Freshwater Pearl Mussel Populations Research Articles

Predicting population status of freshwater pearl mussel (Margaritifera margaritifera, L.) in central Sweden using instream and riparian zone land‐use data

ABSTRACT The freshwater pearl mussel Margaritifera margaritifera (L.) has become scarce and threatened throughout most of its distribution range, and is consequently listed as ‘endangered’ by the IUCN. It has been suggested and used as an indicator species and even as an umbrella species. Despite this, few studies have examined factors affecting its catchment‐level population status. This work tested the hypothesis that it is possible to predict freshwater pearl mussel population viability (reproducing/not reproducing) and population status in central Sweden from a combination of geospatial data describing riparian land‐cover and use, water chemistry data, and electrofishing data describing the abundance of the host fish species (brown trout, Salmo trutta) in 56 streams. Mussel population viability was best predicted by total phosphorus. Using higher‐resolution data (six classes) on mussel population status, host fish abundance (0+ trout) was the best predictor. At abundances below 5 0+ trout per 100 m2 and above a concentration of 8 µg L–1 total phosphorus, only three viable populations existed. Geospatial data could be used to predict levels of total phosphorus and thereby indirectly to predict mussel viability. Geospatial data could not be used to predict abundance of 0+ trout, which is thought to depend more on instream factors. This study demonstrates that geospatial data on riparian land‐use and cover can be a useful tool for screening waters potentially harbouring non‐reproducing mussel populations. Copyright © 2013 John Wiley & Sons, Ltd.

Influence of historical exploitation and recovery of biological resources on contemporary status of Margaritifera margaritifera L. and Salmo salar L. populations in Northwestern Russia

The influence of the historical exploitiation and recovery of biological resources on populations of freshwater pearl mussel and salmon was studied in the Kozha river (Onega River Basin). The freshwater pearl mussel was characterized by a low population density (0.09 ind./m2, on the average) and number of recruits. The role of the anthropogenic factor in the population is negligible; the effect of the pearl fishing is not observed. The dramatic decrease in the number of Atlantic salmon in the Onega River Basin in the late 20th century is a major reason for the slowed down reproduction of the freshwater pearl mussel. The second factor is the reconstruction of the Onega salmon hatchery in 1984, which led to a decrease in the juvenile salmon population density below the critical level on spawning and nursery grounds. As a result, mollusc reproduction has stopped. An artificial increase in the number of recruits must be developed in order to preserve the salmon population in the Onega River Basin. It is also necessary to forbid salmon fishing, including fishing migratory paths (with the exception of trapping for hatchery needs and traditional harvesting performed by locals in historical settlements).

Impacts of flow regulation on freshwater pearl mussel (Margaritifera margaritifera) habitat in a Scottish montane river

The River Moriston in NW Scotland is a cobble–gravel bedded river that has been dammed and regulated for hydroelectric power (HEP) since 1956. The river supports a functional population of the critically endangered freshwater pearl mussel (Margaritifera margaritifera) in the lower part. In contrast the population in the upper reach is sparse and shows no signs of juvenile recruitment, leading to speculation that hydrological and geomorphic changes associated with HEP have degraded the habitat they depend on. A combination of historical analysis, field mapping and geomorphic survey of channel and active bar sites was used to assess habitat changes and current quality. During the post-dam period, the naturally stability of much of the channel has increased, active bars have been stabilised through vegetation colonisation, riparian tree cover has increased and the active channel width has significantly reduced locally (>50%); adjustments that are indicative of a reduction in the incidence of competent flows caused by flow regulation. However area and stability of habitat for freshwater pearl mussels have not been reduced greatly. The channel sites examined are characterised by mixed cobble–gravel substrates (D50 range=46–188mm), predicted to be highly stable, that provide suitable habitat for adult freshwater pearl mussels. However a degree of bed compaction at one site was observed that could be limiting the recruitment of juvenile mussels. It is hypothesised that the sparse, non-functional status of the freshwater pearl mussel population reflects significant historical pearl fishing and the limitation of recovery due to HEP related pressures of fish migration obstruction and bed compaction. The implications of these factors for conservation of the species are discussed.

Impacts of artificial structures on the freshwater pearl mussel Margaritifera margaritifera in the River Dee, Scotland

Freshwater pearl mussels (Margaritifera margaritifera) in the River Dee (north-east Scotland) are in decline, and now occur at low densities with poor recruitment levels. The population's deterioration has been attributed to reduced water quality, degraded river-bed habitat and pearl fishing but the specific importance of each of these impacts has not previously been investigated. This study focused on the impacts of artificial structures on the River Dee's freshwater pearl mussel population. Natural hydromorphological features were also included in the analysis. Relationships between the spatial distribution of the freshwater pearl mussel population and 31 explanatory variables were investigated using logistic regression. Three different model selection strategies were compared and contrasted. An absence of freshwater pearl mussels was found to be significantly associated with the following factors: (1) presence of bank protection; (2) proximity of the nearest upstream bridge; (3) presence of a wastewater treatment works; (4) number of upstream tributaries; (5) proximity of the nearest upstream tributary; and (6) presence of sediment deposition. The results strongly suggest that artificial structures on the main stem of the River Dee have contributed to the decline of its freshwater pearl mussel population. Hypotheses are proposed for the causal relationships involved. Requirements for further exploratory analyses, model testing, and experimentation are discussed. This work opens a new line of investigation into the causes of the decline of freshwater pearl mussel populations, providing formal support at a catchment scale for an impact previously identified only on a site-by-site basis. Copyright © 2012 John Wiley & Sons, Ltd.

Are Scottish freshwater pearl mussel populations recruiting normally?

Natural populations of freshwater mussel species are typically dominated by large numbers of adults. Relatively few juvenile mussels are found during field surveys, even in healthy, unstressed populations. The main factors that may influence this phenomenon are considered for an endangered species, the freshwater pearl mussel (Margaritifera margaritifera). It is important to be able to ascertain whether or not surviving populations are recruiting “normally,” and to determine the critical numbers of juvenile mussels required to sustain these populations in the long term. Based on current knowledge and information available on the biology and ecology of M. margaritifera in Scotland, size-based criteria for determining long-term recruitment success were developed. A sample of 50 surveyed rivers with extant populations was assessed. Only six populations (12%) appeared to be recruiting at high, sustainable levels, and 17 populations (34%) had intermediate levels of recruitment that may be sustainable. The remaining 27 populations (54%) exhibited no or very low recruitment success and these are probably unsustainable in the long term.

A critical reflection on the success of rearing and culturing juvenile freshwater mussels with a focus on the endangered freshwater pearl mussel (Margaritifera margaritifera L.)

ABSTRACT Freshwater mussels are in decline throughout their range. The lack of natural recruitment in freshwater pearl mussel populations and other freshwater molluscs has led to controversies about the usefulness and applicability of captive breeding techniques for their conservation. The current state of rearing and culturing programmes for freshwater mussels in Europe and North America is reviewed here with a focus on the endangered freshwater pearl mussel (Margaritifera margaritifera). Different strategies of propagation and the key factors of success are addressed and conservation management decisions are discussed with respect to risk reduction and the intensity of the culturing system. Based on the rearing techniques applied for multiple species in North America, and for the genus Margaritifera in European countries, sufficient numbers of juveniles can be produced to sustain selected populations. However, captive breeding and stocking should be carefully documented and must not replace the restoration of functional stream habitats. From a conservation point of view, captive breeding of endangered mussel species can be a last‐minute rescue tool in order to retain the evolutionary potential of priority populations which would not persist long enough to benefit from habitat restoration practices. Copyright © 2011 John Wiley & Sons, Ltd.

Suitability of different salmonid strains as hosts for the endangered freshwater pearl mussel (Margaritifera margaritifera L.)

Abstract The complex life cycle of endangered European freshwater pearl mussel Margaritifera margaritifera L. involves an obligatory parasitic phase on a host fish. Knowledge on the host–parasite interaction and on the suitability of different host fish species and strains is required both for the management of wild fish and mussel populations as well as for improving the efficiency of captive breeding methods. In this study, the suitability of different salmonid strains for hosting glochidia was tested, including Danube salmon (Hucho hucho L.) and three brown trout (Salmo trutta L.) strains from inside and outside the freshwater pearl mussel distribution range. All brown trout strains as well as Danube salmon were successfully infected with freshwater pearl mussel glochidia and encystment of mussel larvae was detected. One brown trout strain originating from the natural pearl mussel distribution range was identified as the most suitable host, revealing the highest fish‐weight‐normalized infection rates and highest glochidial growth rates, whereas endemic Danube salmon was least suitable. Under natural conditions, the role of Danube salmon may be attributed to the long‐distance dispersal of glochidia in the Danube system, whereas sedentary brown trout appear to be the most important hosts at a local scale. Successful infection of suitable hosts and the maintenance of these host–parasite systems in calcareous water were demonstrated in this study. These results indicate that neither the infection process nor the encystment phase of freshwater pearl mussels is dependent on low lime concentrations. The results of this study suggest that careful selection and management of appropriate host fish strains is mandatory for sustainable conservation management of freshwater pearl mussel populations. Copyright © 2010 John Wiley & Sons, Ltd.

A case study of a freshwater pearl mussel (margaritifera margaritifera) population in central sweden

.Most of the Margaritifera margaritifera populations in Sweden are not reproducing. Land use such as forestry, eutrophication and water regulation for hydropower are commonly reported causes for the absence of reproduction. A brook with a very dense population of pearl mussels has been investigated with regard to the age distribution of the population and their habitat, in order to discover the reason for the absence of reproduction. The population consisted almost entirely of old mussels with very few young ones. The water flow and chemistry of the water are very stable and well buffered, due to a large glacifluvial deposit in the catchment area. The substrate in the stream is favourable and well aerated to a depth of at least 15 cm. Forestry has been practised with care for at least the last 15 years to avoid siltation of the stream sediments. The most likely reason for the lack of reproduction seems to be the water regulation of the main stream introduced 60 years ago, which has left the main stream dry seasonally. This is likely to have affected the population of brown trout which is the host for the first life‐stage of the mussels, the glochidia.

Ontogenetic morphometrics of individual freshwater pearl mussels (Margaritifera margaritifera (L.)) reconstructed from geometric conchology and trigonometric sclerochronology

This study bridges two conchological approaches to model the growth characteristics of freshwater pearl mussel shell: size-at-age and sclerochronology. We demonstrate a simple numerical model that transfers sclerochronological data into realistic estimates of ontogenetic shell sizing. This model was constructed for a subset of shell growth data dealing with morphometrics and annual shell growth increments. Further, validation of the model was performed using a dataset that was withheld from the calibration. Both subsets of data showed significant correlations between the observed (measured by vernier callipers) and reconstructed size-at-age data, indicating a successful model. The practical applicability of the model was exemplified for the studied Finnish freshwater pearl mussel populations. In accordance with the previously set theory about the plasticity of life history traits of the species, the southern mussels showed higher growth rates than the northern mussels.

Determination of mortality in exploited freshwater pearl mussel ( Margaritifera margaritifera) populations

Estimates of mortality, based on relative numbers of live mussels and empty shells observed were obtained for a sample of seven exploited Scottish freshwater pearl mussel ( Margaritifera margaritifera) populations. Total mortality ( Z) estimates ranged from 0.058 to 0.189 year −1 and fishing mortality ( F) estimates ranged from 0.005 to 0.129 year −1. General trends of increasing Z and F with age were apparent. Pearl fishers generally targeted larger, older mussels. In one heavily exploited population, the Z and F estimates converged, indicating that most of the mortality in older mussels (>50 years) was due to the effects of destructive pearl fishing. Since a complete ban on pearl fishing was introduced in 1998, illegal pearl fishing has continued albeit at a reduced level.

The status of host fish populations and fish species richness in European freshwater pearl mussel (Margaritifera margaritifera) streams

Abstract The status of host fish populations and fish species richness was investigated at 36 sites of 20 extant freshwater pearl mussel populations, including the drainages of the Elbe, Danube, Rhine, Weser, Aulne, Kemijoki and Tuuloma in Germany, the Czech Republic, France and Finland, by carrying out comparative electrofishings. Brown trout (Salmo trutta f. fario) were found to be the available host fish for pearl mussels in all except one of the streams investigated with mean densities of 2861 ha−1 (range 0–8710 ha−1) and a mean biomass of 119 kg ha−1 (range 0–478 kg ha−1). Streams that had been frequently stocked with brown trout had higher trout biomass and densities of host fish than natural populations, but trout stocking had no positive effect in two of the streams investigated. Fish species richness ranged from 2 to 16 species per stream and showed a negative correlation with host fish biomass and host fish densities. Undisturbed oligotrophic pearl mussel headwater streams usually only yielded a low number of fish species. Habitat degradation can reduce competitiveness of specialized trout and result in an increased abundance of ubiquitous or atypical species. A link between the lack of juvenile pearl mussels and a lack of suitable host fish was only rarely observed. Functional pearl mussel populations with relatively high numbers of juveniles had significantly lower densities and biomass of host fish than pearl mussel populations without recent recruitment. This study suggests that 0+ host fish are not necessarily required to sustain functional pearl mussel populations. Low densities of host fish can be compensated by the higher glochidia carrying capacity of older host fish with limited previous contact with pearl mussel glochidia, by the long reproductive period of mussels, and by low mortality rates of juvenile mussels during their post‐parasitic phase. Copyright © 2005 John Wiley & Sons, Ltd.

Genetic diversity and differentiation of central European freshwater pearl mussel (Margaritifera margaritifera L.) populations: implications for conservation and management

Despite the fact that mollusc species play an important role in many aquatic ecosystems, little is known about their biodiversity and conservation genetics. Freshwater pearl mussel (Margaritifera margaritifera L.) populations are seriously declining all over Europe and a variety of conservation programs are being established to support the remaining endangered central European populations. In order to provide guidelines for conservation strategies and management programs, we investigated the genetic structure of 24 freshwater pearl mussel populations originating from five major central European drainages including Elbe, Danube, Rhine, Maas and Weser, representing the last and most important populations in this area. We present a nondestructive sampling method of haemolymph for DNA analyses, which is applicable for endangered bivalves. The analyses of nine microsatellite loci with different levels of polymorphism revealed a high degree of fragmented population structure and very different levels of genetic diversity within populations. These patterns can be explained by historical and demographic effects and have been enforced by anthropogenic activities. Even within drainages, distinct conservation units were detected, as revealed from high F(ST) values, private alleles and genetic distance measures. Populations sampled close to contact zones between main drainage systems showed lowest levels of correct assignment to present-day drainage systems. Populations with high priority for conservation should not only be selected by means of census population size and geographical distance to other populations. Instead, detailed genetic analyses are mandatory for revealing differentiation and diversity parameters, which should be combined with ecological criteria for sustainable conservation and recovery programs.

Timing of spawning and glochidial release in Scottish freshwater pearl mussel (Margaritifera margaritifera) populations

Summary1. The timing of reproduction was investigated in six Scottish freshwater pearl mussel populations from 1993 to 2002. Gravid females were examined and the release of mussel larvae (glochidia) was monitored.2. Annual spawning (oviposition) and spat (glochidial release) events occurred during June to July and June to September, respectively.3. Between‐river differences in timing seem to be related to water temperature. Mussels in the warmest rivers tend to spawn and spat first, and vice‐versa.4. Thermal variations also seem to influence the timing of reproduction within rivers, which can be delayed by several weeks during cold years. At least 3000°‐days occur between annual episodes of glochidial release.5. The timing of spawning is determined gradually, probably by a thermal summation effect.6. The release stage occurs as a sudden, synchronised event, with most of the glochidia spat over 1–2 days, indicating that it is triggered by an environmental cue. Sudden changes in water temperature and/or river level often result in spats, and the underlying mechanism may be respiratory.

The threat of climate change to freshwater pearl mussel populations.

Changes in climate are occurring around the world and the effects on ecosystems will vary, depending on the extent and nature of these changes. In northern Europe, experts predict that annual rainfall will increase significantly, along with dramatic storm events and flooding in the next 50-100 years. Scotland is a stronghold of the endangered freshwater pearl mussel, Margaritifera margaritifera (L.), and a number of populations may be threatened. For example, large floods have been shown to adversely affect mussels, and although these stochastic events were historically rare, they may now be occurring more often as a result of climate change. Populations may also be affected by a number of other factors, including predicted changes in temperature, sea level, habitat availability, host fish stocks and human activity. In this paper, we explain how climate change may impact M. margaritifera and discuss the general implications for the conservation management of this species.

The Threat of Climate Change to Freshwater Pearl Mussel Populations

The Threat of Climate Change to Freshwater Pearl Mussel Populations

Conservation of threatened freshwater pearl mussel populations: river management, mussel translocation and conflict resolution

River engineering work has historically been responsible for the decline and extinction of a number of endangered freshwater pearl mussel [ Margaritifera margaritifera (L.)] populations and potentially remains a significant threat. The impact of different types of river engineering work on a number of Scottish M. margaritifera populations is examined and discussed. M. margaritifera is now fully protected under law in most countries and guidance is urgently needed so that river managers can integrate ecological and socio-economic factors when considering the impact of proposed activities on M. margaritifera populations. To safeguard the remaining important populations, a simple conflict resolution framework is suggested for the appraisal of proposed developments in rivers with pearl mussel populations. Operations likely to harm mussels and permanently damage their river bed habitat should not proceed. In exceptional circumstances (outlined), the translocation of small numbers of adult mussels may possibly be considered as a potential management tool, but mussel translocation has been little used and should be considered experimental and last resort.

The effects of a major flood on an endangered freshwater mussel population

During February 1998, a 100-year return flood occurred in the River Kerry, north-western Scotland. A study was undertaken in order to assess the impact of this event on a large, internationally important freshwater pearl mussel ( Margaritifera margaritifera L.) population. A conservative estimate of 50,000 mussels killed by the flood was made. This represents 4–8% of the total population. Significant channel reformation and large-scale movements of substrata occurred in some reaches, whereas others appeared to be unchanged. Some mussel beds disappeared or were largely depleted whereas others remained intact. Boulder-dominated substrata appear to have provided the most protection from scouring. The relatively high mussel densities and recruitment levels found in the middle reaches are probably due to the fact that this part of the river is hydrologically stable. It is likely that the intact mussel beds in the middle reaches will be the main source of regeneration in other parts of the river as the population recovers. As a result of recent changes in the hydrological behaviour of Scottish rivers, several M. margaritifera populations may now be more at risk from these catastrophic events. Therefore, it is important that conservation managers are aware of this potential threat.