Bottom-trawling techniques leave different traces on the seabed

July 18, 2017 by Hannah Hickey
Trawlers use nets that are pulled through the water or along the bottom to capture fish. Credit: MaxPixel

Fishing fleets around the world rely on nets towed along the bottom to capture fish. Roughly one-fifth of the fish eaten globally are caught by this method, known as bottom trawling, which has been criticized for its effects on the marine environment.

An international group has taken a close look at how different types of bottom trawling affect the seabed. It finds that all trawling is not created equal—the most benign type removes 6 percent of the animal and plant life on the seabed each time the net passes, while the most other methods remove closer to a third. A University of Washington professor is among the main authors on the study, led by Bangor University in the U.K. and published July 17 in the Proceedings of the National Academy of Sciences.

The meta-analysis looks at 70 previous studies of bottom trawling, most in the Eastern U.S. and Western Europe. It looks across those studies to compare the effects on the seabed of four techniques: otter trawling, a common method that uses two "doors" towed vertically in the water or along the bottom to hold the net open; beam trawls, which hold the net open with a heavy metal beam; towed dredges, which drag a flat or toothed metal bar directly along the seafloor; and hydraulic dredges, which use water to loosen the seabed and collect animals that live in the sediment.

"We found that otter trawls penetrated the seabed 2.4 cm (0.94 inches) on average and caused the least amount of depletion of marine organisms, removing 6 percent of biota per trawl pass on the seabed," first author Jan Geert Hiddink at Bangor University said in a statement. "In contrast, we found that hydraulic dredges penetrated the seabed 16.1 cm (6.3 inches) on average and caused the greatest depletion, removing 41 percent of the biota per fishing pass."

Depending on the type of fishing gear, penetration depth and environmental variables such as water depth and sediment composition, it took from 1.9 to 6.4 years for the biota, or marine plants and animals, to recover.

"These findings fill an essential science gap that will inform policy and management strategies for sustainable fishing practices by enabling us to evaluate the trade-off between fish production for food, and the environmental cost of different harvesting techniques," said Ray Hilborn, a UW fisheries professor and one of four co-authors who designed the study.

"There's a common perception that you trawl the bottom and the ecosystem is destroyed," Hilborn said. "This study shows that the most common kind of trawling, otter trawling, does not destroy the marine ecosystem, and places that are trawled once a year really won't be very different from places that are not trawled at all."

But the study doesn't let otter trawling completely off the hook.

"We need to view these results in light of the footprint of each of these activities," Hilborn added. "While otter trawling has the least impact per trawl pass, it is the most widely used of all the bottom types and hence its effects are more widespread than are those of more specialized fishing gears, such as hydraulic dredges."

The study is one part of a larger effort to catalogue the effects of different types of bottom trawling worldwide, known as the Trawling Best Practices Project, which Hilborn leads with co-authors Michel Kaiser of Bangor University and Simon Jennings of the International Council for the Exploration of the Seas in Denmark. The group is doing other work to estimate how much bottom trawling takes place globally and thus determine the overall effect of bottom disturbance on the seafloor ecosystem. A previously published paper looked at how changes to the seafloor ecosystem affect the populations of fish that people are trying to catch.

Ultimately, the team aims to publish a set of fishing-industry "best practices" for the methods, equipment, density and frequency of bottom trawling.

The authors were unsurprised to find that otter trawling techniques are less destructive than hydraulic dredges. Similar findings came before, including a previous study led by Kaiser, but that one looked at a smaller number of trawling studies. The authors since developed a scrupulous protocol and cast a wide net for the studies included in the current meta-analysis.

"This one is therefore somewhat bulletproof to the criticism that you have been choosing the studies," Hilborn said. "Understanding how gear impacts the bottom, and species on the bottom, is important for a scientific understanding of the impacts of ."

Explore further: Trawling makes for skinny flatfish

More information: Jan Geert Hiddink et al. Global analysis of depletion and recovery of seabed biota after bottom trawling disturbance, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1618858114

Related Stories

Trawling makes for skinny flatfish

December 11, 2014

Trawling the seabed doesn't just remove some of the fishes living there; it also makes some of the survivors thinner and less healthy by forcing them to use more energy finding less nutritious food.

Trawling is changing seafloor habitats: study

September 5, 2012

Bottom trawling is dramatically altering the ocean floor and harming habitats, similar to the way that farming has permanently changed the landscape, a study said on Wednesday.

Hong Kong bans trawling to save fish stocks

May 20, 2011

Hong Kong has banned trawl fishing in its waters, a decision welcomed by conservationists Friday as a crucial move to save fish stocks and revive the city's depleted marine environment.

Recommended for you

World's smallest tape recorder is built from microbes

November 23, 2017

Through a few clever molecular hacks, researchers at Columbia University Medical Center have converted a natural bacterial immune system into a microscopic data recorder, laying the groundwork for a new class of technologies ...

A possible explanation for how germlines are rejuvenated

November 23, 2017

(Phys.org)—A pair of researchers affiliated with the University of California and Calico Life Sciences, has discovered a possible explanation regarding how human germlines are rejuvenated. In their paper published in the ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

irsd7
not rated yet Jul 19, 2017
All types of drag trawling must be GLOBALLY BANNED !!! Global marine ecosystems are systematically being obliterated beyond any recovery. Once again; the Corporate Machine is decimating our planet home to further private capital gains. This so-called fishing practice must cease; permanently ! Nature can only regenerate to a certain degree. We humankind are driving mass global extinction by our behavior and enterprise actions. We must leave a healthy, prolific; and bountiful living planet for all future generations of forthcoming life; NOT; a dying hazardous; toxic; and lifeless wasteland !

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.