Reintroductions


Saving the freshwater pearl mussel


Dr Roger Sweeting, Honorary Fellow at the Freshwater Biological Association, explains the decline of this once-abundant mollusc and shares plans for an innovative reintroduction.


Many of the slightly acid, poorly buffered rivers that flow into the North Atlantic from its western, northern and eastern shores are home to the freshwater pearl mussel. This unusual mollusc has a wide geographic range in the northern hemisphere, from the rivers in the eastern US and Canada up into the Arctic and over to the westernflowing rivers of Europe.


Historically, it was present in many rivers in hundreds of millions. Now it is reduced to relatively few across most of its range, with only rivers found in the Kola peninsular in western Russia still supporting tens of millions of individuals.


A threefold decline 1. Historic fishing for its pearls


Like many bivalves, the freshwater pearl mussel accumulates sand and other inorganic particles in its body cavity. To minimise physical and allergenic damage it surrounds them with nacreous material: occasionally a pearl is the result. It is said that one of the reasons the Romans came to Britain was in search of its highly prized freshwater pearls.


2. Changes in river habitat


The flows and continuity of rivers through construction of dams, weirs and channelisation have altered the physical habitat considerably. These changes have caused major problems for the pearl mussel.


3. Pollution


Our domestic, industrial and agricultural effluents have degraded the pristine habitats that these animals require.


20 Cumbrian Wildlife | May 2019 Life cycle & reintroduction


Freshwater pearl mussels have a long and complex life cycle, which requires a host fish for their larvae (glochidia).


Since 2007 the Freshwater Biological Association has been breeding pearl mussels in captive conditions. Some have since been released as juveniles (aſter several years) back into their parent rivers following careful river restoration work.


This restoration needs to include atention to flows and suitable fish hosts (in some mussels brown trout, in others salmon and in some populations arctic charr are the hosts). More importantly, the water quality and substrate quality


must be right or the juvenile mussels will not survive. Organic silt, enriched water, canalised streams and rivers cannot act as suitable environments.


Water quality


In more natural environments the mussels will themselves be water purifiers, transforming the rivers to near-pristine conditions: large mussels can filter water through their gills very effectively. Up to 50 litres of water per day per mussel has been recorded. Formerly, with some rivers holding millions of mussels, the volume of water processed would have been in the tens of millions of litres per day. Hopefully one day we will realise this as part of re-establishing one of our major ‘natural capital’ assets.


Lifecycle of the freshwater pearl mussel 3


Glochidia clamp onto the gills of young salmon, trout or


Arctic charr


Juvenile mussels drop off and bury into the


gravel bed Glochidia


(parasitic larva) released by the female mussel


2


Sperm taken in by the


female mussel 1


Young mussel visible aſter


approx. 5 years 5 4


Photo: Linda Pitkin/2020VISION


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