from Muskellunge, Esox Masquinongy (Mitchell), in Lake St. Clair, USA Reveals a New Sublineage of the North American Genotype. Journal of Fish Diseases, Vol. 29, pp. 611â619. Fenchel, T. 2003. Biogeography for Bacteria. Science, Vol. 301, Aug. 15, pp. 925â926. Finlay, B. J. 2002. Global Dispersal of Free-Living Microbial Eukaryote Species. Science, Vol. 296, May 10, pp. 1061â1063. Fisheries and Oceans Canada. n.d. National Aquatic Animal Health ProgramâFAQs, Viral Hemorrhagic Septicemia (VHS) in Various Great Lakes Fish Species. www.dfo-mpo.gc.ca/science/aquaculture/aah/VHS_FAQ_e.htm. Gollasch, S. 2002. The Importance of Ship Hull Fouling as a Vector of Species Introduc- tions into the North Sea. Biofouling, Vol. 18, No. 2, pp. 105â121.
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