Conservation status of two endangered freshwater mussel species in Bavaria, Germany: Habitat quality, threats, and implications for conservation management

Abstract

1. The freshwater pearl mussel (Margaritifera margaritifera) and the thick-shelled river mussel (Unio crassus) are relatively widespread across Europe, but are strongly declining and are now protected by the European Habitats Directive. In the course of this study, 20 pearl mussel and 14 thick-shelled river mussel streams in Bavaria, Germany, were investigated.2. The mussel populations were mapped to determine population size and age structure. For the assessment of habitat quality, host fish abundance and physicochemical parameters were investigated, e.g. substratum quality, water chemistry, redox potential, and turbidity. Furthermore, potential risks for the populations such as predation or river maintenance were also recorded and assessed.3. The average population size and recruitment rates of M. margaritifera populations were lower than in U. crassus populations, with 3517 (2.2% juveniles) compared with 5566 (41.4% juveniles) individuals, respectively. On average, 22.3% of particles were smaller than 0.85 mm in diameter at M. margaritifera sites, whereas the mean proportion of fine particles at U. crassus sites was twice as high, at 41.3%.Other parameters such as redox potential or electric conductivity also indicated more favourable habitat conditions in M. margaritifera streams. Unio crassus seems to be less vulnerable to adverse substratum texture and increased nutrient levels than M. margaritifera.4. The main threats for U. crassus were physical habitat destruction, predation by muskrat, or a lack of host fish, whereas M. margaritifera mainly suffered from siltation leading to a lack of oxygen supply to the interstitial zone, affecting recruitment. Consequently, conservation strategies need to be species-specific and address stream-specific reasons for decline. As a basis, accurate and comparable monitoring data are necessary, which implies the standardization of monitoring protocols.

Invasive alien mammals of European Union concern

“…This is the case for the American mink, which can exert a negative effect on endangered species such as the water vole (Mori & Mazza 2019) or threaten genetically distinct populations of prey species (Flávio et al 2020). Heavier egg-predation on ground-nesting birds (compared to previous studies) has recently been reported for the raccoon dog (Dahl & Åhlén 2019), and the muskrat was found to be a major threat to endangered freshwater bivalves in Germany (Stoeckl et al 2020).…”

Securing genetic integrity in freshwater pearl mussel propagation and captive breeding

“…Freshwater pearl mussels can reach ages of more than 100 years and have an exceptional life cycle that involves a parasitic life stage on a fish host that can either be Atlantic salmon ( Salmo salar ), brown trout ( Salmo trutta ), or both in European populations 4 , 5 . Degradation of pearl mussel habitats, mostly related to siltation and colmation of interstitial spaces in the stream bed where juvenile pearl mussels burrow, have resulted in a lack of recruitment and severe declines of many pearl mussel populations throughout their European range 6 , 7 . Since restoration of stream beds requires consideration of entire catchments and the restoration of natural flow dynamics 8 , this approach is time-consuming and costly.…”

“…At the same time, many pearl mussel populations are overaged and expected to die out in the near future. Since the first descriptions of the ongoing threats and population declines 9 , the downward trend of populations has even become worse 7 . This development has prompted the creation of a first European CEN standard for a single species 10 as well as several captive breeding efforts in different countries 11 – 14 .…”

How Human Activity Has Changed the Regional Habitat Quality in an Eco-Economic Zone: Evidence from Poyang Lake Eco-Economic Zone, China

“…In recent years, habitat research has focused on two perspectives. One is to evaluate the impact of the external environment or human activity on the habitat quality of individual species [ 26 , 27 , 28 , 29 ]. These studies first clarify the distribution range and characteristics of the target species and then analyze the threat factors that affect the habitat quality of the target species [ 29 ].…”