A female pearl mussel releases millions of glochidia, minute larvae with a hinged shell, into the current. The overwhelming majority perish. A glochidium that is drawn over the gills of an Atlantic salmon or brown trout snaps shut on a filament, becomes encysted, and stays attached through the winter, drawing nourishment while it develops.
In late spring the juvenile releases, sinks into clean gravel, and buries itself for several years before appearing at the surface. The bottleneck is total. No host fish, no recruitment, regardless of how many adults remain.
This is why many European populations are functionally extinct while still visibly present. Surveys repeatedly find beds composed almost entirely of animals many decades old, with no juveniles at all, because the gravel has silted up, nutrient loads have risen or the salmonids have declined. The species is exceptionally long-lived, with ages beyond a century documented, which disguises the failure: a river can hold thousands of mussels and still have had no successful reproduction in living memory.