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.



I’ve been visiting Cassop Vale National Nature Reserve for about thirty years now, mostly to visit the small lake that lies at the foot of the Permian limestone escarpment in County Durham.   I remember one early visited yielding a handful of floating vegetation that included both of Britain’s free-floating liverwort species – the memory is seared into my memory because Rev. Gordon Graham, the local botanical expert, didn’t believe one of my records.  In my ignorance I had recorded Riccia fluitans without knowing that it had never before been recorded from County Durham so I had to go and get another sample in order that he could verify it for himself.


On the cold Saturday morning in January when I visited, the pond was fringed with the dead stems of sedges and the water was icy cold on my skin. Yet there was life here, even at this most inhospitable time of year, including quite prolific growths of floating plants, especially the tiny leaflets of common duckweed, Lemna minor, in amongst the patches of sedge.   The individual plants are just one or two millimetres in diameter, each with a root hanging down into the water for a few millimetres beneath them.

As ever, with my trips, the point of this visit was go beyond what I could see with my eyes even if, as is the case with Lemna, my starting point was, itself, only just visible.   But by the standards of the microscopic world with which I normally deal, Lemna is huge.  So huge, in fact, that I could not use my conventional microscope slides and coverslips and, instead, needed a cavity slide (one which has a shallow, round depression at the centre).   Now I could put it under my microscope and explore the hidden world of Lemna in more detail.


Scattered across the underside of the leaf, I could see many elliptical outlines, each yellow-brown in colour.  These were cells of Cocconeis placentula, an extremely common alga, belonging to a group called the diatoms.  They are fast-growing species that thrive in many habitats, but the surfaces of aquatic plants often present the open spaces where this species can grow and multiply without competition from other algae.  There were also a few needle-like cells belonging to another type of diatom, Fragilaria rumpens, growing upright from the leaf surface.   All these are growing on the underside of the Lemna, so my picture looks as if it is upside down.   This habit means that these diatoms are living in a shady underworld after the Lemna plants have filtered out the light that they need.  But this should be no surprise as diatoms, worldwide, are species that thrive in the cool, dark depths of lakes and oceans.   January in County Durham presents far less of a climatic challenge to these tiny organisms than it does to grown men who should have grown out of pond-dipping years ago.


I’ve watched three episodes of David Attenborough’s new series on the iPlayer so far
and, as ever with his programs, am captivated by the photography and the
stories that unfold.  But there is something disquieting about this series, although it took a while for me to put a finger on what exactly was bothering me.   Then I realised … no humans.  Attenborough paints Africa as if it were a Garden of Eden, before Adam
and Eve came along and started spoiling things.   My own memories of Africa are of places teeming with people, not the vast, empty wildernesses that he portrays.   I found the “Eye to Eye” segments at the end of the program to be the most interesting parts because here, for a five minute segment, humans are allowed to intrude, albeit with the focus on the derring-do of the cameramen  as they collected the footage.  There is something of the colonial era in the footage of a column of native bearers plus one or two Europeans snaking through the bush.  Even in these segments the Africans themselves seem to be relegated to the supporting cast.