Cod, Eels, and the Quiet Power Beneath Our Feet

New research shows the fish we love to eat are also busy engineers of the seafloor, shaping ecosystems and carbon storage in ways we’ve long ignored.

I remember the first time I saw a little fish sweeping the seafloor while snorkeling in the Costa Brava coast where I grew up. It wasn’t fast or flashy. Just this gentle, gliding creature stirring up sand as it passed. 

I watched it long enough to realize something odd: everywhere it moved, it reshaped the seafloor ever so slightly. At the time, I was focused on macrofauna distributions and food webs. But I logged that moment in my mind. Years later, while teaching an ecology course, I pulled it out again when we reached sediment processes. I told my students, “You think fish just swim around, but many of them are also moving the ocean floor under our noses.”

Now there’s a paper that explains how, beyond fun-to-watch behaviors, this is also how fish reshape their landscapes, turning them into ecosystem engineers.

Published in Marine Environmental Research, the study by Dr Mara Fischer and colleagues at the University of Exeter finally puts numbers and names to something long overlooked: fish as bioturbators (aka organisms that stir up and rework the seabed). 

This churning helps cycle nutrients, bury carbon, and build healthy ecosystems. And the results? Eye-opening. Not only do fish play a massive role in shaping the seabed, but many of the most impactful ones are also the species we’re fishing the hardest.

Fish Are Movers, Not Just Swimmers

Bioturbation is usually talked about in the context of worms, clams, and other invertebrates. But this team looked at 288 marine fish species from UK waters and found that 185 of them interact with the seabed in ways that reshape it either by burrowing, building nests, sifting sediment for food, or digging for shelter.

To assess their impact, the researchers created a bioturbation score based on three simple factors: body size, how often the fish touches the sediment, and how it interacts with it. They grouped fish into five types: sediment sifters, lateral excavators, nest builders, vertical excavators, and burrowers.

At the top of the list? Three very familiar names: the European eel, the Atlantic cod, and the Atlantic hagfish, each with the highest possible score of 100 out of 125. But there’s a catch. 

Here’s the interesting part: The eel is critically endangered, the cod is vulnerable, and the hagfish, while not yet commercially targeted, is on the radar.

How They Did It

The team combined species data from ecological databases, FishBase, and conservation sources like the IUCN Red List. The latter has been an incredible tool for so many of my own research outputs. 

For each fish, they calculated its potential to stir up the sediment based on traits like size and behavior. Then they looked at how many of these fish are exploited by fisheries, and how vulnerable they are to extinction.

Of the 185 seabed-disturbing fish, 64% are commercially fished. And not just lightly; many of the most impactful bioturbators are also the most heavily targeted by industrial fisheries.

That overlap is what makes this study more than a curiosity. It’s a warning.

What This Means for the Ocean and Us

The seabed isn’t just a passive dump for marine debris. It’s a dynamic, living interface where carbon is buried, nutrients are cycled, and ecosystems are built. Sediment isn’t static; it’s shaped and reshaped by life.

Fish that dig and churn the seabed do more than make a mess. They create microhabitats, oxygenate the sediment, stimulate microbial processes, and enhance carbon burial. It’s like composting meets excavation, done on a scale that affects the whole climate system.

“Bioturbation is very important for how the seabed takes up and stores organic carbon,” said Dr. Mara Fischer. “Our study is the first to attempt to quantify the bioturbation impact of fish, and it shows they play a significant, widespread role.”

Yet, according to co-author Dr. Callum Roberts, we’ve fished out many of the biggest seabed engineers. “Giant skates, halibut and cod… have all but vanished from our seas,” he said. These losses aren’t just a hit to biodiversity; they’re altering how ecosystems function and how carbon cycles through the ocean.

Even species considered of “least concern” aren’t safe. The black seabream, for example, builds nests in spring. But during that very season, it’s often caught by bottom trawling, a method that can destroy both the fish and the nest in one sweep.

A New Way to Think About Fisheries and Conservation

This study pushes us to think differently about fish, not just as stock or seafood, but as engineers of the ocean floor. Protecting species like cod or eel isn’t just about preventing extinction. It’s about safeguarding a process that underpins the health of the entire seabed.

It also reminds us that conservation isn’t always about dramatic interventions. Sometimes, it’s about noticing the subtle roles organisms play and making space for them to do their thing.

Anyone who has ever spent time underwater knows this intuitively. As study author Dr. Julie Hawkins put it, “It’s hard to believe that such an obvious and important activity has been largely overlooked when it comes to understanding ocean carbon burial.”

Looking Ahead

There’s more to understand — mainly how local fish populations, abundance, and behavior patterns affect sediment processes at larger scales. But the framework offered here is a powerful starting point. With it, we can begin to identify which fish are more closely related to seabed health, and which need more protection, not just for their sake, but for ours.

I think back to that little fix I saw years ago. What seemed like a simple stirring of sand was, in hindsight, part of a much bigger story. One we’re just starting to understand.

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Best,
Sílvia P-M, PhD Climate Ages