Saving Ardnamurchan's Pearl Mussels

Diane Baum, left, and Lucy Ballantyne work for Lochaber Fisheries Trust, a charity which has just received a 2-year grant from the Heritage Lottery Fund, the Landfill Communities Fund, and Scottish Natural Heritage to enable them to spend some of their time working to increase local populations of the endangered Scottish pearl mussel.

Here on Ardnamurchan we are fortunate to have four of the seven Lochaber burns which have Special Area of Conservation (SAC) designation for their pearl mussels, though those in the Sanna burn are far from healthy as almost all of them are over 60 years old, and even in the other burns most of the mussels are 40 or older, with little sign of young ones coming on.

The pearl mussel's life cycle is complex, the first part consisting of releasing into the water glochidia which cling to the gills of fish - usually brown or sea trout. These overwinter in the fish, later to drop off into the burn as young adults, where they will live for as much as a hundred years.  The cycle is summarised here.

Diane and Lucy's objective is to help with the transfer of the glochidia from the pearl mussel to the fish. They start by wading into a burn and finding about fifteen pearl mussels at a time, putting these into a bucket to bring ashore.

This mussel is around fifty years old. Each is very carefully opened....

....to expose the gills - at left in this picture, with the pinking 'foot' of the mussel, which enables it to anchor itself in sand and move around, at the right.

If the mussel looks as if it is carrying glochidia, which cling to its gills, then a syringe is carefully inserted into the gills to draw off a sample of water which is then placed between slides and....

....inspected using a portable microscope. The young stay in the mussel's gills through five stages of development which last about a month. The researchers are looking for glochidia in stage five, where they are all ready to transfer themselves to the fish. At this stage they look like pakman images, circular with a 'mouth' on one side which will grip the fish's gills.

On the day I accompanied the researchers, three of the fifty or so mussels were carrying glochidia, but none had enough glochidia at stage five. Had several mussels been ready, then 10-15 trout or small salmon would have been caught from the river using electrodes (Lucy's picture, above, shows this) and immersed in a container....

....from which the spat of several mussels has been gathered - this is another picture from Lucy showing spatting mussels. However, the fish will only be left in there for as few as ten seconds: that's as long as it takes for enough glochidia to transfer. If the fish are left in longer, they will take on too many glochidia which will adversely affect their health.

Lastly, this picture from Lucy shows a small salmon with glochidia on its gills, something of we'd like to see much more in our burns if our pearl mussels are to return to health.

Lucy and Diane will be returned shortly to Ardnamurchan as it appeared that most of the mussels had not yet produced their glochidia, which, like flying ants, is an event which is supposed to be co-ordinated at roughly the same time.

Many thanks indeed to Lucy and Diane for the time they gave me, and to Lucy for the additional pictures.