In‐Field Cage Infestation Patterns of Host Fish for the Freshwater Pearl Mussel ( Margaritifera margaritifera ) in a Fragmented River Before Restoration

The freshwater pearl mussel ( Margaritifera margaritifera ) is among the most endangered unionids in Europe. Many populations are highly specialized and solely infest specific parapatric populations of either brown trout ( Salmo trutta ) or Atlantic salmon ( Salmo salar ). River fragmentation can prevent mussel populations from completing a generational turnover if their migratory hosts can no longer reach upriver mussel habitats. Here, we tested a seminatural reintroduction method and compared the mussel infestation rates on different salmonid ecotypes in an experimental field scenario where previously separated salmonid populations will be reconnected following river rehabilitation. In our study, upriver resident trout sympatric to the mussels and migratory trout and Atlantic salmon allopatric to the mussels (separated by several migration barriers) were cohabitated in in‐field cages containing M. margaritifera allowing for a seminatural infestation. We demonstrate that all three salmonid ecotypes hosted a significantly different number of larval mussels: the resident trout was the most heavily infested, while the allopatric salmon hosted the fewest. This study demonstrates that, insofar as only the mussels are moved between locations, in‐field cage infestation can be a successful method for mussel conservation as it requires significantly less investment than producing juvenile mussels in a dedicated facility. Further work should be conducted to evaluate the long‐term success rate of this reintroduction method and on how to optimize the husbandry of the fish in the cages to minimize mortality.