Pearl mussels in the River Ehen

On Saturday morning I looked out of my window at deep snow and thought, paradoxically, that Tuesday’s fieldwork in Cumbria was looking promising.  The water would be cold – that’s inevitable in January– but at least the water levels in the River Ehen would be low as most of the water was sitting in the catchment rather than flowing down towards the North Sea.   But the rest of the afternoon saw rising temperatures and a rapid thaw and this, coupled with rain on Sunday night, made any hopes of actually getting into the river fade away.


When we arrived at the River Ehen on Tuesday morning, the water was spilling over the weir that marked the outflow from Ennerdale Water and the flow in the Ehen itself was about four times greater than was safe for wading.  So we stood on the bank, instead, and peered through the swirling waters, trying to catch sight of the river’s most significant resident, the pearl mussel.   These live in dark shells, about five to ten centimetres long and embedded into the bottom gravels.  They are, consequently, hard to spot, if you don’t have a good idea what you are looking for.  And the chances are that you would never have seen them before as the sad fact is that the River Ehen is now the only river left in England with a healthy population of pearl mussels.  There is one river in Wales and a few in Scotland and Ireland with healthy populations, along with a few others where the population is declining.  Elsewhere in Europe, the situation is even bleaker, which is why the UK and Irish populations are so important.

Pearl mussels, to be brutally frank, are poorly adapted to co-existing on this island with 60 million humans.   They live for as long, or longer, than we do (individuals of over 120 years have been recorded) but they are extremely fussy about the conditions under which they live.  They spend their first year or so as parasites hanging off the gills of salmon and trout, then fall off into the gravels. Those that don’t get washed away then need a constant flow of clean, soft, well oxygenated water around them.  Consequently, the slightest change to a catchment – a weir that stops salmon swimming upstream, an occasional pulse of fertilisers washed off a farmer’s field – and the delicate balance that sustains the pearl mussel population is upset.   It is no coincidence that I had to drive so far to get here: the upper Ehen is as remote a location as you can find in England now.  Yet even here the pearl mussels are threatened.   And it was these threats that had brought me to the Ehen: to stare into the river and plan out a campaign of measurements that will help us to understand the Ehen’s Pearl Mussels a little better.

The clouds were low over the fells during our visit, the ground was still soft from the weekend rain and heavy showers punctuated our visit.  However, there was enough of the surrounding landscape on show to make me want to come back in warmer weather, perhaps stay at the Fox and Hounds in nearby Ennerdale Bridge, walk on the fells and enjoy the scenery.   The western fells are a world away from the tourist hubs of Bowness, Ambleside and Keswick but, even here, as the pearl mussels remind us, the influence of man can still be discerned.

The photograph, by the way, is from the Cladagh (Swanlinbar) River on the Northern Ireland / Republic of Ireland border.

More information is available on the websites of Scottish Natural Heritage and the IUCN.

One thought on “Pearl mussels in the River Ehen

  1. Pingback: More about Oedogonium | microscopesandmonsters

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