Flowing seawards from the west end of Ennerdale Water, the short River Ehen supports the largest population of freshwater pearl mussel (Margaritifera margaritifera) in England, which, according to the International Union for Conservation of Nature (IUCN), is one of the world’s 365 “most endangered species” at the same level of the giant panda, on the critical list across Europe, with its strongest populations in northern England and Scotland. Tragically, up to 90% of the Ehen’s mussels were wiped out in 2012 when water levels flowing out of the lake fell, causing the temperature to rise and bring oxygen levels down. This is one of the world’s longest-living invertebrates surviving for up to 150 years but taking 10-12 years to reach sexual maturity so it will take some time for their numbers to recover. Its life cycle is also a complex and easily disrupted one. They begin their lives as glochidia, tiny mussel-like larva, which are ejected from their mothers, several million at a time, into rivers. Here, the glochidia have the almost impossible task of being inhaled by a salmon or trout. These lucky few then snap themselves shut on the fish’s gills and live parasitically for ten months or so, until dropping off and settling down in the gravel bed to grow into adult pearl mussels. To increase their reproductive chances, glochidia are now being artificially inserted into the gills of host fish, which are then released into the River Ehen. Their collapse across Europe has been affected by water quality but also by the fall in wild salmon numbers, with whom their relationship is probably symbiotic as they filter large amounts of water, helping to keep rivers clean.
There are also cultural reasons for protecting the mussels. Pearl farming in Britain has a long history and Julius Caesar’s biographer, Suetonius, suggested, probably exaggerating, that the presence of these black pearls was one of the reasons why Caesar decided to invade the island in 55 BC .