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An investigation of recruitment levels in Scottish freshwater pearl mussel Margaritifera margaritifera (L.) populations was conducted in order to determine their medium-term viability. These mussels represent surviving populations considered to be currently viable (in that some recruitment had occurred during the past decade). Selected mussel beds in 8 rivers were surveyed at ~10 yr intervals between 1984 and 2007. Observed changes in mussel density (1996-2007) ranged from -50 to +55%. These may be due to the effects of pearl fishing activity, sampling bias, and, to a lesser extent, habitat degradation. Marked differences between size distributions and corresponding age distributions were observed. Large mussels (>50 mm shell length L) were predominant at every site, although evidence of recent recruitment (mussels <30 mm L and/or aged <5 yr) was observed in each population. Based on the relative abundance of juvenile mussels observed, medium-term stabil- ity is indicated in 4 populations. One large population appeared to have localised recruitment prob- lems, as relatively few juveniles were found at one site but larger numbers were found in other parts of the river. Three smaller populations had very few juveniles and appeared to be recruiting at his- torically low levels. Continued monitoring of these sites every decade will provide valuable informa- tion about the population dynamics of long-lived freshwater mussels.

Medium-term recruitment patterns in Scottish freshwater pearl mussel Margaritifera margaritifera populations