**6. Solutions to the decline of freshwater mussels**

Because of the growing awareness of the importance of freshwater mussel diversity and freshwater ecosystems in general, there have been increasing efforts to restore and rehabilitate mussel populations and their habitats. Most strategies focus on reversing the root causes of the decline in unionid abundance and diversity listed in the preceding section, along with restoring and protecting existing mussel populations.

#### **6.1 Reduction in commercial harvesting**

148 Biodiversity Loss in a Changing Planet

in small streams where water temperature is more closely linked to air temperature (Hastie

The change in precipitation patterns could also impact mussel populations through increased flooding and prolonged droughts. Although periodic, low-intensity flooding can have beneficial effects on mussel populations such as flushing fine sediments and pollutants out of substrates (Gordon et al., 1992), extreme storm events can dislodge mussels from the sediment and alter mussel bed habitat (Hastie et al., 2001). In a record multi-year drought in Georgia, Golladay et al. (2004) observed a greater than 50% loss in total mussel abundance in some reaches in the study area. As mussels are limited in their ability to move horizontally, they are unlikely to reach refuges in response to complete dewatering of their habitat. Even reduced flows can have negative effects on respiration, feeding, growth, and glochidial recruitment; and can increase predation by terrestrial consumers like raccoons

The response by unionid mussels to climate change will vary depending on several factors. Geographic location will play an important role as climate change is expected to affect different parts of the world differently (Parry et al., 2007). Climate change, as with most types of ecological changes, will produce winners as well as losers (McKinney and Lockwood, 1999; Somero, 2010). Endemic species with restricted geographical ranges are expected to be especially hard hit (Malcolm et al., 2006), as are species that are already close to their upper thermal tolerance ranges (Spooner and Vaughn, 2008). The threat of climate change does not exist in isolation. It also interacts with other disturbances such as land use, direct human-caused flow alterations, and biotic exchange of non-native species; and the severity of these other threats along with geographic location will influence the effects

As serious as the current conservation status of many freshwater mussels are, there most likely exists a substantial extinction debt in many mussel populations (Haag, 2010). Freshwater mussels naturally exist in spatially "patchy" populations separated by areas occupied by no or only a few individuals. These patches remained connected, however, by glochidia transported by host fishes travelling throughout the matrix of mussel beds and unoccupied areas (Strayer, 2008a). Thus, population declines caused by stochastic events such as major floods or droughts could be restored through recruitment from neighboring populations. Many of the threats unionids are facing today, though, such as the building of dams, decline or extinction of host species, increased difficulty of host fish finding female mussels' "lures" or conglutinates due to decreased visibility, and lack of suitable habitat for

As pelagic spawners that release sperm into the water column, it has also been shown that reproductive success declines dramatically with decreasing mussel density, with almost no fertilization occurring at densities below 10 individuals/m2 (Downing et al., 1993). This lack of reproductive connectivity creates a genetic bottleneck in the remaining populations. These life history characteristics, along with the well-documented decline in mussel diversity and abundance, point to significant future losses in even seemingly stable mussel populations unless action is taken to reduce the perturbations causing the initial decline and

juvenile mussels, limit reproductive success and gene flow between patches.

increase connectivity between populations (Haag, 2010).

et al., 2003).

(Golladay et al., 2004; Hastie et al., 2003).

caused by a changing climate (Sala et al., 2000).

**5.7 The extinction debt** 

Although the commercial harvest of freshwater mussels has greatly contributed to the historic decline of Unionids, it is not generally considered to be a major threat to them at present. There are several reasons for the reduction in commercial harvesting of freshwater mussels. The replacement of mussel shell with plastics in the 1940s and 50s in the button industry reduced demand for shells, and more recently the collapse of the Japanese oyster pearl fishery has reduced the demand for pearl nuclei in that industry (Neves, 1999). Enforced regulation on commercial harvesting, as well as low prices for mussel shell, have also provided a respite for mussel populations (Strayer et al., 2004).

#### **6.2 Best management practices to reduce pollution**

Although water pollution has significantly declined in many industrial countries thanks to national-level legislation such as the Clean Water Act in the United States and the Water Resources Act in the UK, it is still a major threat to freshwater ecosystems and unionid mussels in most parts of the world. Acute toxicity studies in freshwater mussels have been performed on only a small number of known organic and inorganic contaminants present in the surface water of North America, and sublethal toxicity studies are even more rare (Keller et al., 2007). More studies are needed on a broader array of substances to provide regulators with better information for setting acceptable pollution standards in surface waters where freshwater mussels are found.

Non-point source nutrient and sediment pollution from agriculture, timber extraction, and urban runoff is regularly cited as one of the most serious threats to freshwater ecosystems (Richter, 1997). Best management practices that control runoff into surface water have been shown in numerous studies to improve the physical and chemical quality of streams (Caruso, 2000; D'Arcy & Frost, 2001; Lowrance et al., 1997). One of the most effective ways controlling sediment and nutrient inputs into streams is an intact, functional riparian zone. Well-vegetated riparian zones slow and reduce surface run-off into streams, capture large amounts of sediment in the runoff, store excess nutrients for uptake into riparian vegetation, and stabilize stream banks which further reduces instream sedimentation (Allan, 2004).

#### **6.3 Restoring natural and adequate stream flows**

Reversing the trend of increasing human control of the flow of rivers and streams worldwide is not likely in the near future. As the human population grows over the foreseeable future, the global demand for domestic and irrigation water is projected to increase correspondingly (Robarts and Wetzel, 2000). Although the world's rivers have been fragmented and controlled by more than 1 million dams (Jackson et al., 2001), there are methods of operating these dams to minimize the negative effects they have on downstream ecosystems. In several case studies in the United States, water managers, conservation organizations, and scientists have attempted to regulate releases from dams to mimic the

Biodiversity Loss in Freshwater Mussels: Importance, Threats, and Solutions 151

possible to directly restore benthic habitat through riparian and instream construction projects designed to stabilize banks and stream channels and increase the habitat heterogeneity that supports high levels of benthic diversity. Several studies in Finland (Muotka et al., 2002), Japan (Nakano & Nakamura, 2006), and the United States (Miller et al., 2009) have found increased macroinvertebrate abundance and richness in streams following stream channel restoration projects, and while these studies did not look at freshwater mussels specifically, they provide a basis of reference for mussel-specific restoration techniques. Osterling et al. (2010) indicated that restoration activities to improve environmental conditions of mussels' habitats should focus on reducing fine material transport into streams, because sedimentation of inorganic and organic materials and high

The ability of freshwater organisms to adapt to climate change is dependent on a particular species' ability to disperse and migrate to cooler environments in higher latitudes or elevations (Poff et al., 2002). As unionid mussels have limited dispersal and reproductive potentials under the best of circumstances, this puts this group at a higher risk than many other groups (Hastie et al., 2003). There are two main approaches to dealing with the threat posed by climate change: (1) to reduce further changes in climate and (2) to manage the consequences of current and predicted changes. To review the numerous methods being debated and currently attempted to control climate change is beyond the scope of this chapter; however it is important to note that a few of these methods (construction of dams for "clean" hydroelectric power, intensification of agriculture for biofuels) have the potential to further degrade freshwater ecosystems beyond their current state if not planned and

As far as managing the effects of climate change on freshwater ecosystems, there are two major aspects to this as well: (1) to reduce pollution, habitat loss, and other anthropogenic disturbances that are already placing stress on freshwater systems, and (2) to establish a network of protected areas based on species' current and projected ranges, and to manage the connecting matrix between them (Hannah et al., 2002; Heino et al., 2009; Poff et al., 2002). Ways of reducing anthropogenic stress on freshwater ecosystems include riparian zone management, reducing nutrient loading, habitat restoration, and minimizing humandriven water withdrawal (Poff et al., 2002), and have already been discussed in previous sections. The concept of freshwater protected areas and dispersal corridors between

Protected areas have been a mainstay of terrestrial and marine conservation efforts for decades, yet have only recently been part of the discussion about conserving freshwater species and habitat (Abell et al., 2007). Freshwater protected areas (FPAs) have been used in the past mostly to protect fish species from overharvesting by providing areas closed to fishing for at least part of the time. FPAs have the potential to do more than just limit fish harvests, though. Effectively planned and executed protected areas can protect specific habitat types against degradation, ensure minimum surface and groundwater flows, protect riparian zones, and protect rare and endangered species (Saunders et al., 2002; Suski and

turbidity can impact mussel recruitment.

managed correctly (Bates et al., 2008).

Cooke, 2007).

populations will be covered in the following section.

**6.7 Protecting and restoring freshwater mussel populations** 

**6.6 Minimizing the effects of global climate change** 

timing, duration, and magnitude of natural flood events, and to minimize the number of low flow days in the rivers downstream (Poff et al. 1997; Richter et al., 2003). In one study in Tennessee, recolonization of mussel populations occurred after hydroelectric dam managers altered their release schedule to ensure minimum flows (Layzer and Scott, 2006). There is also a growing movement for the complete removal of dams. As their ecological implications are being realized by scientists and the public, and as dam managers are facing higher operating costs in maintaining aging structures and complying with federal endangered species laws, dam removal is being seen as a viable option for river restoration in many circumstances (Hart et al., 2002; Pejchar and Warner, 2001).

When water levels drop, either through natural wet and dry cycles or through human withdrawals or regulation, the amount of physical habitat available to mussels and other benthic organisms is reduced. Many states and countries have passed legislation that requires minimum ecological flows in streams and rivers. There are over 200 methods for determining exactly how much water is needed for a particular stretch of river, all of which take into consideration the specific ecological function or species water managers are trying to preserve (Arthington et al., 2006). Most of these methods focus on fish or other vertebrate species, and often flows suitable for the preservation of these target species is not sufficient for freshwater mussels or other invertebrates (Gore et al., 2001, Layzer and Madison, 1995). Obviously, more study into the flow requirements of freshwater mussels along with a greater emphasis on this group by regulators is necessary if the hurdle of inadequate flows is to be overcome.

#### **6.4 Control of non-native species**

Controlling invasive, non-native organisms in freshwater ecosystems has met with limited success for most species, despite passage of laws such as the Non-indigenous Aquatic Nuisance Prevention and Control Act of 1990 in the United States. The zebra mussel is still expanding its range, although the rate of spread has slowed in recent years as the most easily colonized waterways have already been occupied (Johnson et al., 2006). The early spread of *D. polymorpha* was due to physical connectivity of waterways to infected areas, whereas current range expansion is due to overland human-facilitated transport by recreational boaters (Johnson et al., 2001). Thus, it seems, the future distribution of *D. polymorpha* will depend on human behavior, although their ultimate range will be limited to ecosystems with suitable pH, calcium concentrations, and temperature (Strayer, 2008b). Although various chemical, thermal, mechanical, and thermal treatment options have been somewhat successful in controlling *D. polymorpha* near shoreline structures and water intake valves, and consumption by natural predators can be high (Hamilton et al., 1994, Perry et al., 1997), the overall fecundity of the species makes eradication or control in most openwater areas unlikely (Strayer, 2008b).

#### **6.5 Restoring habitat**

Many of the solutions to physical habitat loss have already been addressed in the previous sections, such as restoration of riparian vegetation; the use and enforcement of best management practices in construction, agriculture, and forestry; dam removal; and restoration of natural flow regimes. These practices will reduce terrestrial inputs of substrate-smothering sediment, ensure that adequate amounts of water are present, and restore natural stream channel morphology more suitable for freshwater mussels. It is also

timing, duration, and magnitude of natural flood events, and to minimize the number of low flow days in the rivers downstream (Poff et al. 1997; Richter et al., 2003). In one study in Tennessee, recolonization of mussel populations occurred after hydroelectric dam managers altered their release schedule to ensure minimum flows (Layzer and Scott, 2006). There is also a growing movement for the complete removal of dams. As their ecological implications are being realized by scientists and the public, and as dam managers are facing higher operating costs in maintaining aging structures and complying with federal endangered species laws, dam removal is being seen as a viable option for river restoration

When water levels drop, either through natural wet and dry cycles or through human withdrawals or regulation, the amount of physical habitat available to mussels and other benthic organisms is reduced. Many states and countries have passed legislation that requires minimum ecological flows in streams and rivers. There are over 200 methods for determining exactly how much water is needed for a particular stretch of river, all of which take into consideration the specific ecological function or species water managers are trying to preserve (Arthington et al., 2006). Most of these methods focus on fish or other vertebrate species, and often flows suitable for the preservation of these target species is not sufficient for freshwater mussels or other invertebrates (Gore et al., 2001, Layzer and Madison, 1995). Obviously, more study into the flow requirements of freshwater mussels along with a greater emphasis on this group by regulators is necessary if the hurdle of inadequate flows

Controlling invasive, non-native organisms in freshwater ecosystems has met with limited success for most species, despite passage of laws such as the Non-indigenous Aquatic Nuisance Prevention and Control Act of 1990 in the United States. The zebra mussel is still expanding its range, although the rate of spread has slowed in recent years as the most easily colonized waterways have already been occupied (Johnson et al., 2006). The early spread of *D. polymorpha* was due to physical connectivity of waterways to infected areas, whereas current range expansion is due to overland human-facilitated transport by recreational boaters (Johnson et al., 2001). Thus, it seems, the future distribution of *D. polymorpha* will depend on human behavior, although their ultimate range will be limited to ecosystems with suitable pH, calcium concentrations, and temperature (Strayer, 2008b). Although various chemical, thermal, mechanical, and thermal treatment options have been somewhat successful in controlling *D. polymorpha* near shoreline structures and water intake valves, and consumption by natural predators can be high (Hamilton et al., 1994, Perry et al., 1997), the overall fecundity of the species makes eradication or control in most open-

Many of the solutions to physical habitat loss have already been addressed in the previous sections, such as restoration of riparian vegetation; the use and enforcement of best management practices in construction, agriculture, and forestry; dam removal; and restoration of natural flow regimes. These practices will reduce terrestrial inputs of substrate-smothering sediment, ensure that adequate amounts of water are present, and restore natural stream channel morphology more suitable for freshwater mussels. It is also

in many circumstances (Hart et al., 2002; Pejchar and Warner, 2001).

is to be overcome.

**6.4 Control of non-native species** 

water areas unlikely (Strayer, 2008b).

**6.5 Restoring habitat** 

possible to directly restore benthic habitat through riparian and instream construction projects designed to stabilize banks and stream channels and increase the habitat heterogeneity that supports high levels of benthic diversity. Several studies in Finland (Muotka et al., 2002), Japan (Nakano & Nakamura, 2006), and the United States (Miller et al., 2009) have found increased macroinvertebrate abundance and richness in streams following stream channel restoration projects, and while these studies did not look at freshwater mussels specifically, they provide a basis of reference for mussel-specific restoration techniques. Osterling et al. (2010) indicated that restoration activities to improve environmental conditions of mussels' habitats should focus on reducing fine material transport into streams, because sedimentation of inorganic and organic materials and high turbidity can impact mussel recruitment.

#### **6.6 Minimizing the effects of global climate change**

The ability of freshwater organisms to adapt to climate change is dependent on a particular species' ability to disperse and migrate to cooler environments in higher latitudes or elevations (Poff et al., 2002). As unionid mussels have limited dispersal and reproductive potentials under the best of circumstances, this puts this group at a higher risk than many other groups (Hastie et al., 2003). There are two main approaches to dealing with the threat posed by climate change: (1) to reduce further changes in climate and (2) to manage the consequences of current and predicted changes. To review the numerous methods being debated and currently attempted to control climate change is beyond the scope of this chapter; however it is important to note that a few of these methods (construction of dams for "clean" hydroelectric power, intensification of agriculture for biofuels) have the potential to further degrade freshwater ecosystems beyond their current state if not planned and managed correctly (Bates et al., 2008).

As far as managing the effects of climate change on freshwater ecosystems, there are two major aspects to this as well: (1) to reduce pollution, habitat loss, and other anthropogenic disturbances that are already placing stress on freshwater systems, and (2) to establish a network of protected areas based on species' current and projected ranges, and to manage the connecting matrix between them (Hannah et al., 2002; Heino et al., 2009; Poff et al., 2002). Ways of reducing anthropogenic stress on freshwater ecosystems include riparian zone management, reducing nutrient loading, habitat restoration, and minimizing humandriven water withdrawal (Poff et al., 2002), and have already been discussed in previous sections. The concept of freshwater protected areas and dispersal corridors between populations will be covered in the following section.

#### **6.7 Protecting and restoring freshwater mussel populations**

Protected areas have been a mainstay of terrestrial and marine conservation efforts for decades, yet have only recently been part of the discussion about conserving freshwater species and habitat (Abell et al., 2007). Freshwater protected areas (FPAs) have been used in the past mostly to protect fish species from overharvesting by providing areas closed to fishing for at least part of the time. FPAs have the potential to do more than just limit fish harvests, though. Effectively planned and executed protected areas can protect specific habitat types against degradation, ensure minimum surface and groundwater flows, protect riparian zones, and protect rare and endangered species (Saunders et al., 2002; Suski and Cooke, 2007).

Biodiversity Loss in Freshwater Mussels: Importance, Threats, and Solutions 153

habitat requirements, and valuable shell and pearls have put them at risk to human-driven disturbances, and have contributed to their worldwide decline in both abundance and richness (Bogan, 1993; Vaughn, 1997). The drivers of the decline in unionid biodiversity are the same as those of freshwater diversity in general: pollution, habitat destruction, overharvest, altered flows, invasion by non-native species, and climate change, but because of their lifestyles and high degree of endemism, they are being especially hard hit (Strayer et

The solutions to the decline in unionid biodiversity are simple, but not easy. Reducing pollution (Caruso, 2000; Lowrance et al., 1997), restricting harvesting (Strayer et al., 2004), ensuring ecologically sustainable flows (Arthington et al., 2006; Layzer and Scott, 2006), habitat protection and restoration (Miller et al., 2010; Muotka et al., 2002; Wilson et al., 2011), combating non-native invaders (Strayer, 2008b), mitigating and planning for the effects of climate change (Heino et al., 2009; Poff et al., 2002), creating connected freshwater protected areas (Heino et al., 2009; Saunders et al., 2002) and artificially enhancing wild populations (Cope and Waller, 1995; Strayer et al., 2004) are all necessary to restore freshwater

It is clear that any successful freshwater conservation plans must be large in scale and longterm in scope, and take into consideration the multiple chronic stressors that are causing the alarming decline in freshwater pearly mussels. It is equally clear that failure to take concrete steps to halt and reverse the trend of biodiversity loss in unionid mussels could result in the

Abell, R. (2001). Conservation Biology for the Biodiversity Crisis: a Freshwater Follow-up. *Conservation Biology,* Vol.16, No.5, (October 2002), pp. 1435-1437, ISSN 0888-8892 Abell, R.; Allan,J. & Lehner, B. (2007). Unlocking the potential of protected areas for

Aldridge, D.; Fayle, T. & Jackson, N. (2007). Freshwater mussel abundance predicts

Allan, J. & Flecker, A. (1993). Biodiversity conservation in running waters. *Bioscience*, Vol*.*43,

Allen, D. C. & Vaughn, C. C. (2011). Density-dependent biodiversity effects on physical

Amyot J. & Downing J. (1998). Locomotion in Elliptio complanata (Mollusca:Unionidae): A

Anthony, J. & Downing, J. (2001). Exploitation trajectory of a declining fauna: a century of

freshwaters *Biological Conservation*, Vol.134, No.1, (January 2007), pp. 48-63, ISSN

biodiversity in UK lowland rivers. *Aquatic Conservation: Marine and Freshwater Ecosystems,* Vol.17, No.6, (September/October 2007), pp. 554–564, ISSN 1099-0755 Allan, J. (2004). Landscapes and Riverscapes: The Influence of Land Use on Stream

Ecosystems. *Annual Review of Ecology, Evolution, and Systematics*, Vol.35, No.1,

habitat modification by freshwater bivalves. Ecology, Vol.92, No.5, (2011), pp. 1013-

reproductive function? *Freshwater Biology*, Vol.39, No.2, (March 1998), pp. 351–358,

freshwater mussel fisheries in North America. *Canadian Journal of Fisheries and Aquatic Sciences*, Vol.58, No.10, (October 2001), pp. 2071-2090, ISSN 0706-652X

al., 2004).

**8. References** 

0006-3207

ecosystems and the mussels that occupy them.

permanent loss of this unique and important group of animals.

(2004), pp. 257-284, ISSN 1543-592X

1019. ISSN *0012-9658*

ISSN 0046-5070

No.1, (January 1993), pp. 32-43, ISSN 0006-3568

One of the key aspects that have limited the effectiveness of FPAs against ecosystem degradation, especially in rivers and streams, is that many of the stressors affecting these systems come from diverse, non-point sources upstream from critical habitat and threatened populations. The success of localized protected areas or catchment management strategies can be limited due to the large scale connection of aquatic ecosystems with terrestrial activities, especially where streams with their longitudinal connectivity are concerned (Saunders et al., 2000). Therefore, many researchers have pointed out the need for catchment-scale protection for threatened freshwater ecosystems that truly limit the impacts to sensitive areas (Abell et al., 2007; Dudgeon et al., 2006; Heino et al., 2009). Although there has been little published data on freshwater mussels and protected areas, some researchers have noted the possibility of refuges for some species (Ricciardi et al., 1998; Saunders et al., 2002), and preservation and protection of critical mussel habitat has the potential to significantly aid in the recovery of unionids.

Naturally reproducing unionid populations can take decades to recover after severe and prolonged disturbances. As mentioned earlier, mussels are dependent on critical densities to facilitate successful reproduction (Downing et al., 1993), and many areas where unionids have been extirpated lack access to restocking populations (Strayer et al., 2004). In these situations, artificially stocking mussels can help restore populations and eventually enable them to become self-sustaining (Strayer et al., 2004). Mussel relocation and reintroduction have been met with varying levels of success, mostly due to lack of knowledge of specific habitat requirements and handling techniques (Cope and Waller, 1995). Many successful propagation techniques have also been developed over the last few years (Barnhart, 2006; Henley et al., 2001), and although field trials of lab-reared mussels are limited, artificial propagation techniques hold much promise to enhance unionid populations in the future, provided the degraded environmental conditions that caused the decline in the first place are corrected.
