Over the last 200 years, the fish fauna of the Great Lakes has changed significantly as a result of declines in native species and the deliberate and inadvertent stocking of non-native fishes. These changes have resulted in the global extinction of three taxa and the extirpation of 18 species lowering the number of currently extant native species in the Great Lakes basin from 169 to 148 species. A further 82 species have declined to the point of endangerment in at least one jurisdiction in the basin. The causes of these declines are primarily habitat alterations, aquatic invasive species, and overexploitation. Some significant changes in the fish fauna of the Great Lakes basin have also been the result of the successful introduction and establishment of 35 non-native species. In addition, 34 non-native species have been found in the basin, but have not, or are not thought to have, established reproducing populations. These species have been introduced through a variety of pathways including commercial shipping, dispersal, live trade, recreational boating and angling, and stocking. Many of these species have substantially impacted the Great Lakes ecosystem directly through predation and competition or indirectly through trophic disruption and disease transmission. The relative importance of pathways as a source of new introductions has changed over time, and can be expected to continue to change as a result of evolving regulations and trade patterns. The fish fauna of the Great Lakes basin will continue to change as the result of continuing threats to native species and ongoing introductions of non-native species, and such change will undoubtedly be influenced by climate change and human population growth.

Introduction

Global fish faunas have changed dramatically as a result of losses of species from, and the movement of species beyond, their native ranges (Helfman, 2007). The fish fauna of the Great Lakes is no exception. It has changed significantly since, and as a result of, the arrival of Europeans. Europeans brought stocked introduced fishes (e.g. Common Carp, Brown Trout; common names according to Nelson et al., 2004; see Appendix for scientific names) that they preferred to angle and eat. They also brought technology that directly (e.g. efficient fishing equipment) and indirectly (e.g. dams) impacted native fish species and led to their decline, in some cases, to the point of extirpation. Both deliberate and inadvertent stocking of introduced fishes and a wide variety of other threats to native species have continued into the 21st century.

These changes have resulted in the global extinction of three taxa and the extirpation of 18 species lowering the number of currently extant native species in the Great Lakes basin from 169 to 148 species (Roth et al., Michigan State University, East Lansing, in press). A further 82 species have declined to the point of endangerment in at least one jurisdiction in the basin. The causes of these declines can be broadly summarized as habitat alterations, aquatic invasive species, overexploitation, and other lesser threats. In addition, these threats may act synergistically, and may be exacerbated by evolving threats such as climate change and human population growth.

The significant changes in the fish fauna of the Great Lakes basin have also been the result of the successful introduction and establishment of 35 non-native species. In addition, 34 non-native species have been found in the basin, but have not, or are not thought to have, established reproducing populations. These species have been introduced through a variety of pathways such as commercial shipping, dispersal, live trade, recreational boating and angling, and stocking. Many of these species have had substantial impacts on the Great Lakes ecosystem directly through predation and competition (for food, habitat, space), or indirectly through trophic disruption and disease transmission.

This paper summarizes the changes in the fish fauna in the Great Lakes basin as the result of declines in native fishes and associated threats, and of increases in introduced species and associated pathways. These changes are examined across the five lake basins in the past, present and future.

The fall of native fish species

Conservation Status

Of the 169 species native, or likely native, to the Great Lakes basin (Roth et al., Michigan State University, East Lansing, in press), three are globally extinct (Blue Pike, Deepwater Cisco, Shortnose Cisco), 18 species are extirpated, and 82 are in decline to the point of endangerment in at least one jurisdiction. Species in decline in the Great Lakes basin have been assessed using a variety of methods, and are in decline for a variety of reasons.

Only 17 species, including seven subspecies, were assigned a conservation status by the American Fisheries Society Endangered Species Committee (Table 1; Jelks et al., 2008). Of these 17 taxa, the status declined for one since the last list was published in 1989, improved for three, and did not change for five–the remaining eight taxa were not on the 1989 list (Jelks et al., 2008).

Fifty-nine species have been assessed by the federal Committee on the Status of Endangered Wildlife in Canada (COSEWIC; Table 1). Of the 59 species, 31 were assigned a conservation status, 24 as Not at Risk, four as Data Deficient, and one (Orangespotted Sunfish, introduced into Canada and not eligible for assessment; COSEWIC, 2008a) as Non-active. Of the 31 species assessed a conservation status, 21 are listed in Schedule 1 of Canada's federal Species at Risk Act. Listing, and subsequent protection and recovery, under SARA are based on COSEWIC assessment. Fewer species are listed in Schedule 1 of SARA than assessed by COSEWIC because Extinct species (n = 3) are not eligible for protection under SARA, several species are awaiting the Minister's decision as to whether or not to list them under SARA (Schedules 2 and 3), and the Minister had decided not to list several species (e.g. Black Redhorse, Shortjaw Cisco; Hutchings and Blanco-Feishet 2009). No species in the basin are listed by the United States Endangered Species Act.

Eighty-two species have become rare enough (S-Rank ⩽ 3) to warrant a NatureServe conservation listing of Vulnerable or higher by at least one jurisdiction in the Great Lakes basin (www.natureserve. org; Mandrak and Cudmore, Fisheries and Oceans Canada, Burlington, ON, in press). Of these species, only nine (11%) have a global rank (G-Rank) of 3 or higher concern, indicating that while the 82 species are rare in all, or part, of the Great Lakes basin, most are not rare globally. Of the 82 species, Illinois (n = 37 species), Ohio (n = 37), New York (n = 34), and Michigan (n = 33) have the most species listed by NatureServe at an S-Rank of S3 or higher (Table 2). Only a small portion of Illinois is actually in the Great Lakes basin; therefore, the assessments for that state would be primarily based on Mississippi basin populations of species also found in the Great Lakes basin. Minnesota (n = 10) has the least number of rare species, likely due to its northern location and relatively low species richness.

Based on assessments by individual state and provincial agencies, Ohio (n = 30), Wisconsin (n = 30), Michigan (n = 27) and Ontario (n = 26) assigned a conservation status to the most species, while Minnesota (n = 11) and Pennsylvania (n = 11) assigned a status to the fewest species (Table 3). The differences in the number of species of conservation concern between the NatureServe and jurisdictional rankings are likely the result of differences in the methods used to determine the ranks. NatureServe uses the number of locations at which the species is present, and the jurisdictions primarily use expert opinion. In many cases, the jurisdictions rank a species lower, or not at all, compared to the S-Rank (Mandrak and Cudmore Fisheries and Oceans Canada, Burlington, ON, in press). This is likely the result of expert opinion being able to consider contributing factors other than number of locations. For example, if a species is found at relatively few locations, but is common where it is found, it might receive a lesser conservation rank from the jurisdiction.

Of the 82 rare species, only one species, Lake Sturgeon, received a conservation ranking for all states and Ontario from both NatureServe and the jurisdictions (Table 4; Figures 1 and 2; see Appendix 2 in Mandrak and Cudmore, Fisheries and Oceans Canada, Burlington, ON, in press). Four species received an S-Rank for eight of the nine jurisdictions, and seven species for seven jurisdictions (Table 4; Figure 1). Fewer species received jurisdiction conservation ranks; after Lake Sturgeon, the Pugnose Shiner received a conservation status by the most jurisdictions (n = 8), followed by four species ranked by six jurisdictions (Table 4; Figure 2).

Based on the combined NatureServe and jurisdictional rankings, the species with the greatest number of conservation rankings were Lake Sturgeon (n = 18), Pugnose Shiner (n = 16), River Redhorse (n = 14), Paddlefish (n = 13; extirpated in the Great Lakes basin; COSEWIC 2008b), and three species with 12 rankings (Table 4). The number of jurisdictions for which a species is given an S-Rank or conservation status is influenced by the distribution of the species–species with more limited distributions can only be listed by the jurisdictions in which they are found (e.g. Eastern Silvery Minnow is found only in New York and Ontario); however, most species are found in at least several jurisdictions (Smith et al., 2004). Note that the Gravel Chub is now considered extirpated in the only jurisdiction (Ontario) in which it was present in the Great Lakes basin, but is listed by other jurisdictions based on the status of populations in the Mississippi basin. Similarly, American Eel is listed in many jurisdictions, but other than the Lake Ontario basin, it is not native to the Great Lakes basin and its listing is based on its occurrence in the Atlantic (e.g. Pennsylvania) or Mississippi basins (e.g. Illinois).

Threats

In Canada, the leading causes of imperilment of rare fish species are (in order of importance) habitat alteration and destruction, introduced species and overexploitation (Dextrase and Mandrak, 2006). Of the 82 species considered rare in the Great Lakes basin, habitat alteration is the primary threat for 60 species and secondary threat for two species, aquatic invasive species was the primary threat for 13 species and secondary threat for three species, overexploitation was the primary threat for 12 species and secondary threat for three species, lampricide was the primary threat for 3 species and secondary threat for one species, and pollution was the primary threat for one species and secondary threat for three species (Figure 3; Mandrak and Cudmore, 2010; N.B. some species may have more than one primary threat). However, it is most likely a combination of threats, not any single threat that has caused the decline of many fishes in the Great Lakes.

Current threats to the fish fauna of the Great Lakes are likely to continue into the future, and are likely to be exacerbated by evolving threats such as climate change and human population growth, which may directly impact fishes through physiological stress and indirectly through facilitation of other threats such as water level changes and non-native species (e.g. Mandrak, 1989).

The rise of non-native fish species

Although the discovery of species not native to the Great Lakes basin (non-native) is not a new problem, with a new non-native species being discovered, on average, every 28 weeks, the rate of discovery has been increasing (Ricciardi, 2006). Currently, of the over 180 non-native species that have been found in the basin, 69 are fishes (Ricciardi, 2006; Mandrak and Cudmore, Fisheries and Oceans Canada, Burlington, ON, in press). These species have arrived in the basin by many means, usually as a direct consequence of human-mediated activities (Leach et al., 1999). Although not all non-native species have a negative impact on the ecosystem to which they are introduced, those that do are termed “invasive species” (CCFAM, 2004).While the introduction of non-native species into the Great Lakes basin is likely to continue into the future, so will subsequent impacts to the Great Lakes ecosystem.

To date, there have been 69 non-native fish species (not including three hybrids) introduced to the Great Lakes basin: 35 of which are established in at least part of the basin; 29 were failed introductions (did not establish); and, five are of uncertain status (Table 5; Mandrak and Cudmore, Fisheries and Oceans Canada, Burlington, ON, in press). The Lake Michigan basin has the highest number of non-native species, with 22 established species, six species that failed to become established, and another two species that have been reported, but their status is uncertain. The Lake Huron and Erie basins have the next highest number of non-native fishes (36) with the same number of established species (21). Lake Huron has more failed introductions (13) than Lake Erie (10); however, Lake Erie has more species of uncertain status (five), compared to Lake Huron (two). The Lake Ontario basin has the next highest number of non-native fishes (29); however, only 14 species are established, 13 are failed introductions, and two species are of uncertain status. The Lake Superior basin has the lowest number of non-native fishes overall (26); however, 19 of these are established, five species have failed to establish, and two species are of uncertain status.

Pathways of Introduction

There are a number of pathways, and associated vectors, through which species are introduced to a new system. Pathways are the broad routes by which an invasive species is transferred from one geographic area to another. Vectors are the sub-routes within pathways and are the physical means by which a species is transported, usually by humans, from one area to another. The pathways, and their associated multiple vectors, used by non-native fishes to enter the Great Lakes basin can be grouped into five broad areas: (i) commercial shipping; (ii) dispersal; (iii) live trade; (iv) recreational boating and angling; and, (v) stocking. It is important to note that not every potential vector within a pathway plays an important role for the introduction of fishes; some are of greater importance for invertebrates or aquatic plants. The relative importance of individual pathways has changed over time (Figure 4), and it is anticipated that changes will continue in the future.

Commercial shipping is a pathway that can involve the vectors of ballast water or sediment release, or hitch-hiking organisms found directly on ship structures such as hulls and anchor chains. The ballast water vector is the most likely source of introduction of non-native fishes in the commercial shipping pathway. Non-native fish introductions not only occur via trans-oceanic shipping, but through ballast water release through shipping within North American inland waters or between the Great Lakes. Commercial shipping first influenced non-native fish introductions into the basin in the 1970s, but has stabilized since the 1990s (Figure 4). Overall, the commercial shipping pathway has been responsible for six non-native fish species becoming introduced to the Great Lakes basin, five of which have been successful (Figure 4).

Non-native species, especially mobile aquatic taxa such as fishes, have used man-made canals and diversions to reach previously isolated waterbodies. Dispersal into new areas can also be facilitated by increasing water temperatures associated with climate change (Mandrak, 1989; Crossman and Cudmore, 1999a; Chu et al., 2005). The dispersal pathway was one of the earliest pathways of introduction into the basin used by non-native fishes, with periods of increasing importance in the 1920s and 1950s, following the construction of major canals, and again in the 1980s (Figure 4). Overall, nine non-native fish species have been introduced into the Great Lakes basin through the dispersal pathway, of which six have become established (Figure 4).

Live aquatic organisms are imported into the Great Lakes basin for a variety of reasons: use in aquaria or water gardens, the live food industry, school curricula, or for research (Smith et al., 2008, Cudmore, unpubl. data). These organisms are often imported beyond their native range and are kept live to point of sale or use. Although not usually intended for release into natural ecosystems, the live trade provides a source of individuals, some of which have the potential to become invasive, for intentional (but unauthorized) release into the natural environment. These releases may occur for cultural (Severinghaus and Chi, 1999) or ethical (e.g. animal rights) reasons (Crossman and Cudmore, 1999b).

Another aspect of live trade is the aquaculture industry, which has imported non-native fishes for reasons of the food trade or biological control. These cultured species are not normally intended for release into natural waterbodies, but have unintentionally escaped into natural waters; however, this vector has played a minimal role in establishing non-native fishes in the Great Lakes. Live trade was one of the earliest pathways of introduction into the basin with increases in importance in the 1920s, 1930s, and 1950s, and has been steadily increasing since the 1970s (Figure 4). Nineteen species have been introduced into the Great Lakes basin through the live trade pathway, of which five have been successful (Figure 4).

Both baitfish use and equipment transfers are vectors of introduction for non-native species within the recreational boating and angling pathway. For fishes, baitfish use is the more likely vector of introduction. The potential for capture and movement, and subsequent release, of fishes to a new waterbody by bait harvesters and anglers exists. These releases can occur through culling of unwanted or illegal bait species by harvester, or from bait bucket dumping by anglers (fish species, as well as non-native invertebrates in the bait bucket water (Litvak and Mandrak, 1993). Surveys conducted in Ontario over the last 17 years indicate little change in the public's perception of, and response to, this problem with angler release rates of over 30% (Litvak and Mandrak, 1993; Dextrase and MacKay, 1999; Mandrak et al., 2006). Unintentional escapes of fishes harvested in one watershed and stored in baitfish holding ponds in another watershed can also occur. In the Great Lakes basin, recreational boating and angling became a more important pathway of introduction for non-native fishes beginning in the 1920s, and increased during the 1950s and 1960s, and again in the 1990s (Figure 4). To date, six species have been introduced into the Great Lakes basin through the recreational pathway, of which five have been successful (Figure 4).

Development of a commercial or recreational fishery is the reason for most authorized introductions in North America, such as the Brown Trout, which is one of the earliest introductions for recreational purposes (Crossman and Cudmore, 1999c). Authorized stocking is defined here as stocking practices undertaken, or approved, by government agencies, while unauthorized stocking practices are those by the public without permission from government. Other reasons for authorized stockings include: introduction of a new forage species for economically valuable fishes; creation of new populations of species at risk as a recovery technique; biological control; and, creation of a tourist attraction. Not all of these reasons have been behind authorized introductions in the Great Lakes, but may occur at some time in the future; however, fisheries management is generally moving away from the authorized stocking of non-native species (Helfman, 2007). One unintentional vector of introduction of non-native fishes associated with authorized introductions is contamination by other species and diseases. Some species have been inadvertently introduced, either through mis-identification or careless culling of stock, with an authorized stocking of another species (e.g. buffalo species Ictiobus spp.; Trautman, 1981). Intentional, but unauthorized, introductions by humans have resulted in range extensions of aquatic species into non-native areas. Stocking, both authorized and unauthorized, was one of the earliest pathways of introduction and responsible for the greatest number of non-native fish introductions (Figure 4). However, this has stablized since the 1980s (Figure 4). To date, 27 species have been introduced into the Great Lakes basin through the stocking pathway, of which 12 have been successful (Figure 4).

Overall, stocking has been responsible for introducing the greatest number of successful non-native fish species into the Great Lakes basin, followed by dispersal through canals and diversions (Figure 4). However, there have been no new introductions from both of these pathways over the last couple of decades (Figure 4). Species introduced into the basin through the live trade pathway has greatly increased over time; however, most of these introductions have been unsuccessful to date (Figure 4). Recreational boating and angling has brought in increasing numbers of non-native species, all of which have been successful (Figure 4). While, to date, the commercial shipping pathway has brought in the same number of species as recreational boating, but not all have been successful (Figure 4).

Impacts

The impacts of some introduced species may be relatively benign or unknown; whereas, the impacts of others can be significantly negative. Aquatic invasive species can affect native fish species directly through predation and competition (for food, habitat, space) or indirectly through trophic disruption and vectors of disease transmission (Helfman, 2007). The rise of non-native fishes and the fall of native fishes can range from the unknown to negligible to significantly negative. Direct predation on native fishes can result in a decline of native species abundance and diversity. Competition for prey items, which can lack escape and avoidance mechanisms for non-native species, can also impact native species at higher trophic levels (Helfman, 2007). These impacts can, in turn, result in simplified and unstable food webs, leading to systems less resistant to further introductions of non-native species (Cudmore, 1999; Helfman, 2007). Competition with native species for habitat can also result from the introduction of non-native fishes as they are released from biological limiting factors imposed upon them in their native range (Helfman, 2007). Along with the introduction of a non-native species, pathogens (diseases or parasites) may also be introduced, and may particularly impact native species previously not exposed to the non-native pathogens (Helfman, 2007).

With ongoing introductions of non-native fish species into the Great Lakes basin, some of which will have further negative impacts on the native ecosystem, the fish fauna of the Great Lakes can be expected to continue to change. The importance of primary vectors as a source of new introductions has also changed over time (Figure 4), and this can be expected to continue as a result of changing regulations and trade patterns. These changes are likely to result in different types of non-native fishes being introduced in the future. This may result in different, sometimes unexpected, impacts to the Great Lakes ecosystem. These changes are also likely to be facilitated and exacerbated by climate warming (e.g. Mandrak, 1989).

Although the commercial shipping pathway has resulted in a relatively low number of successful introductions of fishes into the Great Lakes basin, it is the most regulated pathway in the Great Lakes with the implementation and enforcement of ballast water regulations. These regulations require the exchange, or treatment, of water in ballast tanks. Therefore, the overall risk of invasive fishes being released through the ballast water vector is likely to continue to decline with these strict control measures implemented by regional, national and international agencies, such as mid-ocean ballast exchange and no-ballast-on-board (NOBOB). Conversely, with little federal regulation of import of non-native fishes in place in either the United States or Canada, the overall risk of non-native fishes being released through live trade is likely to increase as the composition and volume of this trade increases. The successful practice of the control and management of the ballast water vector has not been expanded to other pathways of greater importance.

The likelihood of Asian carps dispersing into the Great Lakes from the Mississippi basin is of particular concern. Four species (Grass, Silver, Bighead, and Black carps), native to Asia, were introduced in the 1970s into land-based aquaculture in the southern United States for pest control and, subsequently, escaped into the wild (Mandrak and Cudmore, 2004; Kolar et al., 2005). Based on predictive modelling using environmental conditions in their native distribution, all four of the Asian carp species could survive as far north as northern Canada (Herborg et al., 2007) and, hence, could survive in the Great Lakes. Two of the species, Bighead and Silver carps, have been rapidly expanding up the Mississippi River, where they have become the dominant biomass and have outcompeted most native fishes (Mandrak and Cudmore, 2004). These carps are currently being prevented from moving from the Mississippi River through the Chicago Sanitary Canal into Lake Michigan by an electrical barrier built specifically to prevent the movement of AIS between the basins (Dettmers and Pegg, 2003). However, there is concern that the barrier may not be 100% effective, or that the carps will be moved around it through other vectors such as overland flooding or bait use.

To prevent potential future invaders from impacting Great Lakes ecosystems, we must prevent them from becoming established in the first place. Prevention requires a good understanding of the pathways that bring species into the Great Lakes, the species present in these pathways and associated vectors, and the potential impact of these species. This information is best organized in risk assessments for both species and pathways to inform preventative actions against the riskiest species and pathways. The results of these risk assessments will help develop effective policy and direct management actions to prevent the establishment of potential, invaders.

The fall of native fishes and rise of non-native fishes

It is no coincidence that the number of native species declined in each of the Great Lakes basins as the number of non-native species increased (Figure 5). Non-native species not only have a direct negative effect on native species, as noted above, but often do well in degraded habitats that negatively impact native species (Helfman, 2007). The number of successfully established non-native species is highest in the Lake Huron and Michigan basins with the increases occurring in the 1930s, 1970s and 1980s (Figure 5). These basins are followed by the Lake Superior basin, which has shown an increasing rate of establishment in the last several decades. The Lake Erie and Ontario basins exhibit the lowest number of established non-native species with a very low rate of increase as a result of most species becoming established over 50 years ago. Conversely, all lake basins show a similar pattern in the number of native species that have been extirpated and in the rates of loss – the rates of loss being greatest between 1930 and 1970 (Figure 5). When examined cumulatively across the basins, the greatest rate of extirpation occurred between 1920 and 1980 and was particularly steep between 1950 and 1980

(Figure 6). The rate of successful establishment of non-native species was relatively constant from before 1900 to 1910, increased between 1910 and 1990, and then tapered off towards 2000 (Figure 6).

Conclusions

The fish fauna of the Great Lakes has changed significantly as a result of the decline of many native species and the introduction of non-native fishes. Three taxa in the basin are now globally extinct and 18 species are extirpated, lowering the number of the number of currently extant native species in the Great Lakes basin from 169 to 148 species. A further 82 species have declined to the point of endangerment in at least one jurisdiction in the basin. The causes of these declines are primarily habitat alterations, aquatic invasive species, and overexploitation, often working synergistically. Although a total of 69 non-native species have been found in the basin, 35 of these have established reproducing populations. These successful species have been introduced through a variety of pathways such as, commercial shipping, dispersal, live trade, recreational boating and angling, and stocking. Many of these species have had substantial impacts on the Great Lakes ecosystem directly through predation and competition or indirectly through trophic disruption and vectors of disease transmission. The importance of primary vectors as a source of new introductions has changed over time. There has been a decline in species being introduced in pathways that had historically brought in the greatest number of species, such as stocking and dispersal. Pathways, such as live trade and recreational boating, are increasing in importance for introducing non-native species into the lakes; whereas, successful introductions of non-native fishes has been relatively lower through the commercial shipping pathway compared to other pathways in the Great Lakes basin. These changes in pathway importance can be expected to continue as a result of changing regulations and trade patterns. Understanding what species are present in these pathways and associated vectors, and the potential impact of these species, is important for developing prevention strategies.

The fish fauna of the Great Lakes basin will continue to change as the result of continuing threats to native species and ongoing introductions of non-native species, and such change will undoubtedly be influenced by climate change and human population growth.

Appendix 1. List of scientific and common names according to Nelson et al., 2004.

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