Sizes, densities and age structures of Scottish Margaritifera margaritifera (L.) populations

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 e river. 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 ability. 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.