Fish Host Suitability for the Threatened Freshwater Pearl Mussel (Margaritifera margaritifera) and Implications for Reintroductions

Habitat restoration followed by species reintroduction is a key strategy for biodiversity recovery. For species with parasitic life stages, such as the freshwater pearl mussel (Margaritifera margaritifera), host suitability is crucial. Before a planned reintroduction of mussels into a newly constructed nature-like fishway, we tested the compatibility of four brown trout (Salmo trutta) strains from local and foreign drainages as hosts for mussels from a nearby river. The strains included (a) a local sympatric wild strain, (b) a local allopatric wild strain from near the fishway, (c) a local allopatric hatchery strain used for stocking, and (d) a foreign allopatric hatchery strain. After forty days, infestation rates of the parasitic mussel glochidia did not differ significantly among strains, indicating that all could serve as hosts. Glochidia that developed on the local allopatric hatchery strain had the highest growth rate, suggesting the highest suitability for production under laboratory conditions. While stocking hatchery strains can have negative ecological impacts, local wild fish provide a sustainable option without continued introductions of hatchery strains. If wild fish are scarce, carefully chosen hatchery strains may support juvenile mussel production and reintroduction under controlled conditions. Our findings highlight the importance of evaluating host compatibility before mussel reintroduction and fish stocking.