Sedentary host fish: A significant threat to the recruitment of pearl mussels

Recently, the brown trout (Salmo trutta f. fario) has been reported to be the only available native host species for the complete metamorphosis of the critically endangered freshwater mollusc pearl mussel (Margaritifera margaritifera L.) in Central Europe. Pearl mussel glochidia invade host fish in late summer, at the beginning of spawning migrations of brown trout. As spawning migrations are usually considered the key changes in the spatial distribution of the species, we evaluated the hypothesis that the brown trout spreads the pearl mussel along the longitudinal profile of streams. 2. Migrations of 130 brown trout adults were observed over five years in four functional mussel streams with viable freshwater pearl mussel populations and two non-functional mussel streams where the mussel no longer exists and that are considered for its repatriation. In functional streams, brown trout migrations were short and did not extend beyond the boundaries of the occurrence of pearl mussels, a maximum of 1,236 m in length. In the Czech Republic, river continuity is fragmented by numerous lateral obstructions, considerably limiting the free migration of fish along the longitudinal profile of streams. Accordingly, our observations tended to show that glochidia invaded spatially isolated brown trout populations with stationary behaviour.