Seabed Trawling May Be Spewing Huge Amounts of CO2 Into the Atmosphere

New research suggests the controversial fishing method is also contributing to increased ocean acidification, which can harm marine wildlife

Fisherman fixing a net on a boat at dusk
Bottom trawling is a polarizing fishing practice that involves dragging heavy nets and equipment across the seafloor. Davide Pischettola / NurPhoto via Getty Images

A controversial fishing method may be releasing up to 370 million metric tons of carbon dioxide into the atmosphere every year, according to new research published this week in the journal Frontiers in Marine Science.

The method in questions is bottom trawling, a practice that involves dragging heavy nets and equipment across the seabed. It’s effective at scooping up fish, shellfish and crustaceans—responsible for roughly a quarter of wild-caught seafood—but damages the seafloor in the process.

Bottom trawling can destroy habitats, churn up sediment and damage cold-water coral reefs.

“They drag along the bottom and cut through everything in their wake,” Max Valentine, a campaign director at the nonprofit ocean conservation group Oceana who was not involved in the new study, tells Wired’s Matt Simon. “We liken bottom trawling to clear-cutting of a forest. For example, hard corals in Alaska, which have been dated to hundreds of thousands of years [ago], can be destroyed in just a single swipe.”

The sediment stirred up by trawling would otherwise sit untouched for millennia, keeping carbon locked up at the bottom of the sea in the form of organic matter. But when the trawling equipment moves through, it disturbs that organic matter and makes it easy fodder for microbes. When microbes break down the sediment, they expel CO2, which dissolves in the water and moves around the world via currents.

Research published in 2021 found that seabed trawling was releasing large amounts of CO2 into the world’s oceans. But this earlier study didn’t account for how much of that CO2 was making it into the atmosphere.

“Lots of countries and different agencies started asking us about that research,” says Trisha Atwood, an ecologist at Utah State University who co-wrote both the 2021 paper and the recent study, to New Scientist’s Michael Le Page. “But they basically said, if it just stays in the ocean, we don’t really care.”

Using computer models, the scientists estimated that a little more than half of the CO2 released into the ocean by trawling—between 55 and 60 percent—ends up in the atmosphere within seven to nine years, which the researchers describe as a relatively quick timeframe.

370 million metric tons equates to roughly half the annual emissions of the international shipping industry, reports Climatewire’s Chelsea Harvey, or about 0.8 percent of all global emissions, per New Scientist.

That may not seem like much, but as the planet continues to warm because of human-emitted, heat-trapping greenhouse gases, every bit counts.

“Global warming is like death by a thousand cuts,” says study co-author Enric Sala, founder of the National Geographic Pristine Seas project, to National Geographic’s Ayurella Horn-Muller. “It’s lots of different sources producing CO2 emissions. … Everything, everything counts.”

A pteropod shell is shown dissolving over time in seawater with a lower pH
A pteropod shell is shown dissolving over time in seawater with a lower pH. When carbon dioxide is absorbed by the ocean from the atmosphere, the chemistry of the seawater is changed. NOAA

The CO2 that remains in the ocean, meanwhile, is making the water more acidic, the study finds. Increased ocean acidification can harm marine life, including “shell builders”—like coral, crabs and oysters—that need carbonate to form their tough exteriors. As the ocean becomes more acidic, fewer carbonate ions are available. If the water becomes too acidic, these organisms’ “shells and skeletons can even begin to dissolve,” notes the National Oceanic and Atmospheric Administration.

Acidification is especially troubling if it occurs in enclosed seas like the Mediterranean, Atwood tells Wired.

“Those are areas where it’s localized because acidification from trawling, from that CO2, stays in the water and could become problematic,” she says.

To the researchers, the findings add to the growing body of evidence that seabed trawling is harmful and should be more tightly regulated and, in some instances, banned.

“Much like destroying forests, scraping up the seafloor causes irreparable harm to the climate, society and wildlife,” Atwood tells the Guardian’s Karen McVeigh.

But not everyone is convinced by the paper’s results. Last year, a group of scientists publicly challenged the findings of the 2021 study. Writing in the journal Nature, they argued that the researchers relied on a flawed formula that likely resulted in an overestimation of the amount of CO2 being released by trawling. One of these critics, Jan Geert Hiddink, a marine ecologist at Bangor University, again expressed his skepticism upon the publication of the new paper.

But the researchers who conducted the trawling research continue to stand by their findings—and they say there’s no time to waste in reducing greenhouse gas emissions in whatever way possible.

“Any time science suggests that there might be regulatory issues with an industry, there are going to be questions about the science—and there should be. Do we know enough to create regulations?” Atwood asks Inside Climate News’ Georgina Gustin. “The question is: Do we want to science something to death, and do we have time to do more work? We’re up against a climate clock and, unfortunately, we’ve lost the luxury of time.”

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