1. Populations of freshwater pearl mussels (Margaritifera margaritifera (L.
)) were surveyed in 12 Scottish rivers (selected from those known to contai
n viable populations). Overall mussel densities in different rivers ranged
from 0.27 to 30.01 m(-2) Median densities ranged from 2.5 to 14.5 m(-2) and
a maximum of 398 mussels m(-2) was observed in one river.
2. Total population estimates in different rivers ranged from 2000 to 0.9-3
.7 million. One population is particularly dense, with an estimated 0.6-1.2
million mussels in a 4-km stretch of river.
3. Samples from nine populations were taken in order to provide mussel size
/age data. Marked differences between size profiles and their corresponding
age profiles were observed. It is difficult to interpret the former in ter
ms of recruitment.
4. There were signs of recent recruitment in all of the populations investi
gated. Large numbers of young mussels (aged less than or equal to 20 years)
were found in four rivers. The largest proportions at any particular site
were 67/219 (30.6%) aged less than or equal to 10 years and 191/219 (87.2%)
aged less than or equal to 20 years, both values being recorded in the sam
5. The expected predominance of young mussels was not achieved in any popul
ation. Presumably, there is considerable underestimation of the youngest ag
e classes owing to biased sampling techniques. However, several populations
are thought to be recruiting at levels that are high enough to maintain vi
6. One population (River F) is the best example of a 'healthy', moderately
fished, recruiting population and its age profile could be used as a benchm
ark for future comparisons.
7. The results of this study emphasize the international importance of Scot
land in terms of M. margaritifera conservation. However, the small numbers
of juveniles found in some Scottish rivers indicates that many populations
are vulnerable to decline and, therefore, their conservation status may be
threatened in the long term. Copyright (C) 2000 John Wiley & Sons, Ltd.