Article published in The Beacon, magazine of the Brecon Beacons Park Society, January 2024.
Whilst on a BBPS walk on Mynydd Illtyd we happened upon a young man wearing a Natural Resources Wales Top and carrying a white, lidded bucket. Being possessed of naturally inquiring minds, we stopped for a chat. He told us that he was collecting silt as part of a project to restore populations of freshwater pearl mussels to rivers in Wales. Since few of us had even realised there were freshwater pearl mussels in Wales, we naturally had lots of questions, and he was kind enough to tell us about the life cycle of the mussels, their current plight, and the steps that were being taken to recover the situation. I have since done some more background reading on the topic, which I thought may be of interest to members.
Freshwater Pearl Mussels (Margaritifera margaritifera) are bivalve invertebrates – they are a soft bodied creature which is encased within a hinged, two-part shell. They can live to be over 100 years old, grow to be up to 15cm long, and sometimes (about 1 in 5000 mussels) will develop pearls within their shells. They have a fascinating lifecycle: female adult mussels release up to 4 million glochidia (larvae) sometime between July and September. These tiny glochidia can survive for up to 6 days in the water, but the vast majority of them will die. A lucky few will be inhaled by salmon or trout, and their shells will clamp shut upon the gills. They will live on their host for the next several months (this does not seem to have any negative effect on the fish), dropping off in early summer. When they drop off, they will bury themselves completely into the gravel and stones of their habitat, using their “foot” to collect food (algae and bacteria) from the stones and gravel. By the age of 3-5 years, they will develop gills, which allow them to filter their food directly from the flowing water, requiring adults to live with their shells partly exposed above the riverbed. They are ready to produce their own larvae after about 12-15 years.
This lifecycle, and the need for an environment which is sufficiently clean, oxygenated and with gravels for the young mussels to bury themselves in, and in which the right fish exist in sufficient numbers to allow the glochidia to find a host fish, mean that the attrition rate for young mussels is extremely high. Unfortunately, river habitats have, over the last decades, come under increasing pressure, and this has had an impact on both fish and mussel numbers. There are no longer any populations of pearl mussels which are considered “viable” (likely to increase or maintain their current numbers in coming years) in Wales. Indeed, research has shown that there are currently very few mussels below the age of 80 living in our rivers.
Freshwater Pearl Mussels: a “Pearl in Peril”
The reasons for the decline in mussels reaching adulthood are numerous, but have been summarised by Natural Resources Wales into 5 main categories:
Firstly, their habitats have been damaged by dredging and land drainage, which has caused the gravels and stones to become silted and clogged. The faster water flow caused by dredging also means that the gravel becomes unstable and likely to be washed downstream. This means that in many places there are no longer suitable places for juvenile mussels to bury themselves and feed when they leave their host fish.
Secondly, water quality has reduced. Agricultural practices and forestry have led to increased chemical run off into rivers, changing the composition of the river water. The nutrient rich, acidified water both harms the creatures dwelling within it, and alters the ecosystem in which they live. Mussels need low calcium and oxygen rich waters to thrive: river pollution decreases the amount of oxygen within the water, creating an unhealthy environment for the mussels, and indeed for the fish on which they depend.
Thirdly, the proliferation of invasive species which out-compete native plant growth, and can smother rivers, deoxygenating the river, reducing the flow of water and causing instability to the riverbanks.
Fourthly, the migration of salmon (to the sea) and trout (to lakes or within the river) has been made more challenging and time consuming by manmade barriers in the river, such as dams, weirs, etc. Since spawning is a time sensitive process, this can mean that the fish fail to reach an appropriate habitat in which to spawn their young, and thus decreases the population of fish within the river. Without sufficient fish to act as hosts, the infant glochidia are unable to survive beyond their first week.
And finally, climate change, with its promise of warmer weather and increasingly stormy storms also poses a threat to pearl mussels: increased flow of water and flooding following a storm will destabilise the riverbed and potentially wash both mussels and fish eggs downstream. The mussels are also a cold-water species, and thus its range may be affected by warmer water temperatures.
So what can be, or is being, done to reverse this trend and to improve habitats and the chances of the mussels’ survival? Freshwater Pearl Mussels are listed as an endangered species by the International Union for the Conservation of Nature, and are protected against being killed, harmed, or taken from their natural environment by UK law. There are also several initiatives being undertaken which should, directly or indirectly, help to improve matters.
And this is where “our” young man comes in. As an employee of Natural Resources Wales, he works at a freshwater pearl rearing facility at Cynrigg hatchery, near Brecon. Part of the Four Rivers for LIFE Project, uses a new technique of raising the juvenile muscles in boxes containing a bed of sediment and algae during the glochidia stage of their lifecycle, with the eventual goal being to release adult mussels back to the wild at secret sites.
Of course, reintroduction of the mussels will be unsuccessful unless the habitat and water quality are restored to their original state.
A Project called Pearls in Peril focused on a number of UK sites, including Afon Eden (the home to the largest population of freshwater pearl mussels in Wales), and improved 2.4km of the river habitat. Monitoring has proven that this had immediate effects on the water quality and the habitat, and provides hope that that this will increase the trout population and thus increase the chances of glochidia survival.
Following the Pearls in Peril Project, the Four Rivers ForLIFE project has undertaken to work on 776km of river in Wales, choosing to work on 4 rivers which are classed as Special Areas of Conservation due to their important animal and plant life: the Teifi, Cleddau, Tywi and Usk. As part of their efforts, and similarly to the Pearls in Peril Project, they will be removing barriers to fish migration, restoring rivers to their natural state by reintroducing boulders and gravel and correcting artificial changes to the path of the river, planting native trees along riverbanks, removing invasive non-native species and working with the farming community to reduce the impact of agriculture on the rivers water.
Natural Resources Wales are hoping to release the first box bred mussels into an unidentified river in Gwynedd which has been restored to provide an appropriate habitat. I strongly suspect that, having read this article, you and I can make a good guess at which river this is. However, the reason for the secrecy is to prevent people from harvesting the mussels to find pearls, which is a problem in Scotland where the mussels are more common. Let us all hope that these measures are successful, and that Freshwater Pearl Mussels will increase in number and continue live their fascinating lives on Welsh riverbeds for hundreds of years to come.
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