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Margaritifera margaritifera 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Mollusca Bivalvia Unionoida Margaritiferidae

Scientific Name: Margaritifera margaritifera Linnaeus, 1758
Infra-specific Taxa Assessed:
Regional Assessments:
Common Name(s):
English Freshwater Pearl Mussel

Assessment Information [top]

Red List Category & Criteria: Endangered A2c ver 3.1
Year Published: 2017
Date Assessed: 2010-02-02
Assessor(s): Moorkens, E., Cordeiro, J., Seddon, M.B., von Proschwitz, T. & Woolnough, D.
Reviewer(s): Aldridge, D., Bogan, A.E., Cuttelod, A., Vinarski, M. & Ormes, M.
Contributor(s): Aldridge, D., Araujo, R., Killeen, I., Cochet, G., Geist, J., Greke, K., Lopes-Lima , M., von Proschwitz, T., Prié, V., Vinarski, M., Lepitzki, D. & Ormes, M.
Justification:
This long-lived mussel has been in decline for the last century and, even in the areas where it has been considered to be stable in the past, a high percentage of non-recruiting subpopulations are known.

In Europe, population information is available from EU Article 17 reporting, with no country reporting favourable conservation status. In North America population data were estimated by NatureServe, and in Russia data were provided on population decline levels by Henrikson and Soderberg (pers. comm., 2010). Deficiencies in recruitment compared with losses from old age are resulting in steep population declines and population losses on an annual basis.

The generation time for Margaritifera is approximately 30 years, so a three-generation prediction of 90 years has been used to evaluate population reductions. Overall in the last 90 years for global assessment, based on the number of viable and recruiting subpopulations left from the original subpopulations known in 1920, it is estimated that there has been a decline of 61.5%, qualifying the species for the Endangered (EN) category.

The 1994/1996 conservation assessments were based on the number of known subpopulations, without the knowledge that many of these subpopulations had failed to recruit for between 1 and 2 generations (30-60 years) and that habitat conditions were not favourable for juvenile growth. This new assessment reflects the new knowledge, and with this knowledge, the species would still been assessed as Endangered in Europe in 1996.

The unknown element is the number of very large subpopulations that are recruiting as well as they did 90-100 years ago. If enough very large subpopulations (1 million +) are replacing 100 year old mussels with the same numbers of juvenile mussels per annum, the future prospects would be for a large range reduction but some stable remaining populations. However, we do not have sufficient information on the best populations to have confidence that this is the case.

The species has been in decline for the last century, and even in the areas considered to be stable in the past, a higher percentage of non-recruiting sub populations are known.  Given that past conservation assessments were based on the number of known populations, not knowing that many of these subpopulations have failed to recruit for between 1 and 2 generations and that habitat conditions are not favorable for juvenile growth, the current assessment reflects new knowledge, and the species would still been assessed as Endangered in Europe in 1996.

There have been widespread attempts at conservation breeding programmes for this species, with some success, however water conditions are still inadequate in many major rivers, and thus juvenile recruitment remains limited.
Previously published Red List assessments:

Geographic Range [top]

Range Description:This is a Holartic species, found in North America, Europe and through into Siberia. It has been declining throughout the European part of its range.

EUROPE
The following are the most recent revised estimates of current population status of M. margaritifera in Europe. The information is mainly from Geist (2005), and compiled by Geist partly based on data and references in Alvarez-Claudio et al. (2000), Araujo and Ramos (2000), Larsen (2001), Young et al. (2001a), Velasco Marcos et al. (2002), Reis (2003), Morales et al. (2004), Rudzite (2004), Sachteleben et al. (2004), Dolmen and Kleiven (2008) and updated information according to personal communications with M. Porkka, C. Greke, M. Rudzite, D. Telnov, St. Terren, G. Motte, J. Reis, I. Killeen, M. Young, G. Cochet, F. Renard-Laval, E. Holder, P. Durlet, T. Ofenböck, J. Hruška, N. Laanetu, L. Henrikson, T. Von Proschwitz, E. San Miguel Salán, R. Araujo, and from Geist's personal survey work (Moorkens, pers. comm, 2011).

Austria: 29 subpopulations with 50,000 individuals. Only three of these are large subpopulations and there is a strong decline. Less than five of the subpopulations have limited juvenile recruitment.

Belgium: 5-6 subpopulations with 2,500-3,000 individuals. All of these subpopulations are almost extinct with a lack of juvenile recruitment. There has been conservation programmes for these subpopulations since 2002.

Czech Republic: six subpopulations with 80,000 individuals. Three of these subpopulations are at frontier streams, and the three other subpopulations contain more than 20% juveniles, but only one of these is large (60,000 individuals).

Denmark: There is a maximum of one subpopulation with an unknown number of individuals. This subpopulation is probably extinct, as it was last recorded in 1970.

Estonia: One subpopulation with 35,000-40,000 individuals. There has been a lack of juvenile recruitment for at least 40 years.

Finland: 50 subpopulations with 1,500,000 individuals. The largest remaining subpopulation is in the Lutto drainage, Northern Finland. Seventy-five percent of the subpopulations were lost in the 20th century. Eleven important subpopulations remain; some subpopulations have few juveniles, but probably there are only a few functional subpopulations.

France: There are now a maximum of 84 subpopulations with 100,000 individuals. It is scarce in most of its former range; originally the species was abundant in more than 200 rivers. Currently it is found in less than 10 rivers with juveniles; subpopulations are still present in Massif Amoricain (18), Massif Central (57), Morvan (6), Vosges (1) and Pyrenees (2) but there are serious declines; there is one large subpopulation in Dronne (16,000 individuals) with little recruitment and the other subpopulations are mostly small with 10 to 100 individuals, maximum of 300 individuals.

Germany: A maximum of 69 subpopulations with 144,000 individuals. The largest subpopulations (with over 10,000 individuals) are in Bavaria but there are serious declines. There is only one recovering subpopulation with more than 20% juveniles in Northern Germany. Several conservation and breeding programmes have started.

Great Britain: There are more than 105 subpopulations with over 12,000,000 individuals. The best subpopulations are in Scotland but 2/3 of the originally known 155 populations are extinct. Overall there are still over 12,000,000 mussels with one river alone estimated at having 10,000,000 individuals. There are 10 rivers with significant numbers of juveniles and common or abundant adults, and five others with some juveniles but scarce adults. In England, 10 pearl mussel rivers remain (the best subpopulation has over 100,000 mussels but few juveniles and evidence of decline). In Wales there are 10 pearl mussel rivers (the best has less than 1,000 mussels).

Republic of Ireland
: There are 139 rivers holding subpopulations with 12,000,000 individuals. The best rivers have between 2 and 3 million individuals, however most have just a few thousand. There is a serious decline with few recruiting subpopulations.

Northern Ireland: Ninety percent of the subpopulations have been lost.

Latvia: Eight subpopulations with 25,000 individuals. There is a serious decline, as no subpopulation with juvenile recruitment remains.

Lithuania: Possibly one subpopulation which the status of is unknown.

Luxembourg: One sub-population with 1,000-1,500 individuals. This species is almost extinct in Luxembourg.

Norway: 340-350 subpopulations, probably with 300 million individuals. There is a serious decline, especially in the South, however the exact distribution, total numbers and juvenile status is unclear.

Poland: There are no remaining subpopulation in Poland and it is considered Extinct here.

Portugal: Six subpopulations with more than 1,000,000 individuals. There is a severe decline, with three large subpopulations (22,000; 50,000; 1 million) with evidence for juvenile recruitment remaining, but serious declines expected in two of them due to recent construction of man-made dams.

Russia: More than eight subpopulations with more than 100,000,000 individuals. There is a serious decline, however four populations of over 1 million individuals remain.

Spain: 19 subpopulations; at least 17 in Galicia, one in Asturias, and one in Salamanca; but the number of individuals is unknown. There is a serious decline as there is probably no more than two reproductive subpopulations with significant numbers of juveniles.

Sweden: More than 400 subpopulations with more than 8,000,000 individuals. There are serious declines; at least 50 subpopulations have significant numbers of juveniles less than 50 mm in size.

NORTH AMERICA
In North America this mussel is distributed from Newfoundland and Labrador down to Pennsylvania and Delaware and west to the Appalachian mountains. It is widespread in New England and the Canadian Maritime Provinces (Cordeiro 2008). The current status across its range is as follows:

USA: Declining and of special concern but somewhat stable in New England.

Canada: Some large populations remain and threats are few.

In North America over 1,500 subpopulations were originally known, of which an estimated 900 are considered viable and recruiting (J. Cordeiro, pers. comm., 2010).
Countries occurrence:
Native:
Austria; Belgium; Canada (Labrador, New Brunswick, Newfoundland I, Nova Scotia, Ontario, Québec); Czech Republic; Denmark; Estonia; Finland; France (France (mainland)); Germany; Ireland; Latvia; Lithuania; Luxembourg; Norway; Portugal (Portugal (mainland)); Russian Federation (European Russia, North European Russia); Slovakia; Spain (Spain (mainland)); Sweden; United Kingdom (Great Britain, Northern Ireland); United States (Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Vermont)
Regionally extinct:
Poland
Additional data:
Number of Locations:2608
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:This species is a special case in having approximately 200,000,000 mature individuals but, due to habitat degradation, having very little replacement as adults die. Functional extinction is widespread throughout most of the range. Most subpopulations are isolated and genetic exchange between subpopulations is unlikely by natural means. Across most of the range adults that die will not be replaced due to habitat changes that cannot support juveniles, so most subpopulations have not recruited for many years.

The following are the most recent revised estimates of current population status and estimated past (90 years) and future (90 years) population and individual number estimates of M. margaritifera in Europe. The information is mainly from Geist (2005), and compiled by Geist partly based on data and references in Alvarez-Claudio et al. (2000), Araujo and Ramos (2000), Larsen (2001), Young et al. (2001a), Velasco Marcos et al. (2002), Reis (2003), Morales et al. (2004), Rudzite (2004), Sachteleben et al. (2004), Dolmen and Kleiven (2008) and updated information according to personal communications with M. Porkka, C. Greke, M. Rudzite, D. Telnov, St. Terren, G. Motte, J. Reis, E. Moorkens, I. Killeen, M. Young, G. Cochet, F. Renard-Laval, E. Holder, P. Durlet, T. Ofenböck, J. Hruška, N. Laanetu, L. Henrikson, T. Von Proschwitz, E. San Miguel Salán, R. Araujo, and from personal survey work. (E. Moorkens pers. comm. 2010).

Generation length for Margaritifera is approximately 30 years, so a three-generation prediction of 90 years has been used to evaluate the population losses. Overall in the last 90 years for Europe there has been a decline of 81.5%; a loss of 87% for EU countries. The overall global reduction in population size over the same period is 61.5%.

Austria: There are currently 29 subpopulations. The past number of subpopulations is unknown, and the estimated future number of populations is three. There are currently 50,000 individuals; the estimated past number of individuals is 5,000,000 and the estimated future number of individuals is 1,000 (estimated loss of recruiting subpopulations 82% over the last three generations).

Belgium: There are currently 5-6 subpopulations. The past number of subpopulations is unknown and the estimated future number of populations is 0. There are currently 2,500-3,000 individuals; the estimated past number of individuals is 300,00 and the estimated future number of individuals is 0 (estimated loss of recruiting subpopulations 100% over last three generations).

Czech Republic: There are currently six subpopulations. The past number of subpopulations is unknown and the estimated future number of populations is 1. There are currently 80,000 individuals; the estimated past number of individuals is 8,000,000 and the estimated future number of individuals is 1,000 (estimated loss of recruiting subpopulations 0% over last three generations).

Denmark (maximum): There is currently one subpopulation. The estimated past number of subpopulations is more than one and the estimated future number of populations is 0. There are currently an unknown number of individuals; the estimated past number of individuals is 10,000 and the estimated future number of individuals is 0 (estimated loss of recruiting subpopulations 100% over last three generations).

Estonia: There is currently one subpopulation. The estimated past number of subpopulations is more than one and the estimated future number of populations is 0. There are currently 35,000-40,000 individuals; the estimated past number of individuals is 400,000 and the estimated future number of individuals is 0 (estimated loss of recruiting subpopulations 100% over last three generations).

Finland: There are currently 50 subpopulations. The estimated past number of subpopulations is 200 and the estimated future number of populations is five. There are currently 1,500,000 individuals; the estimated past number of individuals is 50,000,000 and the estimated future number of individuals is 100,000 (estimated loss of recruiting subpopulations 78% over last three generations).

France: There are currently 84 subpopulations. The estimated past number of subpopulations is 200 and the estimated future number of populations is 10. There are currently 100,000 individuals; the estimated past number of individuals is 50,000,000 and the estimated future number of individuals is 10,000 (estimated loss of recruiting subpopulations 96% over last three generations).

Germany: There are currently 69 subpopulations. The estimated past number of subpopulations is more than 100 and the estimated future number of populations is one. There are currently 144,000 individuals; the estimated past number of individuals is 25,000,000 and the estimated future number of individuals is10,000 (estimated loss of recruiting subpopulations 94% over last three generations).

Great Britain: There are currently  105 subpopulations. The estimated past number of subpopulations is 200 and the estimated future number of populations is 10. There are currently 12,000,000 individuals; the estimated past number of individuals is 20,000,000 and the estimated future number of individuals is 1,000,000 (estimated loss of recruiting subpopulations 90% over last three generations).

Republic of Ireland: There are currently 139 subpopulations. The estimated past number of subpopulations is 150 and the estimated future number of populations is six. There are currently 12,000,000 individuals; the estimated past number of individuals is 20,000,000 and the estimated future number of individuals is 600,000 (estimated loss of recruiting subpopulations 96% over last three generations).

Latvia: There are currently eight subpopulations. The estimated past number of subpopulations is 40 and the estimated future number of populations is 0. There are currently 25,000 individuals; the estimated past number of individuals is 5,000,000 and the estimated future number of individuals is 0 (estimated loss of recruiting subpopulations 100% over last three generations).

Lithuania: There is currently possibly one subpopulation. The past status of the subpopulations is unknown and it is not possible to estimate the future number of subpopulations. The past, current and future number of individuals is unknown or not possible to estimate (estimated loss of recruiting subpopulations 100% over last three generations).

Luxembourg: There is currently one subpopulation. The estimated past number of subpopulations is more than one and the estimated future number of populations is 0. There are currently 1,000-1,500 individuals; the estimated past number of individuals is 200,000 and the estimated future number of individuals is 0 (estimated loss of recruiting subpopulations 100% over last three generations).

Norway: There are currently 340-350 subpopulations. The estimated past number of subpopulations is 750 and it is not possible to estimate the future number of subpopulations. There are currently 300 million individuals. The estimated past number of individuals is 1,000 million and it is not possible to estimate the future number of individuals (estimated loss of recruiting subpopulations 60% over last three generations, based on 40% loss and 20% not recruiting).

Poland: This species is considered Extinct in Poland so there are currently no subpopulations or individuals. The estimated number of subpopulations and number of individuals in the past 90 years is 0. It is not possible to estimate the future number of subpopulations or individuals (estimated loss of recruiting subpopulations 100% over last three generations).

Portugal: There are currently six subpopulations. The past number of subpopulations is unknown and the estimated future number of populations is three. There are currently 1,000,000 individuals; the past number of individuals is unknown and the estimated future number of individuals is 100,000 (estimated loss of recruiting subpopulations 50% over last three generations, predicted to be another two subpopulations extinct in next three generations).

Russia: There are currently 10 subpopulations. The past number of subpopulations is around 100 and the estimated future number of populations is approximately three. There are currently 100,000,000 individuals; the past number of individuals is unknown and the estimated future number of individuals is 30,000,000 (estimated loss of recruiting subpopulations 90% over last three generations).

Spain: There are currently 19 subpopulations. The past number of subpopulations is unknown and the estimated future number of populations is two. There number of individuals currently is unknown. The past, current and future number of individuals are unknown or not possible estimate (estimated loss of recruiting subpopulations 89% over last three generations).

Sweden: There are currently 400 subpopulations. The past number of subpopulations is unknown and the estimated future number of populations is approximately 50. There are currently 8,000,000 individuals; the past number of individuals is unknown and the estimated future number of individuals is approximately 1,000,000 (estimated loss of recruiting subpopulations 81% over last three generations). Based on present subpopulations of 551, of which only ca 100 are considered viable and recruiting, with the original number of subpopulations in the 1920s being over 1,000.

Total estimate across Europe for 2010: 1,282 subpopulations, 423 million individuals (current date).

Total estimate across Europe for 1920: 2,613 subpopulations, 2,000 million individuals (past view at ca two generations).

Total estimate across Europe for 2100:  204 subpopulations, 47 million individuals (forward projection at ca two generations).
Current Population Trend:Decreasing
Additional data:
Continuing decline of mature individuals:Yes
Extreme fluctuations:NoPopulation severely fragmented:Yes

Habitat and Ecology [top]

Habitat and Ecology:Sustainable populations of the Freshwater Pearl Mussel are restricted to near natural, clean flowing waters, often downstream of ultra-oligotrophic lakes. A small number of records are from the lakes themselves.

Margaritifera requires stable cobble and gravel substrates with very little fine material below pea-sized gravel. Adult mussels are two-thirds buried and juveniles up to 5-10 years old are totally buried within the substrate. The lack of fine material in the river bed substrate allows for free water exchange between the open river and the water within the substrate. The free exchange of water means that oxygen levels within the substrate do not fall below those of the open water. This is essential for juvenile recruitment, as this species requires continuous high oxygen levels.

The clean substrate must be free of inorganic silt, organic peat, and detritus, as these can all block oxygen exchange. Organic particles within the substrate can exacerbate the problem by consuming oxygen during the process of decomposition. The habitat must be free of filamentous algal growth and rooted macrophyte growth. Both block the free exchange of water between the river and the substrate and may also cause night time drops in oxygen at the water-sediment interface.

The open water must be of high quality with very low nutrient concentrations, in order to limit algal and macrophyte growth. Nutrient levels must be close to reference levels for ultra-oligotrophic rivers. Phosphorus must never reach values that could allow for sustained, excessive filamentous algal growth.

The presence of sufficient salmonid fish to carry the larval glochidial stage of the pearl mussel life cycle is essential.

Intact natural catchments prevent fine sediment and nutrient losses to the river. As fine sediment losses become chronic, siltation of the substrate can provide a rooting medium for higher plants. Nutrients can also accumulate in the sediment (and may be chronically or intermittently available in the open water), promoting the growth of algae and macrophytes. This exacerbates the stressful environment for the adult and juvenile mussels, and as more adults are lost, further niches for macrophyte growth become available. There is a resultant trophic cascade in the habitat, where oligotrophic conditions succeed to eutrophic conditions and the suite of invertebrate species changes accordingly. Thus, the conservation targets for mussel populations include maintenance of free water exchange between the river and the substrate and minimal coverage by algae and weed. The  most important requirement is the maintenance of recruitment, i.e., the river bed structure required to breed the next generation. Nutrients in the sediment are problematic as they are used by rooted plants. Dissolved nutrients in the water column tend to lead to algal growth. Both come from chronic and/or periodic loss of dissolved nutrients from the catchment.

Margaritifera is highly demanding of very clean river habitats in order to be self-sustaining, but it lives for over 100 years, and thus non-sustainable populations of adult mussels can persist for many years after negative changes in the habitat have occurred. While a range of possible causes of decline can exist (e.g., direct habitat damage, acidification of rivers (particularly in Scandinavia) and depletion of mussels from pearl fishing activities), the overwhelming majority of population declines in Europe have been due to sediment accumulation in the river bed gravels, cutting off the supply of oxygen to juvenile mussels. New generations of mussels cannot be recruited, while older adults that were born before the habitat deterioration remain alive as they are filtering open rather than interstitial water. The source of pressures that lead to this decline come from the catchment into the river, thus protection and rehabilitation of mussel populations is impossible without effective catchment management that is protective to the juvenile mussel habitat. The best populations are known from low-intensively managed isolated catchments with little influence from man. However, some famous historical populations persist in low numbers in large, lowland rivers, where adults may be living in habitats that could not possibly sustain juveniles.
Systems:Freshwater
Continuing decline in area, extent and/or quality of habitat:Yes
Generation Length (years):30

Use and Trade [top]

Use and Trade: Pearl fishing was carried out both casually and by a group of people that regularly took pearls for profit. Some pearl fishing continues today and is a serious threat to the species. Pearl fishing is illegal throughout the EU.

Threats [top]

Major Threat(s): There are a number of factors leading to the decline and loss of Freshwater Pearl Mussel subpopulations (Moorkens 1999, Araujo and Ramos 2001). Margaritifera is highly demanding of very clean river habitats in order to be self-sustaining, but it lives for over 100 years, and thus non-sustainable subpopulations of adult mussels can persist for many years after negative changes in the habitat have occurred. While a range of possible causes of decline can exist (e.g., direct habitat damage, acidification of rivers (particularly in Scandinavia) and depletion of mussels through pearl fishing activities), the overwhelming majority of population declines in Europe have been due to sediment accumulation in the river bed gravels, cutting off the supply of oxygen to juvenile mussels. New generations of mussels cannot be recruited, while older adults that were born before the habitat deterioration remain alive as they are filtering open rather than interstitial water. The source of pressures that lead to this decline come from the catchment into the river, thus protection and rehabilitation of mussel populations is impossible without effective catchment management that is protective to the juvenile mussel habitat. The best populations are known from low-intensively managed isolated catchments with little influence from man. However, some famous historical populations persist in low numbers in large, lowland rivers, where adults may be living in habitats that could not possibly sustain juveniles.

The loss of Freshwater Pearl Mussel subpopulations mostly occurs from continuous failure to produce new generations of mussels because of the loss of clean gravel beds, which have become infiltrated by fine sediment and/or over-grown by algae or macrophytes. These block the required levels of oxygen from reaching young mussels. Juvenile mussels spend their first five to ten years buried within the river bed substrate.

Other ways in which mussel populations can decline and be lost is through adult mussel kills, or loss of host fish which are essential to the life cycle of Margaritifera. Further details of the life cycle can be found in Moorkens (1999).

Fine sediment, once introduced to a pearl mussel river, can continue to cause very serious effects on a long term basis (Ellis 1936, Marking and Bills 1979, Killeen et al. 1998, Araujo and Ramos 2001, Naden et al. 2003). Direct ingestion of silt by adult mussels can lead to rapid death. Turbidity, particularly from fine peat entering the water, causes adult mussels to clam up (they close their shells tightly and do not filter water through their siphons), a response that provides a protection against ingesting damaging fine particles. If the river water remains strongly turbid for a number of days, mussels can die from oxygen starvation, either from remaining clammed, or from ingesting contaminated water while stressed. During a time of year when water temperatures are high, oxygen depletion in the body occurs more rapidly, and mussels die more quickly. The evolutionarily primitive Margaritifera gills and the annual brooding of young in all four of the gills demand a continuous, high supply of oxygen. Even if the adult mussels survive an initial silt episode, food/oxygen deprivation from clamming will have caused them to become stressed, from which they will take a long time to recover. If during that recovery period, there are further incidents of mobilisation of this or other silt, then the stressed mussels will be more susceptible to death than mussels in a cold river in unstressed conditions. Thus, they may continue to die over a period of several months. Higher temperatures throughout the summer further exacerbate this problem.

Once a silt load enters a river that holds a pearl mussel population, it can continue to cause harm. Silt causes river changes, which in turn change the dynamics of the river into the future (Dietrich et al. 1989, Colosimo and Wilcock 2005, Curran and Wilcock 2005). Increases in fine material in the bed and suspended in the water column, and consequent changes in channel form, may affect mussels in many ways and at various stages in their life cycle. The direct kill to adults is only the first stage in the damage that silt causes to the population. Sediment that infiltrates the substrate decreases oxygen supply in the juvenile habitat, which prevents recruitment of the next generation. The sediment subsequently provides a medium for macrophyte growth, a negative indicator in pearl mussel habitats. Macrophytes then smother the juvenile habitat even further, and the macrophytes trap more sediment, exacerbating the problem in the long term. One of the most essential requirements for pearl mussel conservation is the removal of the risk of any sediment reaching the river, as any one single incident has such long term ramifications.

Silt infiltration of river bed gravels can also have a negative effect on the essential species of fish that host the mussel glochidial stage (Levasseur et al. 2006).

As with siltation, nutrient enrichment can have serious and ongoing impacts on both juvenile and adult mussels. Increased inputs of dissolved nutrients to mussel rivers tend to lead to filamentous algal growth, unless combined with siltation, where macrophyte growth can dominate. Filamentous algae can lead to the death of juvenile mussels, through blocking oxygen exchange with the sediment, and cause adults to become stressed, as a result of night time drops in oxygen. Even if filamentous algae are destroyed in a flood, adult mussels may not make a full recovery before the algae re-grows. Adult mussels may eventually die as a result of oxygen/food deprivation.

Death and decomposition of filamentous algae contributes fine particulate organic matter to the river substrate. This further blocks water exchange between the river and the substrate and causes additional oxygen depletion through the process of decomposition. Decomposition also releases dissolved nutrients, promoting further primary productivity. Inputs of organic material, such as slurry, to the river have a similar effect on the mussel substrate as dying/decomposing algae and macrophytes.

Major pressures that are leading to damage of river bed substrate from infiltration of inorganic silt, organic fine peat and decaying organic detritus and from eutrophication are listed below. These are pressures that are present in many catchments and their cumulative effects have had very severe impacts on mussels.

Agricultural improvement
Explanation:  Any practice that leads to exposure of bare ground and/or fertiliser applications increase can increase the fine sediment and nutrient load to the river. The cumulative effects of such practices can have very severe impacts on mussels. Liming of land has a negative effect on Margaritifera populations, through direct toxic effects, and through increased growth rates leading to shortened life expectancy and, thus, loss of reproductive years (Bauer et al. 1991, Skinner et al. 2003). In some countries, acidification problems are so severe that liming is considered to have a more positive than negative effect (Henrikson et al. 1995). However, water chemistry data from declining Irish pearl mussel rivers indicate high peaks of calcium and conductivity levels that are likely to have been caused by liming.

Use of pesticides

Explanation:  Toxic pollution can have very serious and long term effects on a pearl mussel river. Of particular concern is agricultural, including forestry, pesticides. Chemical sheep dip is considered to be a very serious ongoing risk to pearl mussel subpopulations, and the most likely cause of a number of major mussel kills (Cosgrove and Young 1998, Moorkens 1999, Skinner et al. 2003, Young 2005). Organophosphates and synthetic pyrethroides used in sheep dipping are highly toxic to species that are a lot less sensitive to nutrient and silt pollution than Margaritifera. The Freshwater Pearl Mussel is too threatened to justify specific laboratory toxicity testing, but this should not be used as a reason to be ambiguous about the threat such pesticides present to Margaritifera. Pesticides present the greatest risk when used in a form that requires dissolving in large quantities of water, which is why sheep dip is the most obviously damaging.

Fertilisation

Explanation:  Any applications of chemical fertiliser or manure can lead to direct run-off of dissolved and particulate nutrients, as well as gradual nutrient release from the soil. The vast majority of Irish pearl mussel subpopulations now exceed the recommended range of nutrient levels for this species. The most seriously damaging nutrient is most probably phosphorus, as it is the limiting nutrient in most Irish pearl mussel rivers. Phosphorus promotes algal growth.

Overgrazing by sheep, cattle or other animals

Explanation:  Overgrazing by sheep in mountainous moor and blanket bog habitats in the upper reaches of pearl mussel catchments has led to loss of vegetation and exposure of peaty soils. This problem has been very serious in some catchments, particularly in parts of the west of Ireland. The bare peaty soil erodes easily and releases fine sediment into the river. Similarly, overgrazing by cattle and other animals along the banks of pearl mussel rivers has lead to, and continues to cause, bank erosion. Furthermore, drinking access for cattle causes direct damage and death to mussels, as well as encouraging further bank erosion and sediment mobilisation.

Restructuring agricultural land holding

Explanation:  Removal of hedges, copses and scrub from lands surrounding pearl mussel rivers is linked with possible kills of adult mussels and declines in the quality of juvenile habitat. These land changes lead to exposure of bare ground that causes the release of silt into the river. They are often accompanied by drainage. Drains themselves can continuously erode and be a source of fine sediment. These newly drained areas are more conducive to agricultural practices of greater intensity than before, thus the problem is exacerbated and ongoing.

General forestry management

Explanation:  Forestry planting, drainage, ground preparation, clear-fell, replanting, thinning and all management practices associated with clear fell plantation have been a major source of both silt and nutrients in pearl mussel catchments. The drainage and other preparations of land for planting and the practice of clear felling leads to exposure of bare ground that can erode and release silt into the river. Fertilisation of forestry leads to a release of nutrients into the watercourse, especially on peat and peaty soils. These nutrients, alone or in association with other nutrient sources, raise the trophic level of the river above limits that are tolerable for the mussel. Brash left on site during and following harvesting operations provides further, long-term inputs of damaging nutrients. Ongoing forestry operations do not allow for recovery of the Margaritifera habitat and the future for pearl mussel rivers with continued forestry operations is bleak. Restoration of pearl mussel populations will only be possible if there are significant initiatives to remove clear-fell forestry from Margaritifera catchments. Even given such a commitment, major mitigation works will be necessary during the removal of the forestry and restoration to low-intensity or semi-natural land uses.

Acidification has been well documented as a threat to salmonid populations both internationally (e.g., Maitland et al. 1987, Lacroix 1989, Henrikson et al. 1995) and in Ireland (Allott et al. 1990, Bowman and Bracken 1993, Kelly Quinn et al. 1997). In Ireland, acidification is linked with coniferous plantations in acid-sensitive areas rather than industrial pollution. As salmonid hosts can come from anywhere within the pearl mussel catchment, protection of the entire catchment from acidification is essential.

Acidification has also been noted as a direct threat to Margaritifera from the first international IUCN Red Data Book for Invertebrates (Wells et al. 1983). Work carried out in Scandinavia has provided evidence for pearl mussel decline from acidification (Eriksson et al. 1981, 1982, 1983; Okland and Okland 1986; Henriksen et al. 1995; Raddum and Fjellheim 2004). A lowering of pH directly influences pearl mussels through a gradual destruction of their calcareous shell, and also their genital organs (causing infertility), and through problems with regulation of acid-base mantle fluid homeostasis (Vinogradov et al. 1987).

Stock feeding

Explanation:  The introduction of nutrients to Margaritifera catchments through the importation of artificial stock feed, e.g. silage, allows increases in the stock numbers. This in turn can cause trampling damage, soil erosion and nutrient releases.

Leisure fishing

Explanation:  If anglers are allowed to enter rivers at pearl mussel beds, serious trampling damage can occur. Systematic physical changes to rivers for the purposes of enhancing fish numbers for angling can also be very damaging to pearl mussel habitat, including bank reinforcement, and the installation of weir and croy structures. Damage occurs during construction, and through changes to flow patterns, leading to scouring of stable gravels and loss of mussels and their habitat in some parts of the river. In other areas ponds are created where silt accumulates with further loss of juvenile and adult habitat.

Taking / removal of fauna
Explanation:  Pearl fishing has been a major problem in the past, and kills from pearl fishing have been observed in recent years in spite of the practice being illegal under EU law.

Margaritifera margaritifera has been exploited for its pearls since Roman times, for leisure and commercial gain, and Ireland's mussels were well known sources of pearls for many years (Cranbrook 1976, Lucey 2005). Pearl fishing has been cited as a threat to pearl mussels across most of its range, and in countries with very low numbers of individuals such as Germany, there are historical records of pearl fishing causing population decline. Recent records of pearl fishing in Ireland are anecdotal, and generally involve Scottish visitors, some of whom come from families that traditionally made a visit to known haunts at periodic intervals. The decline in pearl mussels and the lack of sufficient recruitment has made any pearl fishing unsustainable and the use of tongs to open mussels for pearls has been shown to be damaging (Moorkens and Costello 2004). Thus pearl fishing is outlawed in the EU and any illegal fishing is considered to pose a threat to that population.

Quarries/ Sand and gravel extraction
Explanation:  Pearl mussel populations have been damaged in the past and continue to be damaged both directly through removal of gravel from pearl mussel river beds, and indirectly through silt and other pollution from quarrying activities. Severe episodes of silt lead to adult mussel kills, large and small releases of silt destroy juvenile habitat. Another common problem is the release of calcium from limestone quarries, which increases growth rate in adult mussels, thus shortening mussel lives and reducing the long fertile period required for pearl mussel life history strategy.

Peat extraction
Explanation:  Hand and machine cutting of peat, including the drainage channels used in the process, leads to losses of pearl mussel juvenile habitat from infiltration of river bed substrate by fine peat particles released from bare soil.

Mines
Explanation:  Pollution of water courses from open cast and underground mining by mined heavy metals, and chemicals used in the process of extraction of mined products has led to the loss of pearl mussel populations.

Discharges
  • Urbanised areas, human habitation
Explanation:  Margaritifera is a species of near natural conditions. Continuous urbanisation, discontinuous urbanisation and dispersed habitation have all been associated with depressed water and habitat quality in pearl mussel rivers. Lack of appropriate water treatment (water must reach the river at reference levels), including even small elevation in BOD (Biological Oxygen Demand) levels, and even minor increases in ortho-phosphate levels can lead to loss of juvenile habitat. Inappropriately plumbed washing machines can lead to serious nutrient elevations and subsequent filamentous algal growth.
  • Industrial and commercial areas
Explanation:  Pearl mussels have already been lost from intensively industrialised areas, but local and more rural industries such as meat processors and creameries operate adjacent some extant pearl mussel rivers. High BOD levels and other pollutants have led to loss of juvenile habitat and severe depletion of adult mussels.

Land reclamation
  • Disposal of household waste
  • Disposal of industrial waste
  • Disposal of inert materials
Explanation:  There is evidence of reduced habitat quality for pearl mussels in rivers where landfill sites are present in the catchment. Decreased habitat quality is also found in rivers where household and other waste is dumped into or adjacent the river instead of being properly disposed of, and in rivers where inert materials such as builder's rubble have been used as infill within the flood plain area to raise and level the ground for more intensive usage.

Communications networks
  • Paths, tracks, cycling tracks
  • Routes, autoroutes
Explanation:  There is evidence of reduced habitat quality for Pearl mussels in rivers where functioning flood plain has been impeded by hard surfaces of roads or paths. It has been reported (Hruska 1999) that juvenile mussels require a direct connection between the groundwater contributing to the interstitial gravels and the unimproved low nutrient vegetation in the flood plain. Building of hard surfaces can release damaging silt into the river. Hard surfaces near a pearl mussel population can also lead to run-off of pollutants into the river. These are permanent effects, i.e. both from construction and operation so road development is considered to present a significant threat to this species.
  • Bridges, viaducts
Explanation:  There is evidence of reduced habitat quality for pearl mussels in rivers where bridges have been built, even where they have clear spanned the river. In general, the most negative effects have occurred where structures were not spaced wide enough and, thus, not enough flood plain habitat has been left on either side of the river (see above). The damage is exacerbated where flow changes have occurred, and consequent hard measures such as revetments, walls or rock armouring have been built along the banks in the vicinity of the bridge to prevent bank erosion. Building of bridges can release damaging silt and nutrients into the river. The bridge and nearby road can also lead to run-off of pollutants into the river. These are permanent effects, i.e. both from construction and operation. Other permanent effects include excessive shading under the bridge, and disturbance to adult mussels and reproduction on a long term basis. Where the population of mussels is dense, the mussels form an intrinsic part of the river bed structure, and damage at one area can then cause knock-on long term damage to beds of mussels upstream and downstream of the structure.
  • Energy transport
Explanation:  There is evidence of mussel kills where pipe lines have been routed across river beds, from in-stream works in the river causing silt episodes, and also in silt from bare ground where the pipe has been dug on either side of its entry into the river. to the swathe of habitat removal required before and after it crosses the river.
  • Improved access to site
Explanation:  There is evidence of increased dumping and pearl fishing at pearl mussel beds that are in close proximity to sites with easy access.

Sport and leisure structures

Explanation: There is evidence of increased silt and nutrient releases and depressed pearl mussel habitat where golf courses, sports pitches and camp sites have been developed nearby.

Pollution
Explanation:  Water pollution, particularly nutrient pollution, leading to increased primary productivity, is associated with agriculture, coniferous clear fell forestry, industrial effluents and insufficient treatment of domestic, municipal or industrial sewage. Very small increases, above natural background nutrient loads can lead to damage. In particular, the normal background ortho-phosphate  reference level  (generally 0.005 mg/l P in Ireland) is considered to be essential to the maintenance of oligotrophic waters for reproducing pearl mussel rivers (Moorkens 2006). Small increases in ortho-phosphate can lead to deleterious algal and/or macrophyte growth, so maintaining low levels at all times is considered to be essential. One large input of ortho-phosphate can lead to an algal incident, which in turn leads to detritus/particulate organic matter, causing adult and juvenile deaths and increases the trophic status of the river on a long term basis. Growing algae causes problems by blocking oxygen exchange between the substratum and the water column and through night time depletion of oxygen. Decaying algae causes detritus that not only clogs the interstices, but also causes oxygen depletion because oxygen is used up during its decomposition.

An increase in trophic status can lead to major habitat changes, particularly a change from Fontinalis-dominated flora/macrophytes to Myriophyllum and Ranunculus-dominated flora where nutrient pollution is accompanied by siltation. These macrophytes are indicative of poor Margaritifera habitat and provide conditions for trapping further silt and continued loss of habitat as a result of changes of flow, sediment and nutrient dynamics (Barko et al. 1991, Wood 1997, Masden et al. 2001, Clarke 2002). Phosphorus that resulted in macrophyte growth continues to be released and mobilised as the macrophytes decompose (Barko and Smart 1980, Rooney et al. 2003).

Drainage
  • Canalisation
Explanation:  Both arterial drainage of the river and catchment and field drainage associated with agriculture and forestry impact on pearl mussels. Arterial drainage, canalisation, boulder removal, etc. has destroyed river habitat by replacing natural channel reach patterns of pools and riffles with more uniform runs that suit neither the pearl mussel nor its host fish (Moorkens 1999, 1996; Hastie et al. 2000; Valovirta 2001). Bank reinforcement actions often accompany or are deemed necessary following canalisation. They are a response to external damage to river banks at the site of reinforcement or that has taken place elsewhere but has had ramifications at the site of reinforcement. The reinforcement structures in themselves can affect river dynamics both upstream and downstream of the works (Fischenick, 2003, O'Grady 2006). Hard reinforcement measures are considered to be damaging activities in pearl mussel rivers.

The increased drainage network has led to an increase in the release of silt into river channels hosting pearl mussels, with the subsequent destruction of juvenile habitat. Drainage of peaty catchments has been shown to increase run-off rates and flood peaks (Müller 2000). Such hydrological changes lead to instability in mussel habitat and increased disturbance.

Flooding
  • Modification of hydrographic functioning
  • Management of water levels
Explanation:  Destruction and damage to habitats in the catchments of pearl mussels, such as through bog drainage and in-filling of floodplains has changed the hydrology of some rivers. A small number of pearl mussel rivers are also regulated above the populations. Flow regulation can have serious negative effects on pearl mussel populations (McAllister et al. 1999, Araujo and Ramos 2001). These manifest mainly in two ways. Firstly, consistent but unnatural flows, particularly more prolonged low flows can cause stress to adult and juvenile mussels by raising temperature, reducing oxygen, concentrating pollutants and providing conditions for silt deposition and algal growth. Secondly, rapid changes in flow regime such as where sluices or dams are opened and closed regularly, is damaging to pearl mussel populations. This may be due to the energy effort of individuals, concentrated on digging into substrate or moving around leading to a state of continuous stress. Where rapid changes are occurring at very sensitive times of the year, loss of  annual glochidial production or newly dropped juvenile mussels can occur. These phases of the life cycle normally occur at periods of low flow, and losses in a natural system through flooding are rare events.

Dumping, depositing of dredged deposits 
Explanation:  Dredging has taken place in the past in the large lowland pearl mussel habitats, with large numbers of dead mussels being found afterwards. Kills are likely to have included pearl mussels in the range of the dredging through habitat destruction, and mussels downstream, through siltation.

Erosion
Explanation:  Erosion of river banks is a serious cause of silt entering the river. Its cause is rarely natural, even when no immediate explanation is obvious, but rather a knock-on effect from river bed or bank changes elsewhere. Where cattle or sheep are allowed to enter the river, serious erosion can occur.

Interspecific faunal relations
  • Genetic pollution
Explanation:  Loss of host fish is regularly cited as a potential reason for pearl mussel decline (Araujo and Ramos 2001, Anon 2005). A study on the status of host fish populations and on fish species richness in European pearl mussel populations (Geist et al. 2005) characterised typical fish communities in pearl mussel streams and revealed that a lack of host fish only seems to be limiting pearl mussel reproduction in specific areas. Intact and functional pearl mussel populations were found to occur under extremely oligotrophic conditions with lower host fish density and biomass than in disturbed populations without juvenile recruitment. In Ireland, adequate numbers of host fish occur in at least some rivers with inadequate Margaritifera recruitment, however, where nutrient levels have increased, more host fish may be required as compensation for lower glochidial production rates in stressed mussels (Geist 2005). A comparison of trout versus salmon dominated rivers of Ireland quickly shows that 100% of pearl mussel rivers are associated with salmon and sea trout. Thus, while brown trout make an effective host fish, the rivers occupied by Margaritifera in Ireland, are of naturally low productivity dominated by salmonids that went to sea to get nutrition. Salmon and Margaritifera have been cited as symbiotic in their relationship, with both species providing a beneficial role for the other (Ziuganov and Nezlin 1988, Ziuganov et al. 1994). Pearl mussels filter the river water and increase its purity, and salmon gills host mussels during their glochidial stage. Pearl mussels have also been shown to prevent early senility in salmon and thus extend their life expectancy (Ziuganov 2005). It is likely that host fish numbers in ultra-oligotrophic situations were never very high, as pearl mussels are naturally adapted to live in rivers with low food levels and very low productivity (Bauer et al. 1991), but an unnatural decline in host fish will inevitably threaten Margaritifera. As well as habitat decline and acidification, impediments to fish movement from artificial barriers can result in losses of mussel populations (Bogan 1993).

Genetic pollution through the introduction of fish stocks not native to the catchment is considered to be a problem, as there appears to be a strong level of adaptation between genetic mussel and fish stocks.


All of the pressures referred to above are ongoing and will remain as threats to the population in the future, and in some cases are likely to be exacerbated. In addition, the following are likely threats:

Climate change
Explanation:  Climate change is likely to further threaten the survival of Margaritifera. Increased temperatures will lead to a higher metabolic rate and consequently a shorter life expectancy and thus reduced reproductive episodes per individual. This may exacerbate an already lowered recruitment level. The likely scenario of increased summer droughts and winter storm and flood events may negatively affect the species by increasing the frequency of stressful 'natural' events. These may result in increased siltation incidents during flooding. Habitat space may be reduced as a result of loss of river bed in drought conditions, or instability of gravel beds that are currently stable, through frequent flooding. Climate change may have an as yet unforeseen affect on the salmonid host species or on the food web that they rely upon. Changes in sea level may increase the salinity of a higher percentage of the lower reaches of some mussel rivers, which would have particularly serious ramifications for populations that have now become restricted to the bottom end of rivers. Hastie et al. (2003) predict that a number of Scottish populations may be lost as a result of climate change.

Antagonism arising from introduction of species
Explanation:  The potential for exotic species spreading into pearl mussel rivers could result in major declines to the native pearl mussel, such as continued spread of exotic Ranunculus (Laughton et al. 2007).

Conservation Actions [top]

Conservation Actions: To date, many pearl mussel rivers within the EU have been designated as Special Areas of Conservation (SACs), and the species is protected under national legislation in most countries with various levels of restriction. In most cases, both national legislation and SAC status have resulted in some improvement on direct damage to mussels such as pearl fishing, and some protection against plans or projects that would be likely to damage  populations, but conservation actions are generally lacking through lack of real engagement in how such rivers and their catchments need to be managed in order to rehabilitate and sustain their mussel populations for the future. It is obvious that if damaging activities are not removed and prevented into the future within pearl mussel catchments, that designation of SACs for the species will not have lead to any protection whatsoever.

Part of the problem of protection of the species within SACs is the design of the directive, which restricts SACs to habitats of importance, so that buffer zones that would be of great value in conservation action cannot be part of any designation. There is in theory a requirement to protect SACs from ex-situ damage, but this has not been effectively operated to date.

A strong negative indicator of the future prospects of this species has been the very poor response of the various competent authorities to dealing with the damaging effects of intensification of agriculture and coniferous forestry in pearl mussel catchments.

Agricultural and forestry operations continue to intensify in parts of pearl mussel catchments, and need to be reduced to levels that are compatible with the life cycle of the pearl mussel. Recent intensification has resulted from both economic drivers and environmental policy. Pressure on dairy farmers to intensify operations and increase herd sizes has led to use of previously marginal land. A policy for compensation of farmers for more compatible practices should be urgently undertaken, as part of a management plan on a catchment by catchment basis. Any compensation payments made to farmers in Margaritifera catchments should be for practices that are compatible with favourable conservation status into the future.

In general, damage leading to unfavourable Margaritifera status has been from cumulative effects of intensification of agriculture over many years. The most important conservation action for any population has to be the production and thorough implementation of a catchment management plan that undertakes measures at source and/or pathway between the land activities and the habitat within the river.

Other conservation actions include captive breeding in situations where populations have become severely depleted. This should never take place without a detailed catchment management plan to allow for the return of captive bred mussels to the wild.

It is hoped that the Water Framework Directive may help develop policies, legislation and management strategies that could work towards managing damaging land uses and improving water and habitat quality. It is imperative that recoverable pearl mussel populations are given the highest priority and that everyone involved in the implementation of this Directive understands the very demanding habitat requirements of the pearl mussel.

Citation: Moorkens, E., Cordeiro, J., Seddon, M.B., von Proschwitz, T. & Woolnough, D. 2017. Margaritifera margaritifera. The IUCN Red List of Threatened Species 2017: e.T12799A508865. . Downloaded on 17 December 2017.
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