Scientists map the impact of trawling using satellite vessel tracking

Using satellite tracking data, researchers have come up with new maps showing the impact of trawling in 24 regions.

Article by John C. Cannon.

A team of scientists has produced the most comprehensive assessment to date of trawling, a fishing technique that produces a sizable portion of the world’s seafood but is also seen as destructive and indiscriminate.

The research tracked the movements of trawlers in 24 regions of the world, identifying the extent and intensity of their impacts — the industry’s “footprint” — along continental shelves down to depths of 1,000 meters (3,280 feet).

“This is the first study that tried to map the impact of trawling at this global scale,” Ricardo Amoroso, a biologist at the University of Washington, said in an interview.

Amoroso and his colleagues collected data from satellite-linked vessel monitoring systems, or VMSes, and logbooks from the past two to six years. They found that trawlers fished 14 percent of the ocean in the areas they studied, leaving 86 percent untouched by trawling. They published their work Oct. 3 in the journal Proceedings of the National Academy of Sciences.

The figures quoted in the study raised concerns for University of British Columbia marine biologist Amanda Vincent, who was not involved in the study.

“It strikes me as an extraordinary apology for a fishing method that is really devastating,” Vincent told Mongabay. “I would be very concerned if this became a basis for policy decisions.”

Trawling is often criticized for the damage it can do to the habitats and sea life that anchor ecosystems along the world’s coastlines. Bottom trawls and dredges tear up the ocean floor, critics say, and huge nets indiscriminately scoop up nearly anything in their path. At the same time, the United Nations Food and Agriculture Organization estimates that trawling provides almost a quarter of the seafood that feeds humans around the world.

One of Vincent’s concerns was that the study only covered the portion of the ocean over the continental shelf from which the researchers could obtain reliable VMS data, but it ignored large parts of the world where scientists know that trawlers are likely having a big impact.

“The paper over-interprets quite substantially the work that’s actually happened here in that this covers at best 18 percent of the world’s … continental shelf to 1,000 meters,” Vincent said, “and a very selective 18 percent at that.”

For instance, she said, in the Africa region, the team analyzed fisheries data only from Namibia and South Africa. As with many of the other regions included in the research, such as Europe, Chile and Argentina, Alaska, and Australia and New Zealand, “These are all areas with pretty advanced fisheries management capacity,” Vincent said.

Amoroso and his colleagues acknowledge in the paper that their data weren’t comprehensive. They excluded parts of the world where VMS data covering 70 percent or more of the catch either wasn’t available or the countries didn’t provide it. Those areas, including the eastern coast of Canada and the U.S., Southeast Asia, and the Atlantic Ocean off the coasts of France and Spain, probably have a lot of trawlers in operation, they write.

But Amoroso also said the level of detail from the VMS data was critical in accurately assessing trawling’s impact. Their analysis identified the paths of trawlers inside 1-square-kilometer (0.4-square-mile) blocks of oceans, while in previous studies, grids were as large as 3 square kilometers (1.2 square miles).

“What we clearly see is that resolution is very important,” Amoroso said, “because you can end up with an overestimation of the footprint just because you [choose] a very low-resolution to map the impact of trawling.”

The high resolution that the team used allowed them to pluck nuance from the data, including the “diversity” of trawling footprints around the world, Amoroso said. Around Chile, the footprint covered as little as 0.4 percent of the continental shelf the team studied. In contrast, the trawling footprint was as high as 80 percent in the Adriatic Sea, part of the Mediterranean.

When Amoroso and his colleagues looked at locations with well-run, relatively sustainable fisheries, they found that the footprint of trawling was often lower, meaning the impacts were less widespread.

“You can achieve extra benefits by managing these species on the ecosystem,” Amoroso said.

Vincent said the use of VMS data could boost our understanding of the effects of trawling.

“Using technology, using new approaches is to be applauded,” she said. “The trick always in science is to limit your interpretation [and] limit your representation of your work to a level that can actually be sustained by the work itself.”

She said that focusing narrowly just on the tracks that trawling follows doesn’t account for the technique’s broader reach. Those impacts, on the food web, on the flow of nutrients, and on the absorption of carbon, Vincent said, can ripple through the ecosystem, and they can extend beyond the two- to six-year time frame of this study.

“Imagine a forest with a spider web of bulldozer tracks,” Vincent said. “Would you feel that it was OK just to analyze the bulldozer tracks?

“The answer obviously,” she added, “is no.”

Amoroso defended the study, saying, “If you want to have a full assessment of what is happening in the benthic [seabed] ecosystem due to trawling, you really need to have these high-resolution maps.”

But he agreed that more work was needed.

“The footprint doesn’t mean much,” Amoroso said. “You need to put this information together with benthic maps of habitat. You need to address the question of how much is this gear impacting the species that live in the community and how likely are these species to recover if I stop the fishing activity.”

Banner image of an otter trawling vessel fishing by Jan Hiddink/Bangor University.

Citation

Amoroso, R. O., Pitcher, C. R., Rijnsdorp, A. D., McConnaughey, R. A., Parma, A. M., Suuronen, P., … Jennings, S. (2018). Bottom trawl fishing footprints on the world’s continental shelves. Proceedings of the National Academy of Sciences.

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