Chip-shop fish among key seabed engineers

Atlantic cod. Image credit: Alex Mustard

Convex Seascape Survey scientists assessed the role of fish in bioturbation (churning and reworking sediments) in shallow UK seas.

The Atlantic cod – a staple in chip shops – jointly topped the list of these important “ecosystem engineers” (along with Atlantic hagfish and European eel).

In total, 185 fish species were found to play a role in bioturbation – and 120 of these are targeted by commercial fishing.

“Ocean sediments are the world’s largest reservoir of organic carbon – so what happens on the seabed matters for our climate,” said University of Exeter PhD student Mara Fischer, who led 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 and wider ocean ecosystems.

“We have a good understanding of how invertebrates contribute to global bioturbation – but until now, we have been missing half the story.

“Our study is the first to attempt to quantify the bioturbation impact of fish, and it shows they play a significant, widespread role.”

Overfished and overlooked

Co-author Professor Callum Roberts, from the Centre for Ecology and Conservation at Exeter’s Penryn Campus in Cornwall, said: “We also found that species with the highest bioturbation impacts are among the most vulnerable to threats such as commercial fishing.

“Many of the largest and most powerful diggers and disturbers of seabed sediments, like giant skates, halibut and cod, have been so overfished they have all but vanished from our seas.

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

The researchers examined records for all fish species living on the UK continental shelf, and found more than half have a role in bioturbation – sifting and excavating sediment during foraging, burrowing and/or building nests.

These different ways of reworking the sediments – termed bioturbation modes – alongside the size of the fish and the frequency of bioturbation, were used by the researchers to calculate a bioturbation impact score for each species.

Examples include:

• European eel. Bioturbation mode: burrower. Bioturbation score (out of 125): 100. IUCN conservation status: critically endangered. Fished primarily using traps and fyke nets, they are considered a delicacy in many parts of Europe and Asia – commonly prepared as smoked eel or dishes like eel pie and eel soup. Threats include climate change, diseases and parasites, habitat loss, pollutants and fishing.
• Atlantic cod. Bioturbation mode: vertical excavator. Bioturbation score: 100. IUCN status: vulnerable. Primarily fished using trawling and longlining, they are consumed in many forms, including fish and chips, fresh fillets, salted cod, and cod liver oil. Threats include overfishing, climate change and habitat degradation. Populations have declined in several parts of its range, particularly the North Sea and West Atlantic.
• Common skate. Bioturbation mode: lateral excavator. Bioturbation score: 50. IUCN status: critically endangered. Historically targeted by trawling and longlining, this species is now protected in several regions – but often caught accidentally (bycatch). Numbers have drastically declined due to overfishing. The species is vulnerable due to its large size, slow growth rate, and low reproductive rate – only about 40 eggs are laid every other year, and each generation takes 11 years to reach maturity.
• Black seabream. Bioturbation mode: nest builder. Bioturbation score: 36. IUCN status: least concern. Primarily caught using bottom trawling, gillnets, and hook and line. Fishing during the spawning season in April and May can impact population replenishment. Bottom trawling at this time has the potential to remove the fish, nests and eggs.
• Red gurnard. Bioturbation mode: sediment sifter. Bioturbation score: 16. IUCN status: least concern. Historically not of major interest to commercial fisheries, the species has been targeted more in recent years (including in Cornwall). It is mainly caught by trawlers. There is currently no management for any gurnard species in the EU: no minimum landing size, no quota, etc – which could lead to unsustainable fishing.

Dr Julie Hawkins, another author of the study, commented: “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 the urgent effort to slow climate change.

The paper, published in the journal Marine Environmental Research, is entitled: “A functional assessment of fish as bioturbators and their vulnerability to local extinction.”