The characteristics of the infection of juvenile Atlantic salmon with glochidia of the freshwater pearl mussel in rivers of Northwest Russia

The decline of freshwater pearl mussel (Margaritifera margaritifera L.) populations is globally well documented, but there is a lack of knowledge concerning the status of the species and its interaction with host fishes in the rivers of the White and Baltic Seas. This study aimed to identify the host fish of pearl mussel in four rivers of Karelia (Northwest Russia), to analyse the duration of the parasitic phase, and the susceptibility of fish hosts to the infestation. Juvenile sea-migrating and land-locked Atlantic salmon (Salmo salar L.) were identified as the most important hosts. In the Keret’ River, reduced numbers of juvenile Atlantic salmon are likely to result in a decline of pearl mussel. This problem does not appear to be present for the other three studied rivers. Pearl mussel glochidia were mostly found in yearlings (0+), whereas older fish were infected less, suggesting a more complex pattern of host use than commonly expected. The remote location of Karelian pearl mussel rivers, the high number of pearl mussels in them, and their rather pristine state all suggest that these rivers have a high conservation priority. The importance of Atlantic salmon as a host fish and the emerging declines of Atlantic salmon populations, e.g., in the Keret’ River, suggest that the sustaining of this host species should have a high conservation priority.