The endangered freshwater pearl mussel (Margaritifera margaritifera), one of the longest-lived invertebrates, are threatened globally. Scotland, UK, remains a stronghold, however even here the population is declining due to factors such as habitat degradation, pollution and pearl fishing. The study comprised two parts: field surveys of glochidia infection of host salmonid fish, and a novel laboratory flume based study of the mussel’s behavioural responses to changes in flow regime. The intricate life cycle of M.margaritifera includes a parasitic stage as glochidia attached to gills of salmonids. The preferred host in Scotland is thought to be Salmo salar and Salmo trutta in the absence of S.salar. This has not, however, been empirically tested in the field.  Eight rivers in NW Scotland were surveyed using standard electrofishing techniques and encysted glochidia counted. Results suggest S.trutta is the primary host fish for glochidia attachment in the rivers surveyed, which contradicts current accepted knowledge about host specificity of M.margaritifera. Mussel populations are often found in regulated rivers, however little data exists on response to changes in flow regime. The mussel's behavioural response to changes in flow were investigated in an experimental flume. Mussels buried significantly deeper in conditions of gradually increasing water velocity compared with rapid increases or where water velocity was constant. 68% of individual mussels washed out when the water velocity was rapidly increased.  The findings are novel, provide initial recommendations for targeted management actions for the conservation of M.margaritifera both in Scotland and internationally, and highlight more research is required.