Background Margaritifera auricularia is a large bivalve freshwater mollusc. Once common in the large rivers of western Europe and northern Africa, its area of distribution has been drastically reduced; by the 1930s it was considered largely extinct. Fortunately, some populations were rediscovered in the River Ebro, Spain, in the mid-1980s. Today, 99 percent of the known world population is found in the medium and lower Ebro; the remaining one percent lives in the Loire River, France, where a small population was found in the late 1990s. The life cycle of the mussel depends on certain fish species such as the sturgeon or the freshwater blenny. In the larval stages, the mollusc fixes itself on to the fish's gills until it has metamorphosed into a small bivalve. The species’ situation is critical. No hints of breeding activity have been recorded in any of the existing populations, so no recruitment of youngsters has taken place for decades. The host fish are also scarce, with populations being fragmented and unconnected with the mussels. Destruction of the mollusc's habitat caused by canalisation and contamination of the rivers is another important factor affecting the species’ status. Objectives The project aimed to improve the conservation of 60-80percent of the Margaritifera auricularia population by carrying out a set of measures, ranging from habitat conservation practices and control and conservation of the wild populations, to an ambitious plan for captive stocking and breeding. The plan also involved acting to conserve the host fish species Salaria fluviatilis (freshwater blenny) by designing management and breeding protocols. The project also set out to draw up recovery plans and implement awareness-raising campaigns to underpin the future conservation of the two species. Results The project carried out its planned activities: The Recovery Plan for Margaritifera auricularia was approved: Decree 187/2005, published in the Official Journal of Aragón on 07/10/2005. The Recovery Plan for Salaria fluviatilis was drafted. The freshwater mussel population was surveyed during maintenance works on the Imperial and Tauste canals in Aragon, and new colonies were searched for in the different branches of the canals and the river Ebro. The habitat use of the freshwater mussel was characterised. A captive breeding programme for the mussel was put in place, along with a captive breeding programme for the freshwater blenny. Awareness-raising measures were taken (leaflets, posters, website, and promotion at events). However, despite these actions, the project did not achieve its expected results with regard to Margaritifera auricularia. Captive breeding was not successful during the project and many of the planned semi-captive reintroduction measures could not be implemented. These shortfalls were judged to be mainly due to the difficulties inherent to the management of the species. However, the beneficiary showed ongoing commitment towards the species after the close of the project, and the captive breeding programme was successful for the first time in 2008. The experience gained from the project was essential for this. Measures targeting (captive breeding) the freshwater blenny were more successful. Reintroduction of the species in semi-natural controlled conditions led to good results. However a number of factors potentially stand in the way of the species strongly re-establishing itself: the poor condition of the canals on the Ebro, human intervention during canal maintenance work, and the presence of alien species. Further information on the project can be found in the project's layman report (see "Read more" section).