More than just a shell

Oysters and mussels are a climate-friendly source of protein, but they are also powerful players in the fight against climate change and to protect ocean health, finds Anna Turns.

In mid-March, 24 tall, tower-shaped cages were carefully lowered and secured beneath the pontoon at Northern Ireland’s Bangor marina. Each one contains clusters of mature oysters. During May and June, once the sea reaches 15-18 degrees, these 648 native oysters should spawn, and tidal currents will carry the next generation of larvae from this new ‘nursery’ out into Belfast lough. 

Heidi McIlvenny, marine conservation manager at Ulster Wildlife Trust, hopes this pilot regeneration project will allow native oyster beds to re-establish after the stocks crashed following overharvesting. 

The shellfishery in Belfast lough closed in 1903, and oysters weren’t seen here for more than a century. But two years ago, researchers found a couple as they walked along the foreshore, indicating that conditions were suitable for oysters. “That was a good start, but we knew intervention would be required to help them thrive in big numbers,” says McIlvenny. 

Native oyster beds are one of the most endangered marine habitats in Europe and, according to the Wildlife Trusts, UK native oyster populations have declined by over 95 per cent. 

But until the 20th century, oysters and other bivalves (molluscs with two adjoining shells) such as mussels were an everyday staple. 

“They were overfished and we dredged them all out, but they used to be so abundant and cheap. Once they became rare, they got more expensive, and today, most people would only eat them with champagne in a fancy restaurant as a treat,” says McIlvenny, who wants projects like this and others in the Solent, Loch Craignish on Scotland’s west coast and the Humber estuary to “reignite the oyster in our cultural, collective memory.” 

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Shellfish is a uniquely sustainable protein source. Credit J Hatcher. 

Eating habits aside, bivalves are also crucial for healthy oceans through their role as ‘ecosystem engineers’. As they grow and reproduce, they pile on top of each other, creating reefs, which are the foundation for entire communities of marine life. 

These reefs have the ability to lock down what’s known as ‘blue carbon’ (carbon in the ocean) by stabilising the sediment, faeces and dead shells beneath them.

“So if you can protect your reef and make sure it’s not going to get dredged or washed away, then ideally the carbon underneath is being stored and locked away, therefore not being released into the water or atmosphere. Scientists are working on quantifying that,” says McIlvenny. 

Oyster and mussel beds provide shelter and breeding grounds for marine animals such as European eels, colourful sea slugs and fish. They are a key part of the marine food chain, feeding on plankton and providing food for other animals. And because they’re susceptible to sedimentation and sensitive to chemical pollution, they’re also indicators of healthy habitats. 

So regeneration projects like the one at Bangor marina, combined with sustainable shellfish farming, have a powerful potential to boost ocean health. 

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Abundant and cheap, oysters used to be enjoyed by all. Credit David Smyth. 

But to really drive this restoration, the UK needs a stronger domestic shellfish market, according to Dr Vicky Sleight, a marine biologist at the University of Aberdeen. 

“We should be eating more shelled molluscs because they’re sustainable to produce and really good ecosystem engineers. But to scale that up, we need to make eating them culturally more mainstream,” she says. “If nobody is going to buy them, there won’t be an industry to grow.” 

Oyster beds and mussel farms don’t need feeding, and there’s no need for any chemical input. Then there’s the fact shelled mollusc aquaculture is at least a 1,000 times more sustainable than the terrestrial farming of livestock, in terms of greenhouse gas emissions per kilo of edible protein, according to a 2019 study by US scientists. 

But while they are a climate-friendly food source, shellfish could also be impacted by a changing climate themselves. Establishing more mussel and oyster beds could be a vital form of climate adaptation, but right now, nobody knows exactly how bivalves will respond to threats such as rising sea temperatures and increased ocean acidification. 

Helen Scales, marine biologist and author of What a Shell Can Tell (out 8 June 2022), likens this to a giant experiment: “We’re trying to predict what the future is going to hold, but we won’t actually know how changes are going to ripple through ecosystems until it’s playing out,” she explains. 

“Changing sea temperatures could affect spawning, but because larvae and eggs drift around, bivalves do have that ability to shift range, and I do think we’ll see more of that as species try to keep pace with climate.” 

Bivalves could also be vulnerable to ocean acidification because the changing pH makes shell production more difficult. 

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While they help mitigate climate change, shellfish may also be threatened by it. Credit David Smyth. 

And while they tend to be fairly robust, farming shellfish in conjunction with other species, like seaweeds, could help counter some of their joint challenges.  

“Combining mussels or oysters with seaweeds and creating more of a closed system in the open ocean, encouraging the recycling of nutrients between different animals such as sea cucumbers or sea urchins, could use ecology to create really sustainable ways to farm,” says Scales.  

Back in Ulster, McIlvenny hopes her oyster cages will provide a valuable window into this underwater world, and demonstrate that positive transformation is possible. 

“We’ll be inviting schoolchildren and community groups to pull up the cages and help survey the wildlife as citizen scientists,” she says. “It’s rare that people who aren’t researchers can get involved in marine conservation, so that’s pretty great.”  

When is peak shellfish?

While people don’t wait for the shellfish season like they do for English asparagus, it is worth following a few guidelines. 

Only eat foraged shellfish during months that contain the letter ‘R’. This old wives’ tale comes from the ‘Red Tide’ levels that can become high during the summer season, increasing the chances of harmful algae. That said, tides are now carefully monitored and shellfish is rigorously tested if bought from a reputable source, so risks are relatively low. 

Another argument for following the ‘R’ rule is that during the summer shellfish are spawning, which can change their taste and texture and mean they may not be at their best. Reducing shellfish consumption during this time also allows for the species to repopulate. 

For more info, visit fishbox.co.uk or mcsuk.org/goodfishguide.

This article was originally published in the spring print edition of Wicked Leeks. You can read the full magazine for free on Issuu.

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