Coral and sponge gardens of the deep ocean are now being impacted by deep sea trawling.
Photo: NOAA Office of Ocean Exploration and Research

Industrial fishing is rife with examples of wild over-exploitation, but even by commercial standards, deep sea trawling is wasteful and damaging. It’s also much more prevalent than we thought, according to a recent study in Frontiers in Marine Science, which finds that the poorly-studied practice has extracted some 25 million tonnes of fish from the deep ocean in the past 60 years, up to 42 percent higher than what’s officially reported.

In all likelihood, this largely-unregulated industry is having a horrific effect on deep sea species and the habitats they depend on.

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Bottom trawling—the act of dragging a large net along the seafloor—has been around in some form since the 14th century, according to Maria Deng Palomeres, a co-author on the paper and project manager with the University of British Columbia research initiative Sea Around Us. At first, the practice was confined to shallower waters. But as shallow fisheries began declining in the 1950s, trawlers from Europe, North America, and the Soviet Union began prospecting at depths over 400 meters (1,300 feet), in waters along the North Atlantic and the northwest and southwest Pacific.

The deep sea fishing industry remains small, accounting for less than 1 percent of all landed fish worldwide. But the fleets that are trawling the depths are intruding on a world that runs on a different logic from the surface, one where open water gives way to a world of muddy silt bottoms, wrinkled sea-mounts, and long ridge lines. These more topographically-complex areas play host to sponge gardens and thousand year-old cold-water corals, which form delicate forests for fish, crustaceans, sea stars, and other denizens of the abyss. The organisms that live down here grow slowly, breed infrequently, and can live quite a long time if left alone.

It’s a strategy well-suited to their resource-poor environment, but not one that responds well to commercial harvesting. “Bottom trawling is extremely destructive, clear cutting any habitat that the trawl passes through, leaving next to nothing except rubble behind,” Diva Amon, a specialist in deep-water ecosystems who wasn’t involved with the study, told Earther.

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A shark egg sac attached to a deepwater coral. Sharks are among the deep ocean species sought by some commercial fisheries.
Photo: NOAA Office of Ocean Exploration and Research

While the damage that deep-water trawling can cause is fairly well known, until recently, the level of waste involved was not. Curious about the state of deep sea fisheries, Lissette Victorero and her coauthors began by looking at the fisheries data recorded by the Food and Agriculture Organization (FAO) of the United Nations, but soon realized that something was amiss in the numbers. That’s when they teamed up with the Sea Around Us, an initiative focused on gathering accurate, independent data about fisheries management.

“The more I investigated deep-sea trawling, I started realizing that some of the literature didn’t quite match,” Victorero said. “So it became obvious that we would need additional data for discards and illegal landings to get the real picture.”

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Take the Orange roughy, a fish with a wide distribution that’s fairly easy to catch in large numbers. The new study, which takes into account bycatch and discards, reports that 50,000 tons of unreported Orange roughy have been caught from 1950–2015, mainly by Japan, New Zealand, and South-Korea. The discrepancy with FAO numbers is problematic, because managers look at those numbers to figure out how sustainable a fishery is.

Overall, the study estimates that up to 42 percent more deep sea fish have been caught over the past 60 years than have been reported to the FAO. Then there’s the matter of bycatch, or anything swept up in the nets that’s commercially unsaleable and gets returned dead to the sea. Over the study period, bycatch from the deep ocean was estimated at around six million tons.

While the practice of deep sea trawling is regulated by some countries—among them the United States and China—those regulations tend to protect only specific areas within national jurisdictions. An attempt by the EU to impose a complete ban by member countries failed in 2013. On the high seas, the practice faces few regulations.

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The main takeaway, Amon says, is that deep-water trawling is having a serious impact on the ecosystems least able to recover from it, with effects on the wider ocean that can currently only be guessed at.

“We are only now beginning to uncover (through studies like this one) that the impact is actually much greater than thought,” she said.

Asher Elbein is a journalist and short fiction writer based in Texas.

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