Graham Oliver, marine biologist and head of Biodiversity and Systematic Biology at the National Museum of Wales, is calling for a blanket ban on pearl fishing.
"You have to open thousands of mussels to find a gem-quality pearl. While it is possible to do this without killing the mollusc, cowboy fishermen don't even bother to try and return the mussel alive," he said.
"In addition to fishing, juvenile mussels are being suffocated by algae which have increased hugely in quantity because they flourish on the nutrients in the water caused by fertiliser run-off and slurry," he added.
Commercially viable gems have been extracted from the pearl-yielding animal since Roman times. The pearls are produced when an irritant such as a grain of sand enters a mollusc's shell. The creature secretes nacre - mother-of-pearl - to envelop it and prevent irritation.
The pearl fisherman ploughs through the river wearing all-over waders and bearing an ash pole fixed up like a 6ft clothes peg, and carrying a bucket with a glass bottom in his free hand.
He puts the bucket most of the way in the water and sticks the whole of his face in the open end. After that he walks around, chest deep, looking at the river bed, poking his stick around or coming up for air.
The split end of the pole is clamped over the mussel until it is wedged and it can be brought to the surface.
With luck, for the fisherman anyway, the nacre inside has grown into a small ball. Then he has a classic pearl.
Nowadays, pearl fishing is a cottage industry. However legitimate operators, together with ruthless opportunists hoping to cash in on the phenomenon, have all but destroyed the species.
This, despite the Government's decision to strengthen conservation of the pearl mussel earlier this year under the Wildlife and Countryside Act.
The Joint Nature Conservation Committee recommended the changes to John Gummer, the former secretary of state for the environment, last February amid concern about the species.
On the other side of the argument, however, a total ban would threaten the livelihoods of the few who have persisted with this ancient tradition. Graham Oliver acknowledges this, but believes prohibition is the only way in which the freshwater pearl mussel's future can be safeguarded.
"Implementing a complete ban is the only way forward; any other method would be impossible to police. We need to act quickly, research has shown undeniably that heavily fished waters do not recover," he said.
The River Wye, which runs through Mid Wales and Herefordshire, is a prime example of the effects of over-exploitation. In the 1920s it was known to have a thriving population of pearl mussels, but a recent survey commissioned by English Nature recorded only 22.
Adrian Fowles, invertebrate ecologist with the Countryside Council for Wales (CCW), agrees that the prognosis is not good.
"Following a number of studies, the feeling is that the British population is on a knife-edge. The species has a fascinating life cycle - it lives to be a hundred years old, dispersal of the young is achieved when juveniles attach themselves to the gills of passing brown trout, which is an effective means of establishing colonies upstream - I just hope that we are not already too late to save it."
Nerys Lloyd Pierce