Background Margaritifera margaritifera, also known as freshwater pearl mussel, naiad or margaritifera, was probably the most abundant bivalve mollusc in rivers throughout Western Europe in former times. However, during the last century many of its numbers disappeared and it is now considered to be an endangered species. Although still in decline, populations still exist in some European countries. In Spain, the scarce information available indicates fragmented populations in the north-western regions. Recent studies in the Castilla y León region have detected surviving colonies in two pSCIs. These are important findings in the conservation of this highly threatened species. The naiad lives in clean and clear water rivers, and is therefore a bio indicator of the good quality of the ecosystem. The life cycle is characterized by being associated to certain species of fish, principally salmonoids. In the waters of Castilla y Léon the common trout (Salmo truta) is the fish host during the larval stage of the Margaritifera. The microscopic larvae released by the million parasitize the fish's body, where they spend several weeks before emerging as small bivalves. They then fall to the bottom of the river, where they must find a suitable habitat and survive a barrage of predators. For this reason, maintaining natural populations of native fish hosts is essential for their conservation and changes to their habitats are detrimental. The species is also very sensitive to other disturbance of human origin, such as water pollution and habitat loss due to changes of hydrological conditions in rivers. In the Castilla y Léon region the main conservation problems for the freshwater pearl mussel can be grouped in two: 1) Alteration and loss of their optimal habitats: Presence of major hydraulic works in the main Tera River bed. Existence of many obsolete traditional small weirs that divide the river and create large expanses of water. The increase of solid accumulation due to surface runoff from areas where the ground is not protected from the effects of frequent fires, aggressive ploughing and inappropriate construction of paths and firebreaks. If this coincides with the period of naiad pregnancy, it causes the abortion of glochidium. The sedimentation causes the death of young individuals, due to the accumulation of material and the alteration of their microhabitat. Excessive and uncontrolled cutting of riverbank vegetation, mainly alder woods. The aggressive urbanization of the riverbanks in several areas. Deterioration of the water quality by organic contamination from small population centres. 2) Decrease in the population of its host, the common trout, and alteration of the natural composition of the fishing community. This problem is directly related with the facts mentioned above, since both species have similar ecological requirements. Objectives The main objectives of the project were to: Get to know the conservation status of the Margaritifera margaritifera, characterize its habitat, determine the potential of its host fish (the common trout)and the factors that most affect the population dynamics of this endangered species. Contribute to the survival of the current population of Margaritifera margaritifera and the conservation of its habitat in the Castilla y León region. Results The main goals of the project were achieved and the results and after LIFE actions should contribute to the survival of the current population of the Margaritifera margaritifera in the Castilla y León region. The project has helped to increase scientific knowledge about the species, particularly with regard to biological factors such as population size, age, breeding parameters, habitat characterization (granulomety, water speed, shading,alder tree density) etc. The results of this research were considered for the elaboration of the Action Plan for the freshwater pearl mussel, and enlargement of Natura 2000 sites (pSCIs) to include the complete range of distribution of the species in Castilla y León. Although the Plan was not approved during the course of the project, all the guidelines were being applied and used to assess technical projects on Natura 2000 sites. The Action Plan was expected to be approved during the course of 2008. The main objective of the Action Plan is to implement the necessary actions so that the species achieves a more favourable conservation status. The application field includes all the rivers in the Castilla y León region, in which the current or recent presence of the species has been detected, as well as the sections of the rivers located upstream, that would be vital for the survival and reproduction of the host fish, the trout. The main project conclusions regarding the state of the populations of Margaritifera margaritifera in Spain show that the main limiting factor for the successful reproduction is the lack of young trout in the distribution areas. This limits the infection by the glochidium in the fish. The absence of recruitment of the species, reported for many decades, and the fact that the mussel population is quite old (average age of 65 years) does not offer good prospects for the future. Although there is some sexual activity among specimens, reports of fish infected by glochidia are scarce and there is a complete absence of juveniles. Management actions were carried out using bioengineering techniques and natural materials such as rock and wood. The different actions have helped to increase the quality of the habitat and make it more resilient to floods or sediment accumulation. Also fish connectivity has been ensured by the removal of obstacles and their breeding areas have been enhanced. The project has also helped to regulate extracting activities of alder wood and other interventions in the alluvial forests as well as fishing activities (e.g. using exotic mussels as live baits is now forbidden). Finally, awareness raising activities have been well focused on academics, technicians and tourists and have helped to make local residents more aware of the need to conserve this endangered species. The educational activities reached 1700 children and university students. The Natura 2000 network has also become more familiar to inhabitants. The travelling exhibition "Naiads, our river lookouts" and all the materials produced are still available at the Centre of the Natural Park of Sanabria. The project has also networked with LIFE projects in Germany and Belgium for the exchange of information and experiences. Six technical workshops were organised during the course of the project. Good cooperation with other administrations lead to very positive results. The Duero Water Confederation has approved a project for the ecological recovery of the river Negro (€ 400,000). This will give continuity to most of the management actions carried out during the LIFE project. Furthermore, the ‘Diputación de Zamora’ (provincial delegation), has agreed to a yearly grant for the cleaning of septic tanks. Disclaimer: This « results » section should be considered as a draft until the Commission has completed its evaluation .