Background The threatened, thick shelled river mussel (Unio crassus), a once abundant species of slow-running freshwater habitats, has experienced a dramatic decline in its two remaining populations in Luxembourg – in the Natura 2000 sites of the rivers Our and Sûre. A crucial problem of many river systems in central Europe is their heavy load of fine sediments, which cause problems particularly for the young mussels living in river beds, and eutrophication. Objectives The project’s overall objective is to improve the habitat quality of the thick shelled river mussel in the rivers Our and Sûre in Luxembourg, to strengthen the two surviving populations. Specific objectives are to: Reduce the amount of fine sediments in the two Natura 2000 sites by implementing different best practice methods to inhibit erosion close to the river banks. Agro-environmental measures will reduce the nutrient concentration in the tributaries and the main stream; Introduce captive breeding methods for the species, and to release propagated mussels at both project sites to strengthen the elderly populations with juvenile mussels (demonstration); Improve the connectivity of the river system by removing migration obstacles (best practice), which will enhance the host fish population e.g., the bullhead (Cottus gobio) and the brown trout (Salmo trutta fario); Improve the river beds at both sites by adding natural gravel into the river systems; and Control the population densities of the invasive species, the musk rat (Ondatra zibethicus), a heavy predator of the mussels. Expected results: Installation of 2 000 m of fencing and five watering outlets, to reduce the impact of erosion by cattle at the river banks; Installation of 60 water gutters to minimise the sediment runoff at selected sites; Agri-environmental measures (40) to directly reduce the amount of nutriments into the catchment. Farmers will be assisted in the sustainable management of their soil; Creation of cleaner and less clogged gravel within the river will improve the self cleaning capacity of the river and highly improve the ecosystems services provided by the respective rivers; Release of 200 host fish per annum at each of the rivers; After the third year, to release 500 to 1 000 mussels into each river system. A further 100 'host fish will be ‘released’ (in cages) each year, into each river system to directly strengthen the populations in the rivers; The removal of six obstacles to migration (three per project area). This will re-connect some 15 km of river at each site with the main streams. Bullheads and brown trout are expected to directly benefit from the opening of these areas, which should also indirectly benefit the mussel populations; The addition of 100 m3/yr of suitable gravel into each river system will create new mussel habitats. In addition, spawning grounds will be created for other fish species present in the rivers; and Control of the predator musk rat will save hundreds of individual mussels every year. The project will also monitor some 100 km of rivers. As well as an improvement in the conservation status of the thick shelled river mussel, other Annex II-listed species are expected to benefit from the project’s actions, e.g. the freshwater pearl mussel (Margaritifera margaritifera) the bullhead, brown trout, the European brook lamprey (Lampetra planeri) and the European otter (Lutra lutra).