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The river mussel (Margaritifera margaritifera) is an endangered freshwater bivalve mollusc, whose life cycle is characterized by having a mandatory parasitic phase, lasting about 9 months, in the gills of two species of salmonids, the Atlantic salmon (Salmo salar) and the common trout (Salmo trutta).

Thus, prior to the development of any conservation strategy, it is essential to deepen this parasite-host relationship and address unknown aspects of the Iberian populations, located on the periphery of the naiad’s range, according to the University of Santiago de Compostela. The entity also considers it a priority to assess the effectiveness of both species as hosts, identify if there are differences in affinity of naiad populations for one or the other at the basin level, test the success of the cycle in the natural environment or evaluate the effect of artificial parasitization on the well-being of the fish.

In this sense, this initiative gives continuity to the ex situ strategy of the LIFE Margal-Ulla project, increasing the stock of juveniles, improving cultivation techniques through experimentation and extending its objectives to other basins.

The overall objective of the project has been to include fish hosts as one of the key factors in the conservation strategies of the naiad M. margaritifera and to give continuity to the ex situ conservation plan initiated with the LIFE Margal-Ulla project.

The specific objectives were as follows:

• To analyze the degree of affinity of Margaritifera margaritifera for hosts in its own basin and the effect that the origin of the host may have on the results of the crop.
• To evaluate the efficacy of Atlantic salmon versus brown trout as a host for M. margaritifera in ex situ and in situ conservation.
• Obtain information on the efficiency of the life cycle of the species in the natural environment.
• To study the viability of juveniles of M. margaritifera of different origins, in different sections and basins, in order to evaluate their suitability in future plans for reintroduction or population reinforcement.

• Cultivation of juvenile river mussels in captivity, introducing improvements in the rearing protocol in order to increase their viability and the stock for population reinforcement.
• Introduction of naiad juveniles from the Eo and Ulla rivers, reared in the growing season until they reach optimal conditions, in different river stretches with current presence of the naiad.
• Assessment of the habitat quality of the river stretches where juveniles are introduced in different basins through physical-chemical analysis of the water.
• To analyze the efficacy of a non-invasive technique for the diagnosis of parasitosis in the host.
• Control of the water quality of the cultivation facilities.
• Analysis of the impact of artificial infestation on fish throughout parasitization through the evolution of the condition factor and gill recovery mechanisms after parasitism from a histological point of view.
• Control of salmonid infestation in the wild.
• Dissemination and communication of the project.

The “MargaSalmo” project has focused on the relationship that is created between the river mussel and its hosts (Atlantic salmon and brown trout), trying to obtain data on the specificity of the relationships in different populations, the effectiveness of trout against salmon as hosts and the welfare of the fish during infestation. The data obtained underline the need to have populations in a good state of conservation of hosts to ensure the viability of the naiad in the natural environment, providing trout and salmon aptitudes that optimize the results of their life cycle, although in farming conditions a better performance of salmon has been observed, which adapts better to the conditions of captivity. In addition, according to the entity, river connectivity continues to be a key factor for the regression of the current demographic status of Margaritifera margaritifera, since it favors the mobility and contact of both host species with the populations of naiads, ensuring their dispersion and optimal functioning of this complex life cycle.

The project has closed with a considerable increase in the stock of juveniles (85,500 specimens) at the facilities of the University of Santiago de Compostela, which, thanks to this project, has been able to substantially improve its equipment. This, together with the increase in knowledge and all the experience acquired by the work team over the years, has resulted in the improvement of the techniques used and the cultivation conditions, translating into remarkable results in relation to survival and growth parameters.