Pearl Mussel Ark Project
In 2007, a partnership project between the FBA, Natural England and the Environment Agency was set up to create an ‘ark’ facility at Windermere to conserve dwindling populations of the freshwater pearl mussel Margaritifera margaritifera from English rivers, and to develop a captive breeding and reintroduction programme. Over the past 18 months, the FBA has received populations from six rivers across England. The aim, after five years, is to have sufficient juvenile mussels from each population in England for reintroductions to be made with adequate numbers of young. The overall project aims are to:
1) Create an ark facility to maintain rescued populations from rivers;
2) Develop a captive breeding programme and;
3) Develop a reintroduction strategy for these populations.
About the species
The freshwater pearl mussel Margaritifera margaritifera is classified as Endangered on the IUCN Red List of Endangered Species due to its unprecedented, worldwide decline during the latter part of the 20th Century. Many factors have contributed to the decline including pearl fishing, pollution, siltation, and declines in host fish populations.
There are still several million individuals living in the wild with English and Welsh populations making up over 500,000 of these. Scotland is an international stronghold with an estimated 12 million pearl mussels in the cleaner rivers, but most of these occur in one river. Pollution and siltation cause particular problems for pearl mussels. Despite the adults reproducing successfully, there continues to be a near total loss of juveniles annually due to siltation of their gravels. The result is aging populations with the youngest individuals in some rivers being over 40 years old.
M. margaritifera have a very interesting and complex life cycle which requires a host fish for their larvae (glochidia).
Life History
The freshwater pearl mussel has a very long life-span, commonly reaching ages of over 120 years! The species lives buried in clean gravel in oligotrophic rivers and feeds by taking water in through the inhalant siphon and sifting out food-material using the gills before expelling the water through the exhalant siphon.
M. margaritifera is normally dioecious (has separate sexes), with males releasing sperm into the water column in June – July. Sperm is inhaled by the females and fertilises eggs which are in a specialised sac next to the gills. Depending upon water temperature, larvae (known as glochidia) are released into the water column some time between July and September. A single female can release up to 4 million glochidia pear year (measuring 60-70µm) which require a salmonid fish host (Atlantic salmon, Salmo salar and brown trout, S. trutta in the UK) for the next stage in their development. Glochidia are inhaled by a host and as water passes over the fish gills, the glochidia snap shut onto the gill filaments. Glochidia become encysted within the gill tissue and grow there until the following spring when they drop off the fish in May or early June (around 400µm in size). They are now known as seed mussels, and these juveniles must land in clean, fine gravel substrates where they will burrow into the interstices.
Mussels reach sexual maturity between 10 and 15 years of age, when the shell length exceeds 6.5cm. It is thought adult mussels do not have an upper age limit for reproductive activity and will continue to reproduce until they die.
First Year Progress
The first year (April 2007- March 2008) was spent acquiring populations of mussels from various rivers and moving them to the hatchery at Windermere. The mussels were put into rectangular steel mesh cages filled with river gravel. These were then placed in circular tanks supplied with water from the lake. In other captive breeding programmes, both salmon and brown trout have been used as host fish with different levels of success. Salmon parr were trialed as the host fish for this year, and were added to the tanks with mussels from the river Ehen, river Irt and Dubbs Beck. There was some success with these fish and small numbers of seed mussels were collected for the rivers Ehen and Irt. However, an interesting observation with the Dubbs Beck glochidia was that they excysted (dropped off) before they reached maturity. This may be because they were on an unsuitable host fish, as only brown trout (not salmon) are known from Dubbs Beck. Another problem we had was that the salmon parr smolted (the metamorphosis they undergo which prepares them for their sea-living phase) earlier than expected leading to the loss of most of the fish and therefore, potential seed mussels. To combat these problems this year we have used brown trout as the host fish. These were introduced to the tanks containing mussels in June 2008.
Looking Forward
By the end of 2008, we hope to receive a population of mussels from the River North Tyne to add to our other six populations. Now that we know we can keep the mussels alive using water from Windermere and that the temperature regime of the lake is sufficient for the critical periods of glochidial development, we are hopeful for success on a much larger scale. Our first objective is to keep the first-year seed mussels alive. Our second objective is to successfully infect the brown trout from the six populations currently at Windermere (rivers Ehen, Irt, Brathay, Torridge, Esk and Dubbs Beck) and also the expected River North Tyne mussels. Hopefully this will give us much greater numbers of seed mussels in our second year.
By successfully breeding year-on-year, we can begin to establish a more ‘normal’ population demography for these rivers and pull this species back from the edge of extinction in England. Once our captive-bred juveniles are 5 years old they are less susceptible to siltation and can be released back into their native rivers (or at least tributaries in the original catchment). There is much work to be done in the meantime to restore their natural habitat to the condition it once achieved. Only then will captive breeding programmes become less necessary and wild populations can once again support themselves.
