Many of the fish we eat and those we favour in our fish ‘n chip shop order play a key role in maintaining the health of the seabed and, therefore, stabilising our climate, a new report collated by scientists from the Convex Seascape Survey has shown.

Out of the 185 species of fish found to play a vital role in a process known as bioturbation – the churning and reworking of sediments on the seafloor – within shallow UK seas, 120 of them are currently targeted by commercial fishing, including some Friday night takeaway favourites: Atlantic cod, Atlantic hagfish, and European eel.

“Ocean sediments are the world’s largest reservoir of organic carbon, so what happens on the seabed matters for our climate,” said Mara Fischer, a PhD student at the University of Exeter and lead author of the study.

“Bioturbation is very important for how the seabed takes up and stores organic carbon, so the process is vital to our understanding of how the ocean absorbs greenhouse gases to slow the rate of climate change. Bioturbation is also important for seabed – as well as wider oceanic – ecosystems.

While researchers have always shared a ‘reasonable understanding’ of how invertebrates contribute to global bioturbation, this is the first attempt to actually quantify the bioturbation impact of fish, filling in – according to Fischer – the “missing half of the story.”

“And it turns out they play a significant, widespread role,” she said.

Not only are they a favoured species of fish on the menu, they – the fish with the highest bioturbation impacts – are in fact among the most vulnerable to threats such as commercial overfishing – some even to the point of now being considered at a critical state of existence.

“Many of the largest and most powerful diggers and disturbers of seabed sediments, like giant skates, halibut, and cod, have been so overfished that have all but vanished from our seas,” said Professor Callum Roberts, a co-author on the study, an Oceanographic columnist, and head of marine biology at the University of Exeter. 

“These losses translate into big, but still uncertain, changes in the way seabed ecosystems work.”

To draw their conclusions, the researchers behind the study examined records for all fish species living on the UK continental shelf to find that more than half have a role in bioturbation through either sifting and excavating sediment during foraging, burrowing, or building their nests. These different methods of reworking the sediment – termed ‘bioturbation modes’ – alongside the size of the fish and the frequency at which bioturbation takes place among them, were used by the researchers to calculate a bioturbation impact score for each species.

As an example of how the fish species were categorised, Atlantic cod, a fish ‘n chip shop menu favourite, was noted to practice vertical excavating of the seabed, giving it a bioturbation score of 100 out of 125. Its current IUCN status is ‘vulnerable’ and it is a species primarily fished using trawling and longlining to be consumed in many forms, including fish ‘n chips, fresh fillets, salted cod, and cod liver oil. The threats this species faces include overfishing, climate change, and habitat degradation.

At the other end of the scale is the red gurnard, a sediment sifter with a bioturbation score of 16 and an IUCN status of ‘least concern.’ This species is historically not of major interest to commercial fisheries, though it has been targeted more in recent years – including in Cornwall – where it is mainly caught by trawlers. There is currently no management for any gurnard species in the EU, no minimum landing size, and no quota – which could all lead – eventually – to unsustainable fishing.

Julie Hawkins, another author on the study, said: “Anyone who has spent time underwater, whether snorkelling or diving, knows that fish are constantly digging up the seabed. It’s hard to believe that such an obvious and important activity has been largely overlooked when it comes to understanding ocean carbon burial.”

The Convex Seascape Survey is a partnership between Blue Marine Foundation, the University of Exeter, and Convex Group Limited. 

The ambitious, five-year global research programme is the largest attempt yet to build a greater understanding of the properties and capabilities of the ocean and its continental shelves in the Earth’s carbon cycle, in an urgent effort to slow climate change. 

The research paper itself – titled ‘A functional assessment of fish as bioturbators and their vulnerability to local extinction’ – has been published this week in the scientific journal, Marine Environmental Research.

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