This video says about itself:
The mysteries of the freshwater pearl mussel
3 September 2014
In the northern regions of Norway, Sweden, Finland and Russia you can find pearl mussels in the rivers. Research scientist Paul Eric Aspholm, Bioforsk, tells more about it in this movie. Aspholm is working in a project funded by the Fram Centre.
From Birders Against Wildlife Crime in Britain:
Freshwater Pearl Mussels
The Law and Protecting Freshwater Pearl Mussels
The source of some of the world’s most beautiful pearls (they’re in the crown jewels in the vaults of Edinburgh Castle and were even one of the reasons that Caesar invaded Britain), Freshwater Pearl Mussels Margaritifera margaritifera were once widespread across Europe, but are now one of the UK’s most threatened species.
Restricted in Britain to a scatter of functionally extirpated populations in England in Wales, and a few small extant populations in southern Scotland with some more abundant populations further north, in 2011 Scottish Freshwater Pearl Mussels joined the Giant Panda and Javan Rhino on a list of 365 of the world’s “most endangered species”.
Larger than their marine relatives, Freshwater Pearl Mussels can live to over 100 years old and only mature after ten or so years. Dependent on wild salmon and trout for part of their life cycle, the species is dark brown to black and lives in the bottom of fast-flowing streams and rivers where they may be completely or partially covered in sand or gravel. Freshwater Pearl Mussels feed by filtering food particles out of the river water (adults can filter up to 50 litres of water a day) and help keep freshwater streams clear benefiting other aquatic wildlife.
Scotland has a total of 21 designated sites for Freshwater Pearl Mussels, and Scotland’s rivers now contain around 50% – 60% of the entire world’s population. Conservation work by Scottish Natural Heritage and other organisations is taking place to improve habitats for mussels, to tackle pollution and reintroduce animals to rivers where they have become extinct. However, wildlife criminals searching for pearls threaten to undermine this work – just a few hours pearl-fishing can wipe out whole populations. As recently as 2012, several entire breeding colonies were stolen from remote areas of Harris in the Western Isles.
For more information on the ecology of the Freshwater Pearl Mussel please see Ecology of Margaritifera margaritifera in Scotland (pdf).
Most illegal pearl fishing takes place when rivers are low in the spring and summer, and generally in remote locations or areas hidden from view. It has been illegal since 1998 to collect these rare molluscs – even touching a Freshwater Pearl Mussel can be punished with a £10,000 fine! – and any incident involving Freshwater Pearl Mussels is likely to be a crime. Tackling illegal pearl-fishing is one of six ‘priorities’ of the National Wildlife Crime Unit.