More or less hidden at the bottom of the water, a group of animals is constantly working to keep our waters clean.
It is our native freshwater mussels. Unfortunately, this group of animals is now highly endangered and many populations in Europe are already severely reduced or have disappeared. The number of individuals has decreased by up to 90% in some cases. Of the former seven species found in Luxembourg, two species are now considered extinct in the wild. The freshwater pearl mussel and the depressed river mussel.
An analysis of the freshwater mussels in the border rivers Our, Sauer and Moselle more than 17 years ago showed already back then that only small populations were left. In order to re-evaluate the situation, natur & ëmwelt / Fondation Hëllef fir d'Natur sampled the same rivers again last year. This was carried out with financial support from the “Gestion de l'eau” (Boundary Fisheries Commission) and the Environmental Administration (ANF).
Fortunately, the situation has improved slightly in recent years. Although the freshwater pearl mussel is now extinct in the upper part of the river Our, the highly protected thick shelled river mussel (Unio crassus) population has recovered slightly. The LIFE Unio Project (www.unio.lu) has certainly contributed to an improvement of this species in the river Our. In 2002, living specimens of native mussel species were found on only three spots in the lower Our, Sauer and Moselle. In 2019, there were again four species of mussels at seven spots. The duck mussel and the painter mussel were the most common species. The swan mussel and the swollen river mussel, on the other hand, were found only rarely. In addition to the native species, the Moselle also hosts the invasive Corbicula clam and the zebra mussel. In the past few years, the Corbicula clam has also invaded the lower river Sure and the lower river Our.
Although there has been a slight improvement, it must be mentioned that the populations are still very small and far from its initial abundances. The pollution of our waters with nutrients, pollutants and fine sediments, as well as beside stream and in stream construction still cause considerable problems to many species. In addition, our native mussel species need specific fish species for their reproduction, which must also be present. The ecological role of the freshwater mussel in rivers and lakes is enormous, as they constantly filter the water (up to 40 litres per day and animal) and help to keep our waters clean.
Meanwhile, there is awareness at the European level of the enormous ecological and economic benefits that freshwater mussels have for society. This year, a COST programme (CONFERMUS: CONservation of FREshwater MUSsels: a pan-European approach, CA18239) funded by the European Union will be launched. Representatives from science, authorities and environmental organisations from 25 countries will meet to discuss how to protect our 16 European large freshwater mussel species best. In addition, the ecosystem services provided by freshwater mussels will be studied and quantified in order to disseminate this information to the general public.
Freshwater bivalve molluscs are neither cute nor furry, nor can they be eaten and thus do not attract much attention. Nevertheless, they are among the best bioindicators in the aquatic environment. The presence of native bivalves indicates that a water system and its catchment area is in a good state of health. Healthy mussels mean that the water systems are in good condition and we humans are highly dependent on this.