Where have all the oysters gone?
Research shows native oysters at Loch Sween face functional extinction from unregulated gathering.
Beneath the waves at Loch Sween, a vital habitat is teetering on the brink. Once a backbone of biodiverse marine life in Scotland, native oysters (Ostrea edulis) are more than just a delicacy—they filter water, stabilise seabeds, and provide shelter for countless species.
In 2013, a report for NatureScot (then Scottish Natural Heritage) described the Native oysters at Loch Sween as of national importance for their density and the relatively high quality of a nationally rare biotope. Highlighting the habitat and species as one of four priority marine features for which this area should be designated as a Marine Protected Area (MPA).
Yet recent research from Highlands Rewilding has found that the oysters at Loch Sween are missing, with numbers so low and fragmented that the resident population could be considered functionally extinct within this MPA.
Why are the oysters disappearing, and why does it matter? We sat down with Dr David Smyth, Marine Rewilding Lead and native oyster expert, to find out.
For David, the fight to bring back native oysters is both a scientific challenge and a personal mission. His journey began during his PhD research in Strangford Lough, Northern Ireland.
“At the time, the loch had a small population of about 3,000 to 5,000 oysters,” he recalls. Then, something extraordinary happened—a spawning event from an aquaculture facility led to an explosion in numbers, increasing the population to 1.5 million within just three years. As David describes, “it was the transformation in the marine habitat along the intertidal zone that really caught my attention […] When the oysters start to settle and form their clumps and assemblages, biodiversity just exploded. Areas of the Lough went from being a very boring piece of shoreline to a very exciting biodiversity hotspot.”
Oysters can be considered the coral reefs of our temperate seas due to the important role they play in marine ecosystems. Some of David’s past research has recorded over 120 species on one oyster. When oysters settle out in high densities they become habitats in their own right, forming complex structures which act as nursery grounds for juvenile fish and crustaceans. As the reef develops it will become a feeding station for many important commercial species. Beyond providing habitat, they act as natural water purifiers; one oyster can filter up to 200 litres of water a day. As David explains, this "takes all the floculant out of the water column, [an oyster] will remove micro-plastics as it sifts the water through its gills, wraps it in mucus and expels it into the sediment. They are a fantastic tool for clearing up water.”
Loch Sween Marine Protected Area (MPA)
David recalls his excitement when he was offered the chance to work at Loch Sween, known for its high density of oysters, “I thought ‘I’ll see something really cool here’,” he says. But when he started surveying his anticipation soon turned to dismay, “unfortunately when I started doing my survey work, that wasn’t the reality. [There were] very few, oysters on the intertidal zone. Subtidally in the Linne Mhurrich there were some oysters, but certainly nothing that you would consider prolific. In fact, as a biologist, I would have said it was nonfunctional. There were just too few oysters to create a proper spawning event; to produce larvae, they were too spread out.
David’s 6 kilometers of shoreline surveys (and across 20km of wider surveys) found only 48 oysters. He goes on to say, “you should be finding 100,000 oysters. So, that’s the densities you're talking about. They are not there.”
“The reality of it is that over the years something's happened. They're no longer there in the densities that they were, which is quite alarming because the fact that it is a priority marine species or habitat in Scotland, and as part of a marine protected area. It's a matter of concern, really, to the authorities. It's something that should really be addressed.”
A survey commissioned in 2013 for NatureScot prior to Loch Sween’s MPA designation, records native oysters along 11 relevant sites within the loch, with densities ranging from 23 to 582 oysters per 100 meter transect (0.23 – 5.82 per m2). With a mean of 1.89 per m2 this is described as a high density in comparison to other Scottish sites at the time of the survey.
What is an MPA?
Loch Sween is designated as an MPA for the presence of four priority features: native oysters, burrowed mud, maerl beds, and subtidal mud with mixed sediment communities.
There is a network of 233 MPA’s in Scotland. The MPA is a designated region of the ocean where human activities are restricted to protect and conserve marine habitats, species, and ecosystem processes from damage caused by human activities, aiming to maintain health and resilient marine environments by limiting destructive practices, protecting processes essential for healthy, functioning marine ecosystems.
What is causing the oysters dramatic disappearance in Loch Sween?
In over 20 years of research, David has seen many oyster populations in collapse,
“In most cases unregulated gathering is the main problem. Oysters are quite a valuable commodity. There's a grey area in the law where you can collect for your own use. I have encountered people on the shore in parts of the UK with two or three thousand oysters in specially adapted back packs on their backs. […] It earns a lot of money for people, and a lot of it is within the grey economy. So it's a difficult thing to police,” he explains.
“The reason that I think this might be the case at Loch Sween Marine Protected Area is that if it had been disease, there would have been evidence left by empty shells - empty valves of the oysters, and that's not the case.” In 20km of shoreline surveys, David found fewer than 100 empty shells, removing natural mortality from disease or environmental pulse events as an explanation of population decline. Daivd continues, “They've been removed by something. And the most likely culprit is man. There are no major predators out there that would eat a couple of hundred thousand oysters.”
The size of the remaining population suggests that the oysters are around five to six years old, with no real recruitment—no young oysters coming through, which suggests that there was likely a couple of very heavy gathering events three or four years ago.
The urgency of restoration
Across Europe, restoration efforts are proving that, given the right conditions, native oysters can bounce back.
David believes that with swift, decisive action, the population at Loch Sween can recover too. “It's not all doom and gloom, it can be repaired,” he explains, “we can recover that population through very simple methods. If it was possible to gather the remnants of what's there and clump them together so that they are more fecund and can produce more larvae per spawn, that could be one step.” There are other easy actions we could take too, to prevent native oyster gathering, these areas can be better policed and managed, and people can be helped to understand the impact and damage they are doing.
David cites inspiring examples of restoration work from the Billion Oyster Project restoring oyster reefs to New York Harbour, the cultivation of mini reefs at Bangor University, and the Scottish community oyster and seagrass restoration project at Loch Craignish led by Seawilding. “This work is happening all over the UK, and it’s working,” David says. “There are projects happening everywhere, and they are seeing real results.”
Restoring a healthy native oyster population
The key to restoration, David stresses, is understanding what a healthy oyster population actually looks like, “it's not 20. It's 500,000. This can be considered a high density population; that's what we should be really aiming for.”
Despite the challenges, David remains hopeful . The science is clear, the solutions are known. Now, it’s about taking action.
Highlands Rewilding have issued a press release, supported by several local organisations and individuals concerned about the worrying decline of native oysters within the Loch Sween MPA.