March 14, 2007

All photographs and diagrams on this page are the copyright of Keith Scrivens

The freshwater pearl mussel (margaritifera margaritifera) is a listed endangered species. At one time the freshwater pearl mussel was a common sight in European rivers. In the last 100 years or so populations have declined to the point where it is now extinct in most of Europe. In the UK it still exists in small numbers in a few rivers. In Wales there are possibly less than two dozen rivers which still hold a population, and of these only one is known to have a population with juvenile recruitment.
The life-cycle of the freshwater pearl mussel (margaritifera margaritifera)

The freshwater pearl mussel has a complex life-cycle which involves salmonids. In the summer, millions of mussel larvae, (known as glochidia) are released into the environment by the adult mussels. At this point the glochidia are transported downstream with the river current with the aim of encysting on salmonid gills.  It has been estimated that for every 1 million glochidia released only 10 will survive to become adults. The glochidia will use the host fish for approximately 10 months before dropping off the fish into the river gravel.  The freshwater pearl mussel can live for up to 100 years, and the females can have a reproductive lifespan of 80 years, which may see an adult female produce 200 million glochidia. Despite these high numbers, the freshwater pearl mussel is now under serious threat of complete extinction.  The causes for this rapid decline are a result of a number of factors, which include, reduced numbers of salmonids in the rivers, river engineering, eutrophication, pollution, siltation, and over fishing by pearl fishers.

The Mawddach Hatchery captive breeding programme was initiated in the Spring of 2005 when a small number of adult mussels were brought in to the unit from three different Welsh rivers. There is little information on captive breeding and rearing techniques associated with this mussel and therefore the Hatchery staff have been rapidly developing the rearing system for these mussels.  Much of the work at the Mawddach is unique and to date the hard work being put in by the staff is certainly paying off!  In the Autumn of 2005 approximately 6,000 juvenile salmon and sea trout were successfully infected with glochidia from the adult mussels that were brought in.  In the summer of 2006 70,000 juvenile mussels were collected as they dropped off the infected fish, and these are now being successfully reared in the Hatchery

As the mussels grow slowly they will not reach 4mm in size until the age of between 4 and 5 years, and it will be at this point where some of the mussels will be seeded out back to their natural environment.

The captive breeding project at the Mawddach Hatchery is very important, as to date, attempts to successfully rear them in captivity elsewhere have been disappointing.  Looking ahead the work being done at the Mawddach may mean the difference between survival or extinction of this species of freshwater pearl mussel.