Background The river Our has a transboundary basin in the Ardennes across Belgium, Germany and Luxembourg. In Luxembourg, the upper part of the basin is a site of Community interest (SCI) within the Natura 2000 network. It features 11 habitat types listed in the annexes of the EU Habitats Directive, three of which are priority. Of these, the pearl mussel (Margaritifera margaritifera) is also one of the most endangered species within the site. It is endangered mainly due to poor water quality and management of the river. In addition, populations of the trout (Salmo trutta forma fario), which are necessary to the complex mussel life cycle – as the pearl mussel larvae live some months in the gills of the trout – are very low. Furthermore, the remaining pearl mussel population is very small (1 500 individuals) and old, which makes reproduction quite difficult. For these reasons, the species is endangered across most of its European distribution. One LIFE project for the restoration of the habitat of the pearl mussel (LIFE02 NAT/B/008590) has already been successfully carried out in the Belgian part of the Our basin. Three EU supported Interreg projects were also devoted to improving knowledge and the ecological conditions of the streams and the Our valleys. This LIFE project was a direct follow up to those first steps. Objectives The main thrust of the project was to set up and manage a breeding station to ensure the regular reintroductions of young pearl mussels, in order to reinforce the size of the surviving population. Pearl mussel conservation is also heavily dependent on assuring the good status of the river ecosystem, and especially of very good water quality. Building on extensive contacts with local stakeholders, the project therefore, also aimed to carry out a series of measures to improve the ecological conditions at the site. These would include: removing dams and barriers or fitting them with fish passes; monitoring and reinforcing the trout population (as part of the pearl mussel breeding and release actions); restoring gravel banks free of silt or clay; monitoring water quality and identifying and removing local sources of pollution; restoring natural riverbanks by cutting exotic conifers (4 ha) and planting indigenous trees (4 ha); and preventing cattle entering the river with 2.5 km of fencing. A band of land at least 4-metres wide along the Our and its tributaries, totalling 5 ha, would also be purchased to ensure an appropriate restoration and management. Finally, the project aimed to undertake a series of actions to enhance awareness about the project and its objectives among the local population and stakeholders, including fishing and hunting clubs, recreational operators and farmers. Results The project achieved its principal objective of setting up and managing a breeding station for the highly threatened pearl mussel population. Moreover, regular reintroductions of young pearl mussels have been carried out into a semi-natural environment (replicating some of the Our/tributaries conditions). Eventually, it is hoped that the rearing techniques being tested should start to help boost the existing population in the wild. On the downside, however, despite carrying out numerous practical measures targeting habitat improvements, the project did not result in a significant improvement in the very good water quality status required by the pearl mussel. Pearl mussel breeding: Although difficult, it is possible to successfully rear pearl mussels in a breeding station, as the project results have started to indicate. However, the process is very slow and due to the youth of the surviving mussels (none are yet over 4 years) and their extreme vulnerability to water quality, none has so far been introduced into the Our or its tributaries at the project location. An interesting aspect of the project’s breeding methodology is the rearing of the young mussels in a semi-natural environment (until reaching the size of 1.5 mm) followed by planned releases into the wild. This latter stage, however, has not yet been reached, as the breeding techniques are still being tested and will continue to be trialled in the after-LIFE period. The methodology was developed thanks to project networking and knowledge-exchange between pearl mussels experts worldwide. To date, the results are a very promising up to 1.5 mm size i.e. 60% survival. But then fall off sharply with only 0.1% survival up to 15 mm in size: four pearl mussels survive aged four years; 86 individuals are aged 2 years; 372 individuals are aged 1 year. A hoped for production of 7 200 young mussels bred via this new technique in the final year of the project should see an improved survival rate in the future. Practical Habitat improvement measures: All the planned practical measures aimed at improvements in habitat were carried out, with some actions exceeding expectations. 1. A band of, more than the forecast, 5-metres wide land along a 8.99 ha stretch of the Our and its tributaries was bought for the planned restoration and management actions and 18 private land-owners compensated; 2. Clearance and removal of exotic conifers was carried out along the river banks on 8.31 ha and planting of indigenous trees was carried out on 4.2 ha; 3. More than 2.5 km fencing was erected to stop cattle entering the river and contributing to water pollution; 4. Restoring of gravel banks free of silt or clay (over 2 500 cubic meters/per annum); 5. 12 obstacles blocking the migration of trout were removed from the river/ streams in the project area. River Our water quality issues: Project monitoring indicated that the water quality within the targeted area, although ‘good’ is nevertheless, not good enough for the demands of the pearl mussel and therefore still needs to be addressed in order to ensure the long-term survival of the population. The main problem is over-high levels of nitrates from farming activities – a sensitive issue that nevertheless the project was able, via its various awareness raising activities, to draw attention to notably among local administrators and Luxembourg politicians. Further information on the project can be found in the project's layman report (see "Read more" section).