Information and Advisory Note Number 2 Back to menu
1.1 The freshwater pearl mussel [Margaritifera margaritifera L) is the largest
freshwater mussel found in Scotland and it the only one that occurs in
non-calcareous running water. The habitat, and the heavy, rough, dark shell,
readily distinguish it from the duck and swan mussels, which live in rich
streams, canals and lochs
Adult and juvenile pearl mussels
1 2 Pearl mussels feed by drawing m river water and sieving out fine organic
debris, some researchers think that this clarifies the water and so benefits
fish Despite having a heavy calcareous shell, it lives only in mildly acidic
waters in Scotland, but it grows very slowly and may live for 80-100 years.
1 3 Since Roman times the mussel has been fished for its pearls and there are
still a few people who make a living from them. Most pearls are brown and
valueless, but the best have a faint and subtle smoky lustre and are highly
prized.
2.1 The original British distribution of the pearl mussel mirrors that of
non-calcareous rock. The mussel was once found in rivers and streams of
south-west England, Wales, the north-west Midlands, northern England and all of
Scotland, except the Tweed catchment, parts of the Central Belt and the
sandstone area of Caithness Several of the larger islands have populations but
there are only old, questionable records from Orkney and Shetland. Detailed
records are held at the Biological Records Centre at Monks Wood, at Glasgow
Museum (Kelvingrove) and at the Zoology Department, Aberdeen University (all of
whom would welcome extra records).
2.2 The pearl mussel is still found in all the above areas but is reduced to a
few sparse and scattered remnant populations in England, Wales and southern
Scotland. In the Highlands mussels are also extinct m some rivers and have shown
a dramatic decline in numbers in others, but a few rivers and streams retain
abundant populations, especially in the remote north west. The situation is
worse in North America, where only a few small populations remain, in France the
mussel may now be extinct, the reduction in Germany is reported to be over 90
per cent and in northern Spain there are only three populated rivers. The
position in central Europe is not monitored effectively, but pollution has
certainly led to substantial losses. This means that the Scottish populations,
and those which survive in parts of Scandinavia, are of international
importance.
The distribution of the freshwater pearl mussel in Great Britain
2 3 The losses have been caused by
industrial pollution or siltation in a few rivers,
such as the lower Almond in Midlothian,
organic enrichment in others, such as the
River Ythan in Aberdeenshire, and
overfishing in many places, such as the River
Kirkaig in Highland.
3.1 Freshwater pearl mussels live buried in
coarse sand or fine gravel. They can occur in
wholly sandy streams, but prefer places
where rocks or stones provide shelter and are
typically found in groups behind the rocks,
with two thirds of their shell buried in the
sand.
3 2 Mussels mature around 12 years old and may live for 80-100 years. The sexes
are separate and in early summer the males shed sperm into the water, and these
are then inhaled by the females. The fertilised eggs develop until mid-late
summer, when each female squirts out an average of three million active larvae,
called glochidia. These resemble two tiny gaping shells, less than 0 2mm across, and they are swept off in the current Well over 99.99 per cent die
but some are inhaled by suitable fish and may then snap shut on the gill
filaments where they live as parasites until the following spring. Only brown
trout and salmon are suitable hosts and small fish are most susceptible. These
may have hundreds of glochidia on their gills in autumn, but some are soon
sloughed off and no wild fish are known to die from the infection.
A typical mussel stream in Wester Ross
3 3 In the spring the young mussels, then
around 0 4 mm across, drop off the gills and
settle onto the sand. Most die, but those that
survive burrow into the top layer and grow
quickly at first, reaching about 4 mm after one
year, 12 mm after three years, 2 cm after four
to five years and about 6 5 cm at maturity.
Old mussels may finally reach 12 -15 cm in
length Young mussels are yellowish-brown,
becoming darker when reaching maturity,
when they also begin to show shell erosion at
the hinge.
4 1 The main threat throughout the range of
the mussel is pearl fishing, because most
fishermen are still killing the specimens that
they collect. In fact, regrettably the first
evidence of mussels is often the piles of
widely gaping shells left by pearl fishermen.
Legislation has failed to prevent this problem.
A secondary, but substantial threat in lowland areas is enrichment of streams
and rivers by agriculture and/or sewage works. This leads to clogging of the
spaces between the sand grains and prevents young mussels from becoming
established.
4 2 In some places fish farms are located on rivers where pearl mussels occur,
and this can cause several problems. The river engineering needed to establish
the farm intakes may kill mussels directly or by causing siltation, which
eventually smothers the adults or prevents the young from settling successfully
in the sand. Alternatively glochidia may be sucked into the farms and then
infect the fish, which may die or show reduced growth in the crowded tanks, so
that there is pressure from farm owners for mussels to be moved away form the
intakes. It has been suggested that screens could be fitted to farm intakes to
intercept the glochidia, but as glochidia are very small this would require
unacceptably fine meshes on the screens, leading to rapid clogging.
Pearl mussels killed by pearl fishermen
4 3 Very few mussels are still found in rivers where industrial pollution
occurs, as this is no longer a real threat. However, river engineering for
hydroelectric schemes, for flood protection, or to improve fishing pools may
cause local extinctions.
4 4 Mussels can only survive where
salmonid populations are healthy, and there
has been speculation that acid deposition may
pose a serious long-term problem because of
its effect on fish. No-one has studied the
direct effect of increasing acidity on pearl
mussels.
5 1 The most worrying aspect of the current
situation is that, although there are still some
pearl mussels throughout much of their
traditional range, most populations lack a
proper complement of young specimens. In
addition, there are almost no places where
mussels are still present at natural
abundance, even in the most remote areas.
However, mussels live so long that senescent
populations can survive long after effective
reproduction has ceased, and consequently it
is difficult to be sure of the true position in
any stream. Nevertheless, even sparse
populations remain fertile and so it is possible
that a slow recovery may take place if the
threats are removed. Even so the freshwater
pearl mussel is clearly seriously threatened in
Britain.
5 2 Rehabilitation attempts must ensure
that water quality is high and that healthy
populations of young salmonids are present to
provide potential hosts. In Germany, sewage
effluent and farm drainage diversion has been
used to protect one abundant stock.
Translocation of adults has never been shown
to be effective and anyway would reduce
already depleted donor stocks. Laboratory
infection of fish with glochidia is possible, but
techniques for rearing the resultant young
mussels have not yet been developed and so
this method is not available. The best
strategy is to try to protect existing adults
from further fishing and to ensure optimum
habitat conditions for them.
6 1 Since 1991 the freshwater pearl mussel
has been listed in Schedule 5 of the 1981
Wildlife and Countryside Act
6 2 It is illegal to kill or injure a mussel, unless a licence has been
obtained, but it is
permitted to take a mussel, examine it for pearls using specially designed
tongs, and return it unharmed to its environment. This legislation was designed
to allow the continuation of pearl fishing but unfortunately there is ample
evidence that mussels are still being killed in large numbers and it is only a
minority of fishermen who bother to use tongs. Although locations records are
still being collated, there is no proper monitoring of mussels and so the scale
of the continuing killing is unknown.
Mussel tongs in use
6 3 International legislations, which can only be applied in Britain through
domestic legislation, also makes provision for the protection of species. The
Convention of Conservation of European Wildlife and Natural Habitat (the Bern
Convention) lists the freshwater pearl mussel in Appendix III. This permits some
exploitation of their population. The EC Directive on the Conservation of
Natural and Semi-Natural Habitats and of Wild Fauna and Flora lists the
freshwater pearl mussel in Annex V as a species whose exploitation must be
subject to management. It is also listed in Annex II as a 'species of community
interest whose conservation requires the designation of Special Areas of
Conservation' (SACs).
Young, M R 1984. Scottish freshwater pearl fishing - a dying tradition Scottish
Field Studies, 1984 25-32
Young, M R 1991. Conserving the freshwater pearl mussel (Margaritifera
margaritifera L ) m the British Isles and continental Europe, Aquatic
Conservation Marine and Freshwater Ecosystems 1, 73-77
Young, M R and Williams, J C 1983. The status and conservation of the freshwater
pearl mussel in Great Britain Biological Conservation 25, 35-52
Young, M R and Williams, J C 1984. The reproductive biology of the freshwater
pearl mussel (Margaritifera margaritifera L ) in Scotland. Archiv fur
Hydrobiologie 100, 405-422
Dr Mark Young, Department of Zoology, University of Aberdeen\par Contacts for
further advice and information
Dr Philip Boon, Head of Aquatic Environments Branch
Dr Willie Duncan, Freshwater Conservation Officer
Dr Martin Gaywood, Habitats and Species Directive - Species Co-ordinator
Dr David Phillips, Invertebrate Ecologist
John Ralston, Licensing Officer
All can be reached at:
Scottish Natural Heritage
2 Anderson Place Edinburgh
EH6 5NP
Tel 0131-447 4748