An investigation of salmonid host utilization by the endangered freshwater pearl mussel (Margaritifera margaritifera) in north‐west Scotland

Abstract

Abstract The complex life cycle of the globally threatened Margaritifera margaritifera includes a parasitic stage, where glochidia attach to the gills of fishes of the genus Salmo; however, the species of Salmo used appears to vary across its range. In previous literature the reported primary host in Scotland, home to a high proportion of the world's remaining M. margaritifera populations, is the Atlantic salmon Salmo salar, and in its absence the brown trout Salmo trutta. In this study, the prevalence of infection in putative Salmo hosts in eight rivers in north‐west Scotland was determined. At a selected site on each river, where both S. trutta and S. salar were collected in abundance, S. trutta was the preferred host. At sites where S. salar were abundant but S. trutta were at low density, however, S. salar showed a high prevalence of infection (with the exception of one river where neither S. salar nor S. trutta were infected). Thus, the primary host appears to be very site‐specific in the rivers sampled. We speculate that this may be because M. margaritifera have population‐specific responses to cues for attachment to a host. Alternatively, it may be that host population‐specific immune responses mediate infections by glochidia. In addition, larger fish were less likely to be infected than smaller fish, and gills 1 and 5 were less heavily infected than gills 2–4. One consequence of this finding, for both national and international conservation management of this globally endangered species, is that any current or future management activity must take into account local population host preferences, otherwise conservation efforts may be in vain.