Why oysters, the natural world's 'marine engineers', are vitally important to Scotland's seas

Scotland’s native oysters (Ostrea edulis) may be small and unassuming in appearance – with not a lot to differentiate between them and your average small muddy stone – but they bring myriad benefits to our inshore ecosystems, and the species that rely on them, including us humans. It is hard to exaggerate just how much oysters matter. Simply by going about their quiet business, these ‘ecosystem engineers’ can shape the world.

Oysters are sessile (they don’t move much), bivalve molluscs; the only time they are mobile is just after the female has spawned, when the larvae motor about with the other plankton for a week or so, searching for something suitable to settle on for life – preferably an adult oyster’s shell, as that is a good indicator that the location is suitable for their survival.

Oysters feed by filtering plankton and particulate matter from the sea that surrounds them, in doing this, they remove nitrates, phosphates and other pollutants, and clarify the water, improving the situation for species that are sensitive to pollution such as maerl and seagrass, which are themselves vital fish and shellfish habitats.

Oysters may look unassuming, but they play a huge role in improving water quality and creating habitats for other marine life | Ailsa McLellan

Habitat creators

Oysters once existed in extensive beds around our coasts, in such vast quantities that they dictated the very nature of Scotland’s coastal seas. When allowed to aggregate in large numbers, they stabilise sediments, and protect coasts from erosion, which is increasingly important as climate change brings progressively more volatile weather.

They create fantastic three-dimensional habitats which are vital for biodiversity; Pieter Korringa, a great oyster scientist, listed 250 species living on the shells of native oysters, and that doesn’t include the hundreds more species that live amongst communities of oysters, including commercially important fish species.

Oysters are important culturally; they have been food for people for millennia as is evidenced by the contents of Mesolithic shell ‘middens’ around our coasts. During the 1800s and 1900s, the development of rail networks meant that oyster beds could be exploited by distant companies who were able to transport vast numbers of oysters, longer distances, to larger markets than was previously possible.

Letting nature recover by itself

This is where it started to go very wrong for the oyster. Overfishing and pollution began to take their toll, leaving weakened populations that were vulnerable to diseases and parasites, until populations plummeted to levels that marine environment body Ospar describes as “critical” today. In Scotland, and across Europe as a whole, it is thought that only around 5 per cent of the original oyster population remains – but this story does not have to end on a downer.

Whilst not as easy to farm as the pacific oysters (Crassostrea gigas) that are the main species farmed in Scotland, the native oyster can be cultivated, so their numbers can be boosted. There is often a case for removing pressures and letting nature recover itself – which it often does more effectively than with our ‘help’.

However, the oyster’s reproductive strategy (the female must be close enough to a male to take sperm into her mantle and ‘brood’ it for around a week) means that once densities fall below a certain level, populations cannot recover without the number of breeding age adults being boosted. This is where we can help.

350,000 oysters – and more

Around the coasts of Scotland, community groups and charities are raising the funds and garnering the many permissions needed to buy in juvenile native oysters from hatcheries, grow them on to ‘refuge size’, which means they won’t be eaten by every passing starfish or crab, and releasing them.

One such charity is Seawilding (which I work for), a community-led, marine habitat restoration charity that was spawned in Loch Craignish five years ago with the aim of restoring degraded coastal habitats at scale. We aim to put millions of oysters back into our coastal waters, and will work towards that goal for as long as we have breath in our lungs.

To date, Seawilding has returned 350,000 oysters to Loch Craignish, started releasing oysters in Loch Broom this year, and provided thousands of oysters for other restoration projects. Last year, we started to see ‘spat settlement’ in Loch Craignish – evidence that the oysters we have put back are breeding – which is incredibly rewarding, but our work is not finished until the population becomes self-sustaining, and measures are put in place to protect seabed habitats from the issues that caused the degradation in the first place.

A beautiful, interconnected ecosystem

Scotland’s seas are a public asset and are supposed to be managed for the common good of the Scottish people. The seabed is the foundation of coastal productivity, it and its associated habitats sink more carbon than all our peats, forests, and terrestrial soils combined; we need it to be healthy now more than ever and, for that to happen, we must take a holistic approach to environmental recovery.

Oysters are one small part of a complicated, beautiful, interconnected ecosystem, and whilst communities do what they can to protect and restore this on a local level, the buck stops with the Scottish Government when it comes to addressing the pressures that are wrecking habitats.

They have said that they will halt biodiversity loss by 2030 and reverse it, with large-scale restoration, by 2045, but they are glacially slow to act. Ten years after their designation, we still have ‘marine protected areas’ with no protective measures in place, bottom-dragged fishing gear has legal access to over 90 per cent of our inshore waters, including within ‘protected’ areas, and nutrient and chemical pollution are increasing due to overwhelmed sewerage works and a burgeoning, government-backed salmon farming industry.

It is communities that are raising the funds and doing the hard graft that could edge us back towards the ‘good environmental status’ that the Scottish Government say they are committed to achieving. Now we need Scottish ministers to play their part in making restoration a success.

Ailsa McLellan works for Seawilding, a community-led marine habitat restoration charity