Lampsilis radiata Action Planguillaumechagnon.clg.qc.ca/cegep/101-NYA/RMI... · Lampsilis radiata ,...

Status Assessment and Conservation Action Plan for Lampsilis radiata

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Guillaume Chagnon and Véronik de la Chenelière Biology Department, McGill University

1205 Dr. Penfield Avenue Montréal, Québec, H3A 1B1

Canada


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Table of Contents

Table of Contents ................................................................................................................ 1

List of Figures ..................................................................................................................... 3

Abstract ............................................................................................................................... 4

Introduction to the Species Assessed, Lampsilis radiata (Gmelin 1792) ........................... 5

Biology and Ecology ................................................................................................... 5

Distribution and Taxonomy ......................................................................................... 6

Species Importance ............................................................................................................. 6

Ecological Importance ................................................................................................. 6

Cultural Importance ..................................................................................................... 7

Commercial Importance .............................................................................................. 7

Threats to the Species ......................................................................................................... 8

Habitat Degradation ..................................................................................................... 8

Species Introduction (Dreissena polymorpha) ............................................................ 9

Status Assessment ............................................................................................................. 10

Interpretive Problems ........................................................................................................ 12

Projections......................................................................................................................... 13

Existing Management ....................................................................................................... 13

Action Plan........................................................................................................................ 15

Limitation of D. polymorpha Invasion outside the Great Lakes ............................... 15

Listing Habitats less Suitable to D. polymorpha Invasion ........................................ 16

Minimising Anthropogenic Sources of Threat .......................................................... 16

Relocation Programs for L. radiata ........................................................................... 17

Future Research ................................................................................................................ 17

References ......................................................................................................................... 19

Figures............................................................................................................................... 23

Appendix 1 ........................................................................................................................ 28

Appendix 2 ........................................................................................................................ 29


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List of Figures

Figure 1- Distribution of L. radiata (Gmelin, 1792), following Turgeon et al. 1988 and

Williams et al. 1993. ............................................................................................. 23

Figure 2 - Typical life cycle of an unionid (from Moser 1993). ....................................... 24

Figure 3 - Distribution of D. polymorpha in the United States in 1993 (from Jacquaz et al.

1994). .................................................................................................................... 25

Figure 4 – Mean density of living unionid bivalves and infestation (number of D.

polymorpha per unionid) at East section of Soulanges Canal between 1992 and

1995. (Adapted from Ricciardi et al. 1995.) ......................................................... 26

Figure 5 – Mean densities of L. radiata and D. polymorpha in Northwestern Lake St.

Clair between 1986 and 1994. (Adapted from Nalepa et al. 1996.) ..................... 26

Figure 6 – Relative age-frequency distribution for two populations of L. radiata, in Lake

St. Clair (data from Nalepa and Gauvin 1988) and Lake St. Louis (data from

Magnin and Stanczykowska 1971). ...................................................................... 27


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Abstract

IUCN categories and criteria were used to assess the conservation status of

Lampsilis radiata, a North American unionid (Mollusca: Bivalvia). Unionids are most

diverse in North America. They are the most imperilled animal group in North America,

being threatened by habitat degradation and, recently, invasion by Dreissena

polymorpha. L. radiata was assessed as Endangered with a projected population decline

of approximately 50% over three generations, or 27 years (IUCN criteria A2ace).

Proposed management include: 1) limit the spread of D. polymorpha outside the Great

Lakes, 2) identify areas less susceptible to D. polymorpha invasion, 3) protect

populations less susceptible to D. polymorpha invasion from anthropogenic sources of

threat, and 4) consider L. radiata relocation programs.


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Introduction to the Species Assessed, Lampsilis radiata (Gmelin 1792)

One-third of the freshwater mussels of the family Unionidae (Bivalvia) is found in

North America (Ricciardi et al. in press). Threats, including habitat degradation,

commercial harvesting, and species introduction (the zebra mussel, Dreissena

polymorpha), have accelerated the background rate of extinction of unionids (Ricciardi et

al. in press). Out of the 297 North American unionid species, 21 are considered possibly

extinct and 213 as endangered, threatened, or of special concern by the American

Fisheries Society Endangered Species Committee. Unionidae is the most imperilled

animal group in North America (Williams et al. 1993). This document presents the

assessment of the conservation status of a common unionid, Lampsilis radiata (Gmelin,

1792), occupying the St. Lawrence River system and the American Atlantic drainage

north of the Pee Dee River (South Carolina; see Figure 1). Its common name is the

eastern lampmussel.

Biology and Ecology

Unionids have separate sexes and they reproduce several times in their life. Their

life cycle (Figure 2) is characterised by a parasitic larval stage, the glochidium

(McMahon 1991). L. radiata is a long-term brooder, the female keeping the developing

larvae for a year inside a specialised structure in the mantle called the marsupium

(McMahon 1991). The glochidia are released in July and they must attach to the gills of a

host fish to develop into adults (McMahon 1991). Amphibian hosts might also be

important (Watters 1997). Glochidial attachment therefore allows for some dispersal,

both upstream and downstream. L. radiata fish hosts are unknown (Clarke 1981), but

northern pike (Esox lucius), yellow perch (Perca flavescens), and pumpkinseed sunfish

(Lepomis gibbosus) are among potential fish hosts (J. B. Rasmussen, pers. comm.). The

glochidia remain as cysts on the gills of the host fish for about 3 weeks, after which they

detach themselves and settle on the sediments (Mateson 1948). They then bury

themselves deep into the sediments, grow as detritivores, and migrate to the surface about

two to three years later (Mateson 1948). This stage of the life history of unionids is

poorly understood (McMahon 1991).

Huge numbers of glochidia are produced (range for unionaceans is 0.2-17 x 106

glochidia/female/year), but juvenile survivorship is extremely low, with a female


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typically producing 0.002-0.17 successfully settled young per year. Recruitment of the

released glochidia is limited by host attachment, encystment, full development without

being rejected by the host immune system, and settlement on a suitable substrate

(McMahon 1991).

After reaching maturity (about 3-6 years; Mateson 1948, McMahon 1991), growth

rate diminishes, following a typical von Bertalanffy growth curve (Hinch et al. 1986,

McMahon 1991). Adult unionids are long-lived and have a high relative survivorship.

They are filter feeders and active burrowers (McMahon 1991). L. radiata is restricted to

permanent bodies of water, prefers low flow regimes, lakes and wide rivers with forested

riparian zones (Di Maio & Corkum 1995, Morris & Corkum 1996). Where it occurs, it

tends to be a dominant member of the unionid assemblage (Clarke 1981, and see Nalepa

& Gauvin 1988, Ricciardi et al. 1996).

Distribution and Taxonomy

For this assessment, the distribution of L. radiata follows the taxonomy of

Turgeon et al. (1988) and Williams et al. (1993; see Figure 1). The taxonomy of this

species is problematic in the Great Lakes where it interbreeds with L. siliquoidea

(Barnes, 1823; Clarke 1981). These species are virtually indistinguishable in the Great

Lakes (A. Ricciardi, pers. comm.), and they are interchangeably referred to as L. radiata,

L. radiata siliquoidea, and L. siliquoidea in the literature (e.g. Clarke 1981, Hebert et al.

1991, Salman & Green 1983, Schloesser 1996). Therefore, in this report, L. siliquoidea

and L. radiata are considered to be the same species in the Great Lakes portion of their

range (Schloesser & Nalepa 1994, Schloesser 1996).

Species Importance

Ecological Importance

The ecological role of unionids revolves around their filtering and burrowing

activities. They filter suspended material, clearing the water and making planktonic

biomass available to the benthic food chain. For example, before their extirpation from

Lake Erie, unionids had the capacity to filter every day between 1.4 and 5.3 % of the total

lake volume (Vanderploeg et al. 1995). Strayer et al. (1994) demonstrated that, in the

Hudson River, filtering by unionids was equivalent in magnitude to downstream flushing.

Biodeposition is a factor in structuring the benthic community of lakes (Sephton et al.


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1980). In Lake St. Clair, before the introduction of D. polymorpha, the unionid

assemblage filtered about 13.5% of the total phosphorus load of the lake, and over 60%

of this amount was sedimented via biodeposition (Nalepa et al. 1991).

Unionids further influence the benthic community through their burrowing

activity. Unionids can migrate horizontally and vertically into the substrate, increasing

the water and oxygen contents of the sediments, altering the profile of various elements,

and creating heterogeneity in microbial activity (McCall et al. 1979).

Freshwater mussels are preyed upon by several fish, invertebrate, reptile,

mammal, and bird species, but large adults are relatively immune to predation and act as

nutrient sinks (McMahon 1991). Unionids release glochidia in large numbers, but few

will successfully attach to a host fish (see above; McMahon 1991). Therefore, the larvae

probably constitute a food source for the pelagic and the benthic communities.

These ecological roles are derived from the literature on unionids in general.

Considering that L. radiata tends to be a dominant member of the unionid assemblage in

its range (Clarke 1981), this species probably contributes an important fraction of the

processes ensured by unionids.

Cultural Importance

Native peoples in North America harvested unionids for food, source material for

tools, and ornamental objects (Williams et al. 1993). Prehistoric sites in eastern North

America often contain large mounds of empty shells, and freshwater mussels were

probably exploited as a supplement to more nutritious resources, without showing any

species preference (Parmalee & Klippel 1974). Therefore, L. radiata has a cultural

importance not as a species but as a member of the unionids.

Commercial Importance

Until the advent of plastics, unionids were commercially harvested in the United

States to manufacture buttons (Williams et al. 1993). In the 1980’s, a new market

developed for the commercial harvest of unionids: the shells of North American

freshwater mussels, grounded into beads, make ideal nuclei for high quality cultured

pearls (Williams et al. 1993). About 80% of the world’s cultured pearls are made in

Japan, and almost all their production starts with a bead from a North American unionid

shell (Gubernick 1990). The Tennessee Shell Co Inc. shipped 2,800 metric tonnes of


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unionid shells to Japan in 1989, about 35 % of the total market, and its projected revenues

for 1990 were U.S. $ 16 million (Gubernick 1990, Williams et al. 1993). Unionids are

also harvested for their pearls, and the rarity of the product makes their price about 100

times higher than cultured pearls (Gubernick 1990). The Tennessee Shell Co Inc. is

perfecting a way to produce freshwater pearls cultivated from unionids (Gubernick 1990).

This commercial harvest of unionids is presently restricted to the Midwestern United

States (Pecharsky et al. 1990), and therefore L. radiata is not, for now, a commercially

valuable species.

Threats to the Species

Habitat Degradation

During the past century, the single most important threat to the survival of

unionids in general has been habitat degradation through impoundment, siltation, and

contamination (Moser 1993, Williams et al. 1993). Impoundment, i.e. damming,

dredging, and channelisation of rivers, transforms the biological, chemical, and physical

environment (Williams and al. 1993). Changes in water level, oxygen content, and

temperature can affect the survival of adult and juvenile freshwater mussels such as L.

radiata, as well as adversely affect host fish (Fuller 1974, Moser 1993, Williams et al.

1993). The importance of this threat is particularly well illustrated by two case studies

showing that impoundment resulted in 30% to 60% destruction of the mussel fauna

(Layzer et al. 1993 and Williams et al. 1992 cited in Williams et al. 1993).

Siltation can occur through many watershed disruptive practices, including

impoundment, logging, agriculture, and road construction. It is especially damaging to

freshwater mussels because silt clogs their gills and siphon, and because it can, through

the same mechanism, affect host fish populations (Fuller 1974, Williams et al. 1993).

Chemical pollution is also an important threat to freshwater mussels (Fuller 1974,

Williams 1993, Moser 1993). Contaminants can destroy mussel fauna directly through

toxicity or indirectly by contributing to the elimination of prey organisms or host fish

(Havlik & Marking 1987). Potassium, zinc, copper sulfate, cadmium, and other pollutants

as well as heavy eutrophication are known to be toxic to freshwater mussels (Havlik &

Marking 1987, Moser 1993). The presence of contaminants in the sediment can remain a

problem even after water quality has improved (Moser 1993).


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Species Introduction (Dreissena polymorpha)

L. radiata has survived in the Great Lakes-St. Lawrence River system despite

severe habitat degradation, but the invasion of D. polymorpha (zebra mussel) has

extirpated this species, along with sympatric unionids, in portions of its range. D.

polymorpha, a freshwater mussel native to Eurasia, was first mentioned in North America

in 1988 by Hebert et al. (1989), in Lake St. Clair (between Lake Huron and Lake Erie). It

is thought to have come in the ballast waters of a ship in 1986 or 1987 (Griffith et al.

1991). It reproduces via planktonic larvae, veligers, and it secretes byssal thread to attach

to hard substrate, two unique features considering native North American freshwater

mussel fauna. These characteristics confer definite advantages to D. polymorpha over

unionids: they have tremendous dispersal capabilities and they attach preferentially to the

shells of unionids (Lewandowski 1976 cited in Gillis & Mackie 1994, Lapierre et al.

1994). D. polymorpha rapidly took over Lake St. Clair and Lake Erie, reaching maximal

densities of 700,000 per square meter (Griffiths et al. 1991). Its range is continually

extending. There are established populations in all of the Great Lakes, the St. Lawrence

River, the Lake Champlain/Hudson River system (Strayer & Smith 1996, Caraco et al.

1997), the Mississippi River basin (Mongeau & Jacquaz 1991; Figure 3), and the Rideau

River (Martel 1995).

D. polymorpha represents a very serious threat to unionids (Figures 4 and 5). As

its density increases in an area, its density per unionid increases, and the proportion of

dead unionids increases as well (Ricciardi et al. 1995). For example, two years after its

first mention in Lake St. Clair (east basin), D. polymorpha had reached a density of

11,655 individuals per meter square, the mean density per unionid was about 5,500

individuals, and the proportion of dead unionids in the total found was 98% (Hebert et al.

1991, Hunter & Bailey 1992).

L. radiata is particularly sensitive to fouling by D. polymorpha, and females are

more sensitive than males (Haag et al. 1993). This sensitivity is linked to the long-term

brooding strategy of L. radiata. D. polymorpha affects L. radiata by competing for food,

preventing normal respiration, and interfering with burrowing activities (Haag et al. 1993,

Byrne et al. 1995, Baker & Hornbach 1997). Furthermore, D. polymorpha profoundly

changes ecosystem processes such as water clearing, sedimentation rate, and metal


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cycling in lakes (Klerks et al. 1996, Caraco et al. 1997, Horgan & Mills 1997, Klerks et

al. 1997). In the Great Lakes, shifts from a pelagic to a benthic food chain have been

documented (reviewed in Nalepa et al. 1996). This profoundly changed the composition

of the aquatic communities and could have indirect effects on L. radiata and other

unionid species.

Up until now, L. radiata has been extirpated from Lake St. Clair and most of Lake

Erie (Nalepa et al. 1996, Schloesser & Nalepa 1994), and other populations in the St.

Lawrence River have been extirpated or are declining (Ricciardi et al. 1996, A. Ricciardi,

pers. comm.).

Status Assessment

IUCN categories and criteria (IUCN 1996) were used to assess the conservation

status of L. radiata. This species was found to be Endangered, according to a projected

population decline of approximately 50% within three generations. This projection is

based on direct observation, a decline in the extent of occurrence, and the effects of an

introduced taxon, D. polymorpha (IUCN criteria A2ace).

For this assessment, the projected population decline was quantified using only

the effects of the most pressing threat on L. radiata, namely the expanding invasion of D.

polymorpha. Therefore, this assessment is not exhaustive, and the projected population

decline is likely to be underestimated.

Generation time was estimated to be 8.1 years, using the age distribution of an

established population in Lake St. Louis (St. Lawrence River; Magnin & Stanczykowska

1971), and 9.6 years, using another population in Lake St. Clair (Nalepa & Gauvin 1988;

Figure 7). In both studies, the authors counted the numbers of growth annuli on the outer

surface of the shell, a widely used method (e.g. McCuaig & Green 1983, Tessier et al.

1994). The population decline of L. radiata was assessed over three generations,

therefore 24.4 to 28.8 years, or about 27 years.

L. radiata has been extirpated from Lake St. Clair and most of Lake Erie due to

heavy infestation by D. polymorpha. This introduced species is rapidly spreading,

therefore it is reasonable to predict that, within about 27 years, D. polymorpha will be

well established in all the shallow habitats of the Great Lakes.


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To calculate the population decline of L. radiata, it is assumed that 1) it will be

extirpated from the Great Lakes and their drainage basin and 2) it is evenly distributed

throughout its extent of occurrence. Assumption 1) is reasonable considering the

expansion of D. polymorpha’s range and the sensitivity of L. radiata to infestation by D.

polymorpha. There is no data on the density of L. radiata in streams and lakes outside the

Great Lakes- St. Lawrence River system, therefore assumption 2) was formulated based

on the information available in the literature. The effects of this assumption on the

estimate of the future population decline will be discussed below.

Considering the above statements, the projected population decline of L. radiata

in 27 years was calculated as the fraction of the total extent of occurrence the Great Lakes

and their drainage basin represent. The ‘cut and weigh’ method was used to calculate the

area of the Great Lakes and their basins and the total area of the extent of occurrence of

L. radiata (J. Kalff, pers. comm.). An independent reference was used for the area

covered by the Great Lakes and their basins (Government of Canada & U.S.E.P.A. 1995),

and that allowed us to estimate the relative error on our measurements. According to

these calculations, the projected population decline of L. radiata over 27 years is 49 ± 5

in percent (value ± absolute error; see Appendix 1).

There are three scenarios concerning the assumption of the relative distribution of

L. radiata throughout its range. 1) If it were evenly distributed, the estimate of the

projected population decline would be as above. 2) If it were relatively more abundant in

the Great Lakes portion of its range compared to the rest of its range, it would increase

the estimated projected population decline. 3) If it were relatively less abundant in the

Great Lakes portion of its range compared to the rest of its range, it would decrease the

estimated projected population decline. To evaluate which scenario is most likely, the

ratio of suitable to unsuitable habitat for L. radiata was estimated for different portions of

its range. L. radiata being restricted to shallow parts of the Great Lakes (McMahon

1991), the ratio of area shallower than the thermocline (about 18 m; Patalas 1984) to

area deeper than the thermocline was used to estimate the ratio of suitable to unsuitable

habitat. For the rest of its range, this ratio was estimated as the ratio of area covered by

water (streams and small lakes) to area being land. The ratio of suitable to unsuitable

habitat in the Great Lakes was higher than in the rest of L. radiata’s range (see Appendix


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2). There is no indication of unionids occurring in higher densities in streams compared

to large lakes, therefore L. radiata is likely to be relatively more abundant in the Great

Lakes portion of its range than in the rest of its range. Nevertheless, to avoid being

unnecessarily precautionary, the assumption that L. radiata is evenly distributed

throughout its range was retained.

Forty nine percent is at the boundary between the categories Vulnerable and

Endangered. Since these calculations are likely to underestimate the future population

decline of L. radiata, it is appropriate to assign this species to the category Endangered.

Interpretive Problems

This assessment of the conservation status of L. radiata relies on two major

assumptions and rough estimates. However, these assumptions are converging toward a

same end point: the effects of threats on the persistence of L. radiata and, hence, the

future population decline of the mussel throughout its range were underestimated.

Therefore, assessing L. radiata as Endangered is appropriate.

Most scientists use the growth annuli present on the outer shells of mussels to

evaluate age. Iseley (1914, cited in Nalepa & Gauvin 1988) considered that the method

was reasonably accurate after an extensive study on over 1,100 specimens. However,

recent studies that used either direct observations (Downing et al. 1992, Kesler &

Downing 1997) or δ18O (Veinott & Cornett 1996) showed that the growth annuli

counting method was likely to underestimate the age of freshwater mussels. Therefore,

the calculated generation time for L. radiata was also underestimated. This is likely to

underestimate the population decline of L. radiata over three generations.

Furthermore, L. radiata is susceptible to threats other than infestation by D.

polymorpha. This conservation status assessment did not attempt to quantify and include

these threats in its predictions. However, threats, such as pollution-induced acute stress,

impoundment, and deteriorating host fish populations due to habitat destruction, have

negative effects on unionids populations and had started to affect unionids prior to D.

polymorpha invasion (Fuller 1974, Havlik & Marking 1987, Moser 1993, Williams et al.

1993). Since these deleterious effects were not included in the assessment, the predicted

population decline is likely to be underestimated. This is amplified by the fact that these


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threats occur throughout L. radiata’s range and affect populations that were not taken

into account in the assessment.

Also, despite the fact that D. polymorpha is known to be present and expanding in

numerous North American basins, the deleterious effects of D. polymorpha on L. radiata

were not considered outside the Great Lakes and their drainage basin. The lack of

species-specific data on the effects of D. polymorpha on unionids precluded the inclusion

of these areas in our analysis. Nevertheless, it is likely that invasion of D. polymorpha in

the rest of L. radiata’s range will have similar deleterious effects on the native mussels as

those observed in the Great Lakes.

Projections

The population decline of L. radiata is primarily due to infestation by D.

polymorpha. Several sources predict that D. polymorpha will be present in most of the

United States and southern Canada (e.g. McMahon 1991, Strayer 1991, Williams et al.

1993; Figure 8). The only basin within L. radiata’s range in which D. polymorpha has

not been mentioned yet is the Atlantic drainage (except the Hudson River). Only human

vectors could allow D. polymorpha to spread to this basin. Therefore, further population

decline of L. radiata will depend on the efficiency of measures to prevent the spread of

D. polymorpha in the Atlantic drainage basin.

Habitat degradation through pollution, impoundment, and siltation has detrimental

effects on L. radiata. As L. radiata declines, these threats will have to be closely

monitored or they could undermine conservation efforts such as preservation of natural

refuges and relocation programs.

Die-offs of unknown causes have been reported for several unionid species,

including L. radiata, prior to D. polymorpha’s introduction (Nalepa & Gauvin 1988,

Moser 1993). This is a difficult factor to include into our projections, but it is important

to recognise the risk of a die-off when managing a declining population.

Existing Management

There is no existing management specifically directed at the conservation of L.

radiata. However, there are measures to try to control the invasion and the effects of D.

polymorpha, a major threat for L. radiata. Moreover, other unionids, such as the dwarf


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wedge mussel (Alasmidonta heterodon), have been the object of recovery plans or

management plans that can help in the design of an action plan for L. radiata.

D. polymorpha probably arrived in the ballast waters of a transatlantic vessel;

since 1989, Canada has had a voluntary program that recommends ballast water exchange

at sea for all sea vessels entering the St. Lawrence sea way (Griffiths et al. 1991). Early in

its implementation, the Canadian Coast Guard reported an 80% compliance rate (Malia

1990). This voluntary program was established as a result of growing awareness of the

costly effects of the zebra mussels for companies with water intake pipes in the Great

Lakes (Malia 1990). The first company to have reported problems due to D. polymorpha

invasion is the Monroe Water Works, Michigan, in January 1989, a water treatment plant

for a city of 24,000 people. Between 1989 and 1991, this community had several water

outages due to clogging of the raw water pipelines by D. polymorpha invasion. Control of

the invasion was achieved by scrubbing followed by continuous chlorination of the

pipelines. The costs of developing, implementing, and running the control strategy were

U.S. $ 300,950 for 2 years, and more research was underway to find a control strategy

less toxic to non-target species and less damaging to pipes (Lepage 1993).

Different chemical and biological controls of D. polymorpha have been

investigated with little prospect of success. Chemical controls include treatments such as

chlorination of water intake pipes (Lepage 1993), control of spawning using serotonin,

and local application of molluscicides (Ram & Nichols 1993; see Nalepa & Schloesser

1993 for a review). Biological controls include predation by fish, invertebrate, reptile,

mammal, and bird species (e.g. French & Bur 1993). Both biological and chemical

controls of D. polymorpha are risky measures, and they should not be the focus of

conservation efforts for L. radiata.

Because the introduction of D. polymorpha does not seem to be reversible,

measures focused on the protection of L. radiata have to be considered. The American

Fisheries Society Endangered Species Committee (Williams et al. 1993) lists the status of

L. radiata as ‘currently stable’, and this species does not have a recovery plan.

Nevertheless, other unionid species have been the object of recovery plans. For example,

the U.S. Fish and Wildlife Service published a recovery plan for the dwarf wedge mussel

in 1993 (Moser 1993). This plan recommended the protection of the remaining


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populations and considered the re-establishment of populations within its historical range.

Protection of remaining populations included actions such as identification of essential

habitats and utilisation of existing legislation and regulations to protect the species

(Moser 1993).

Action Plan

The assessment of L. radiata as an Endangered species is based on their predicted

extirpation from the Great Lakes and their drainage basin in the next 27 years. There is

little that can be done to stop D. polymorpha from spreading in the Great Lakes or to

minimise its effects on L. radiata. The objective of this action plan is therefore to

maintain existing populations of L. radiata outside the Great Lakes. To do so, it is

proposed to 1) limit the spread of D. polymorpha outside the Great Lakes, 2) identify

areas less susceptible to D. polymorpha, 3) protect populations less susceptible to D.

polymorpha invasion from anthropogenic sources of threat, and 4) consider L. radiata

relocation programs.

Limitation of D. polymorpha Invasion outside the Great Lakes

Rapid downstream dispersal of D. polymorpha can be attributed to high female

fecundity (104-106 eggs.yr-1) and a pelagic larval stage (Griffiths et al. 1991). Dispersal

upstream and across watersheds can be explained by human agents such as discharge of

ballast waters from Lake Erie in the upper Great Lakes, movements of small boats, and

sales of live baits harvested from Lake Erie and Lake St. Clair (Griffiths et al.1991).

Therefore, limiting the spread of D. polymorpha outside the Great Lakes will involve

raising the public’s awareness about the economic and environmental problems linked to

such activities. For example, Griffiths et al. (1991) stated that boat hulls provide good

habitat for D. polymorpha and that the attached mussels are able to survive outside water

for at least a few days. A particularly good dispersal route for D. polymorpha would be

Interstates 77 and 79, used every autumn to move American pleasure boats from Lake

Erie to waterbodies in the south-eastern United States. This makes L. radiata in the

Atlantic drainage basin susceptible to invasion by D. polymorpha. It is proposed to focus

a public education campaign on preserving the Atlantic drainage basin from D.

polymorpha introduction since it can only disperse there through human vectors. A good

medium for this campaign would be posters, short conferences, and direct interventions


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by Fisheries agents (American and Canadian) in marinas around Lake Erie and the other

Great Lakes. This campaign would promote the conservation and economic advantages

of limiting D. polymorpha’s expansion in North America, and could be partially

supported by funding from industries depending on water intake in the Atlantic drainage

basin.

Listing Habitats less Suitable to D. polymorpha Invasion

D. polymorpha’s requirements in terms of pH and calcium concentration differ

from the requirements of unionids. Therefore, not all unionid habitat is susceptible to

heavy invasion by D. polymorpha (Neary & Leach 1992, Mellina & Rasmussen 1994,

Bergeron 1995). For example, of 152 lakes south of the St. Lawrence River and Anticosti

Island in Quebec, only 31.5 % were susceptible to D. polymorpha proliferation (pH > 7.4

and Ca >10mg.l-1; Bergeron 1995). Within the range of L. radiata, habitats that are

recognised as less susceptible to invasion by D. polymorpha should be prioritised as

potential refuges for the species. The published studies on the requirements and potential

distribution of D. polymorpha are based on known ecological needs derived from

European publications and known water chemistry of North American water bodies

(Neary & Leach 1992, Mellina & Rasmussen 1994, Bergeron 1995). Nevertheless, other

habitats seem to be less susceptible to D. polymorpha invasion, because of synergism

between physical properties of the water and native biological controls of the introduced

mussel (Tucker & Atwood 1995). Knowledge of the suitability of habitats to invasion by

D. polymorpha should be refined in order to have a more accurate list of habitats to

prioritise as potential refuges for L. radiata.

Minimising Anthropogenic Sources of Threat

Anthropogenic sources of threat to L. radiata should be minimised in habitats

recognised as less susceptible to D. polymorpha invasion. Protection of these habitats

could be achieved at the local level through raising community awareness and focusing

on land use planning, both for public and private development. Aquatic habitats have

many values that could be emphasised to promote their protection. For example, a

municipality could decide to protect its aquatic habitats because of the recreational and

social values of a creek or a lake on their territory (Wm. P. Lucey, pers.comm.).

Protection of aquatic habitats can also receive attention through the growing awareness of


17

the importance of preserving freshwater resources. Therefore, L. radiata and other

species could receive protection through the conservation of their habitats.

Relocation Programs for L. radiata

To supplement existing populations of L. radiata occurring in habitats less

susceptible to D. polymorpha, relocation programs could be proposed. Within the natural

range of L. radiata, small lakes and reservoirs may be devoid of L. radiata. These

habitats could be recipient for relocated individuals under threat of imminent elimination

by expanding D. polymorpha populations. These habitats could then be protected against

D. polymorpha’s introduction and other sources of threat, and act as refuges.

Relocation is a widely used management tool for conservation of unionids.

Nevertheless, few projects report the detailed methods used or the success of the

relocation program. Indeed, 60% of the projects surveyed by Cope and Waller (1995)

were not monitored after the relocation effort or were monitored for less then a year.

Prior to extensive relocation programs of L. radiata or other unionids, a workshop should

be organised to provide a forum for people with an array of expertise and experience in

the field of freshwater mussel biology, ecology, and conservation. One objective of the

workshop would be to produce a widely distributed document with recommendations for

relocation programs of unionids. Questions to be examined include: ‘In what

circumstances would relocation be an advisable management tool?’, ‘What type of

habitat, in terms of physical, chemical, and biological environment, is suitable for a

relocation program?’, ‘What measures should be taken to minimise the chances of

contamination by D. polymorpha?’, ‘What should be measured to assess the success of a

relocation program?’, ‘How many populations of a species are needed to ensure its

persistence?’, ‘What is the effect of relocation of unionids on the community of the

receiving environment?’. Participants to the workshop could include a selection of

interested people among the authors of the attached references, as well as people who

have carried relocation programs in the field (see references of Cope & Waller 1995).

Future Research

There is a need to provide distribution and density information about L. radiata

and other unionids in the Atlantic drainage basin. This would provide baseline data to


18

monitor population trends over time. It would also help in revising the status assessment

of L. radiata.

Moreover, the method used to age unionids should be improved to account for the

fact that growth annuli are not necessarily produced annually. This could allow to

accurately quantify age-dependent parameters, such as generation time.

Other areas of research could include monitoring habitat susceptibility to D.

polymorpha invasion and standardising relocation strategies, as mentioned in the action

plan. Efforts to confirm the fish species capable of acting as hosts for the glochidia of L.

radiata may be necessary to ensure the success of relocation programs, and molecular

tools could be used to help resolve the uncertainty (White et al. 1994).

Acknowledgements We are grateful to Dr. Amanda C. J. Vincent and Jessica Meeuwig for their help throughout the

preparation of this document. We are thankful also to Dr. Joseph B. Rasmussen, Dr. Jacob Kalff, and Dr. Mary Seddon,. Finally, a special thanks to Dr. Anthony Ricciardi for his help and his passion for unionid mussels.


19

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Figures

Figure 1- Distribution of L. radiata (Gmelin, 1792), following Turgeon et al. 1988 and Williams et al. 1993.


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Figure 2 - Typical life cycle of an unionid (from Moser 1993).


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Figure 3 - Distribution of D. polymorpha in the United States in 1993 (from Jacquaz et al. 1994).


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Figure 4 – Mean density of living unionid bivalves and infestation (number of D. polymorpha per unionid) at East section of Soulanges Canal between 1992 and 1995. (Adapted from Ricciardi et al. 1995.)

Figure 5 – Mean densities of L. radiata and D. polymorpha in Northwestern Lake St. Clair between 1986 and 1994. (Adapted from Nalepa et al. 1996.)


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Figure 6 – Relative age-frequency distribution for two populations of L. radiata, in Lake St. Clair (data from Nalepa and Gauvin 1988) and Lake St. Louis (data from Magnin and Stanczykowska 1971).


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Appendix 1

Measured values: - Area of the Great Lakes and their drainage basin (GL): 692, 541

km2

- Total area of the extent of occurrence of L. radiata (Total): 1, 548,

126 km2

Value from the literature (Government of Canada and U.S.E.P.A. 1995):

- Area of the Great Lakes and their drainage basin (GL): 765, 990 km2

Relative error of the ‘cut and weigh’ method:

(Literature GL - Measured GL) ÷ Literature GL X 100

= 10 %

Calculation of the population decline (%):

Literature GL÷ Total X 100

= 49 %

Calculation of absolute error on population decline:

Population decline X relative error

= 5 %


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Appendix 2

A) The ratio Suitable:Unsuitable habitat for L. radiata in the Great Lakes

Area of suitable habitat (km2) *1

Area of unsuitable habitat (km2) *2

Ratio of suitable to unsuitable habitat (%)

Lake Ontario 3,199 15,761 20 Lake Erie 12,601 13,099 96 Lake Huron 7,152 52,448 14 Lake Superior 4,351 77,749 6 Lake Michigan 7,283 50,517 14 Total Great Lakes 34,586 209,574 17 *1 Derived from hypsographic curves and a threshold depth of 18 m; Ristic and Ristic 1981, Ristic and Ristic 1985a, Ristic and Ristic 1985b, Ristic and Ristic 1989, Ristic 1989. *2 Data from Government of Canada and U.S. E.P.A. 1995. B) The ratio Suitable:Unsuitable habitat for L. radiata in the rest of its range*3 Portions of extent of occurrence, the Great Lakes removed

Area of suitable habitat (km2) *4

Area of unsuitable habitat (km2) *4

Ratio of suitable to unsuitable habitat (%)

Canadian extent of occurrence

27,820 492,180 6

Maine 5,910 80,117 7 New Hampshire 702 23,395 3 Vermont 868 24,019 4 Massachusetts 1,098 20,287 5 Rhode Island 427 2,717 16 Connecticut 360 12,613 3 New Jersey 787 19,508 4 New York 4,421 123,979 4 Pennsylvania 798 116,614 1 Delaware 194 5,101 4 Maryland 1,777 25,617 7 West Virginia 251 62,445 < 1 Virginia 2,528 103,188 2 South Carolina 2,007 78,425 3 Total without Great Lakes 49,948 1,190,205 4 *3 Here, suitable habitat includes all interior water except the Great Lakes. The portion of salt water is too important in North Carolina, therefore this State was excluded from the analysis. *4 Canada: estimated using data from Energy, Mines and Resources 1974. U.S.: estimated using data from Geraghty et al. 1973 and Larousse 1987.

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