July 2013, Dr Roger Sweeting, Honorary Fellow of the Freshwater Biological Association
Several of the members of the Lune Rivers Trust visited our facilities at the Freshwater Biological Association in December to learn more about the freshwater pearl mussel and our Ark project to protect them. How did freshwater pearl mussels get to the British Isles then, and what is their life cycle?
During the last 12,000 years – since the retreat of the glaciers of the last Ice Age – our native freshwater animals and plants have become established in our rivers and lakes by a variety of means. In our area, the depth of ice was reduced from perhaps 800m above the land to zero: imagine how much water rushed through Cumbrian and Lancashire rivers from the Lake District and the North Pennines. As the ice melted in the (now) temperate parts of the American and Eurasian land masses, the rivers became larger and the coastal areas became brackish. This allowed many primarily freshwater animals to move round the edges of the northern ice cap and enter the developing rivers. Salmon, sea trout and charr battled their way through the melting ice as pioneers looking for new freshwater spawning grounds. We believe that this is the mechanism by which the freshwater pearl mussel also arrived in our local rivers, including the Lune.
At a very early stage in their lifecycle, after the female pearl mussel has released them by the million, microscopic pearl mussel larvae are sucked into the gills of salmonid fish and take up residence within their gills. For 8 to 10 months they live here, benefitting from the protection and nutrients provided by the fish. In colder water with slower-growing fish their development would have been slower and consequently their fish hosts may have carried them further downstream, around the coast and up adjacent rivers as they looked for food or areas to spawn.
As the European land mass warmed and the river flows diminished, the salmonid fish hosts developed more defined migration patterns that allowed less ‘hitch hiking’ to new rivers round our now saltier shores. Hence, we now have a number of different populations of pearl mussels in our rivers. The distribution of pearl mussels mirrors that of salmonid fish – cooler, cleaner, less industrialised rivers with less intensive agricultural practices.
Damage to the river environment has caused and continues to cause major problems for the endangered pearl mussel. Any interference with the movement of their migratory fish hosts such as weirs, barrages, major abstractions, hydroelectric schemes and intensive drainage practices also affect the pearl mussel. Pearl mussel fishing was also a major factor in the decline of this animal, but throughout Europe any interference with pearl mussel populations is now illegal.
The mussel is, however, still in decline and there are no rivers in England and Wales that have stable or increasing numbers of them. In Scotland perhaps half the populations are in decline – a sobering thought considering it is recognised as a European pearl mussel stronghold.
With this background we showed the members of the Lune Rivers Trust how our Ark project has been set up. Our aim (in partnership with the Environment Agency and Natural England) is to breed pearl mussels in captivity with their natural hosts in order to produce young mussels that can be returned to their natural rivers. The Lune is one of these, so we were delighted when you visited our facility. At one time the River Lune may have supported several million pearl mussels. We would like to work with the Trust to restore the river and its catchment to a quality that can support one of its oldest inhabitants. The pearl mussel is the longest-living native animal that we have: individuals well over 250 years old have been recorded in Germany.
Today securing funds for long-term environmental projects is not easy, but we believe that the freshwater pearl mussel really is a gem worth fighting for!
The Lune Rivers Trust are currently working with Dr. Sweeting to put together a project to help the Lune Freshwater Mussels.