Bottom Trawls Create Deserts in the Deep Sea

We already knew that bottom trawl fishing—in which large, heavy nets are dragged along the ocean floor—was damaging to ocean life and marine ecosystems. Numerous studies have shown that trawling damages fragile marine habitats and harms many species because the nets indiscriminately scoop up all animals in their path. But new research shows this type of fishing can have “devastating consequences” for the deep sea floor, creating “highly degraded seascapes.”
 
Published May 19, the study in the Proceedings of the National Academy of Sciences compared untouched portions of a sea canyon in the Mediterranean with areas that had experienced trawling. The scientists were especially interested in effects on worms and other small creatures in the sediment, many of which play important roles in the seafloor ecosystem. 

The research found that the diversity of species in the trawled areas was cut in half, and overall abundance of the small life forms in the sediment dropped by 80 percent. The trawled areas also had only about half as much organic matter in the sediment compared to the control areas.

The study’s authors reasoned that slower growing life forms in the deep sea (the study areas are in 1,500 to 6,500 feet of water) are especially vulnerable to disruption. They likened the effects of bottom trawling to the “desertification” of farmland, in which overuse from plows or livestock turns arable land to wastes. 

The authors wrote that intensive and chronic bottom trawling will "transform large portions of the continental slope into faunal deserts,” and concluded that trawling “represents a major threat to the deep seafloor ecosystem at the global scale.”

Fisheries managers should take note. As inshore fish populations decline and the technology of industrial-scale fishing advances, many trawling fleets are moving into deeper water. This study indicates we should take a precautionary approach to the seaward expansion of fishing practices known to cause lasting damage.

University of Hawaii, Manoa, biology professor Les Watling, a Pew marine fellow and a renowned expert on deep-sea life, commented in the journal that the study challenges some commonly held assumptions. 

“There has been the sense that trawling on open mud bottoms has little or no impact on the resident species,” Watling wrote, but the study shows otherwise.

Here on the Atlantic coast, the Mid-Atlantic Fishery Management Council is considering restrictions on bottom fishing gear in canyons along the edge of the continental shelf and beyond. The council is rightly concerned about possible damage to fragile deep-sea corals and the animals that live among them. But managers now have solid information that the seafloor itself is also vulnerable, as bottom trawling moves to deeper regions.

Peter Baker directs the Northeast U.S. oceans program for The Pew Charitable Trusts.