Freshwater pearl mussel (Margaritifera margaritifera) glochidiosis in wildand farmed salmonid stocks in Scotland

Samples of salmonids were taken from six Scottish rivers and examined for f reshwater mussel (Margaritifera margaritifera (L.)) glochidiosis. The preva lence and magnitude of natural infections observed were comparable to those reported elsewhere. In most rivers, older fish seem to be less susceptible than 0+ fish, possibly due to an acquired immunity resulting from previous exposures. Initial infection loads may be greater on older fish due to gre ater ventilation rates and/or gill surface areas. However, this host size e ffect appears to be transitory. In a stock of farmed 0+ salmon, an entire p arasitic stage was monitored. This took up to 11 months and only 5-10% of t he initially attached glochidia managed to metamorphose and excyst as juven ile mussels. There are apparent differences in host utilisation between sal mon and trout in certain rivers. Some trout stocks appear to be under-utili sed by M. margaritifera, possibly due to differences in behaviour and/or sp awning site. 0+ salmon are the most important hosts in several rivers. Howe ver, there are a number of mussel populations located in small streams whic h have no salmon, and these are entirely trout-dependent. This may be impor tant in terms of conservation, with regard to the recent collapse of migrat ory trout stocks in Scotland.