Freshwater Pearl Mussel
A recent survey of Scotland which visited rivers known to support pearl mussel populations 100 years ago found that almost two-thirds of populations were extinct or on the verge of extinction. Now there are only 57 populations in Scotland that show signs of reproduction, which represents about half the known remaining worldwide populations. Several of these rivers support abundant populations, but many are in apparent decline. The principal causes of this dramatic decline were identified as pearl fishing and pollution.
Pearl fishing has occurred at virtually every Scottish river and it has been a regular sight to find hundreds to thousands of pearl mussel shells forced open and left on river banks. Easier access to most of the Highlands by road, where the majority of pearl mussel populations remain, has meant that rivers which were previously extremely remote are now accessible and the populations are vulnerable to destructive pearl fishing. It has been estimated that, if the recent rate of decline in pearl mussels continues, there will be no Scottish populations left in 25 years' time.
Water pollution has also been responsible for the early decline in many populations, particularly in lowland areas of Scotland where pearl mussels have now largely disappeared. As pearl mussels are filter feeders and pass considerable amounts of water through their digestive system they are very vulnerable to water pollution. The presence of increased nutrient levels from sources such as agriculture and sewage effluent is a particularly recent problem and several important pearl mussel populations have been lost because of it.
A further threat comes from declines in sea trout and salmon stocks in recent years, which pearl mussels rely on during their first year of life. There is concern that the decline in salmon and trout stocks to historically low levels in several rivers in the North West Highlands could threaten the long-term survival of many pearl mussel populations. Further research is under way to investigate the decline in salmon and trout numbers and the coincident decline in pearl mussel populations.