Freshwater pearl mussels: Scotland's little-known royal gems

It is in these waters sluicing through the Cairngorms that a particularly prized part of the environment exists: freshwater pearl mussels, which are among the planet's most critically endangered creatures. The jewels in the mussels are rare – about one in 5,000 contains these milky-white treasures – and, as such, they have historically been valued. According to Julius Caesar's imperial biographer, Suetonius, the leader was an obsessive pearl connoisseur who allegedly had the Roman army invade Britain in 55 BCE in part because of these iridescent gems. Appropriately, the Crown of Scotland – the oldest piece of crown jewellery, dating to 1540 and including materials from an even older diadem – features pearls from these molluscs. 

"The funny thing about pearl mussels, if you ask anyone in Scotland, most of them don't know they exist," said Annie Armstrong, founder of nature tourism company Wild Braemar that organises excursions for Fife Arms guests. "They're kind of this secret, unknown thing." 

Today, these imperilled mussels are on the verge of extinction – a particularly troubling reality given that they're a keystone species. A single mussel filters 50 litres of water a day and their presence indicates good water quality; if they are at risk, it's a harbinger of a decline in the river's health. Scotland has always been a stronghold for the species, which can live up to 100 years, and their presence in the Dee has led the river to be granted a Special Area of Conservation designation. 

Despite a ban on pearl fishing or buying and selling Scottish pearls since 1998, poachers still stealthily try their luck. There's a more daunting adversary, though: warming waters caused by climate change and exacerbated by a lack of tree coverage that would help mitigate rising temperatures.

The abundance of trees that once existed here as part of the ancient Caledonian Forest helped the mussels survive by shading, and thus cooling, the River Dee and its tributaries. These lower temperatures increased the dissolved oxygen available to Atlantic salmon, in whose gills pearl mussel larvae live as they mature. But deforestation ran rampant here in the mid-19th Century, when Queen Victoria moved to Balmoral and made open, unforested estates fashionable. Trees further disappeared because of fuel needs during the two World War efforts. 

Now the River Dee Trust and stakeholders like Artfarm are fighting back with the One Million Trees campaign, an initiative started in 2020 that aims to plant one million trees in the River Dee catchment by 2035. The programme takes a macro approach to protecting salmon and freshwater pearl mussels through this strategy. 

The Fife Arms even has a weighty bronze cast of a freshwater pearl mussel attached to its hotel room key fobs to honour the molluscs and bring awareness to their current predicament. Guests can tour the planting sites on off-roading excursions and sponsor a single tree or a "bouquet" of trees. They can also roll up their sleeves and take an active role in mussel preservation by having the hotel's ghillies (outdoor guides) arrange opportunities on certain days to plant alder, birch, willow and rowan saplings and weed out invasive species from Cairngorms National Park. Through these rewilding immersion outings – which can include warm tea in thermoses, baskets of Scottish shortbread cookies and drams of whisky to be consumed in bothies – Fife Arms guests see first-hand how the future of the park will take shape and, in turn, protect salmon and mussels.

As I romped through the 95,000-acre Invercauld Estate – a part of Cairngorms National Park that Artfarm leases and manages near Balmoral Castle – on one of The Fife Arms' off-road experiences, Flora Grigor-Taylor, a Dee River Trust habitat adviser, took me on a tour of her "mob planting" experimental tree patch along a tributary of the Dee.

"We put in a high-density number of native broad-leaf trees without any fencing to see how they would do planted amongst the heather," she said. "In a couple of years' time, they'll be big enough to create some shade over the river." 

In June, the Dee's waters recorded their highest temperature to date: 23.5C. Salmon typically need a water temperature between 6C and 16C to thrive. As the water heats up and has less dissolved oxygen, salmon are forced to expend greater energy. When they migrate out to sea, they're less fit and have trouble evading predators. If the salmon can't survive and procreate, that, in turn, damages the pearl mussels. 

In addition to providing shade, Steff Ferguson of the River Dee Trust explained that trees also help stabilise the riverbanks and create a habitat for a host of insects. Those insects and leaves fall into the river and provide nutrients for worms, krill and smaller fish that the salmon then eat. While waiting for the trees to grow, the River Dee Trust has lined tree-like wooden structures every 100m across certain channels to create a micro habitat for salmon featuring deeper (and cooler) pools of water, spawning gravels and refuge areas from predators. 

"There's a phrase that salmon live in woodland, which we don't think straight away," Ferguson said. "But they need that woodland habitat around them for a healthy ecosystem in the river."

That should, in turn, benefit the pearl mussels. "Certainly for pearl mussels, salmon are intrinsic to their lifecycle," Ferguson said.

During a hike and foraging journey I booked with Wild Braemar through the Fife Arms, Armstrong guided me through one of Queen Victoria's beloved wilderness picnic spots near the elevated viewpoints of Creag Chòinnich and the Cromlins while we picked wild thyme and wood sorrel that tasted of sour apple candy.

Armstrong said while trees are what will ultimately help cool the rivers and save the mussels, allowing travellers to immerse themselves in the mussels' habitat as we were doing will cultivate a connection with that environment and encourage people to be stewards of those spaces and the imperilled species.

By the beginning of 2022, the One Million Trees campaign had reached a quarter of its goal, and a year later, it was nearing the half-way mark. Part of the reason for the project's growth was due to support from a seemingly unlikely ally: Scottish whisky distillers.

"The whisky industry in Scotland depends on the rivers, the fresh water that they supply," Dr Lorraine Hawkins, River Director at the River Dee Trust, told me inside the Burn O'Bennie Distillery in the town of Banchory. Whisky is ceremonially used to bless the Dee, a particularly beloved river of King Charles III, but freshwater pearl mussels have a practical role in the creation of spirits since they act as a natural purifier for the waters. More mussels means better-tasting whisky. 

In a recent charity fundraising programme, Burn O'Bennie Distillery sold 30 sherry-seasoned 250-litre specialty casks and used £100,000 of its proceeds to help support the Million Trees Project. 

"We live here on … the River Dee – it's the whole area that's benefitting from the river being here and all the improvements they're making for the wildlife that are native to the river or live nearby in other parts," said Mike Bain, Burn O'Bennie's co-founder. "Fresh[water] pearl mussels in the River Dee – they're the last bastions in Europe. They became our first heroes."