Global impacts of trawling quantified in new study

Trawling accounts for 20 percent of global fish landings, provides food for millions of people and is among the fishing methods most criticized by conservationists.

A new study that puts numbers to the impact of trawling, however, finds that some types of trawls cause significantly more damage to the seabed than others. Additionally, the most common type of trawl – otter trawling – has a lower environmental impact.

Fishermen use bottom trawls to catch fish, crustaceans and bivalves that live near seabeds. Trawls kick up sediment, flatten the seafloor and, in places that have them, damage coral and other structures.

The study – a global meta-analysis of 70 studies on bottom trawling – estimates depletion and recovery rates for the seabed post-trawl.

“The net result is that places that are trawled once a year or less will still have very high benthic biomass,” Ray Hilborn, a professor at the University of Washington and a co-author of the study, told SeafoodSource.

The study finds a close relationship between the depth of trawl penetration and the depletion of marine life. Otter trawls cause the least damage, removing only six percent of the biomass per pass and digging into the seabed only 2.4 centimeters. Hydraulic dredges, by contrast, cause the most damage, removing 41 percent of the biomass and penetrating the seabed 16.1 centimeters.

It takes the seabed 1.9 to 6.4 years to recover from a trawl, depending on the type of seabed and the trawl gear used.

But the study’s depletion and recovery figures measure only total biomass – not the diversity or type of organisms. Different organisms are impacted differently, and organisms with shorter life cycles recover quicker.

“Of the most concern to many are corals and sponges, which are both more vulnerable to trawl gear – a higher proportion killed per pass – and have much slower recovery times,” Hilborn said. “However, these make up a very small fraction of biota in most trawled areas, as they are found mostly on rocky substrate and almost all trawling takes place on mud, sand, and gravel.”

Gib Brogan, fishery campaign manager at the conservation group Oceana, doubts that seabeds with 100-year-old corals can recover from trawling so quickly, if at all, especially in areas with repeated heavy fishing. Structure-forming marine organisms and animals that live above the sea floor can be impacted by trawling no matter how shallow the trawl penetration, he said.

“The cumulative effects of intensive fishing by multiple boats in the same area will likely magnify the effect and prolong the recovery time,” Brogan told SeafoodSource. “There are undoubtedly places where trawls and dredges aren’t appropriate, such as coral and sponge habitats.”

Fishermen are exploring some alternatives to bottom trawling, Brogan said, such as bottom longlines, mechanical jigging and fish traps for bottom-dwelling species. These could be good alternatives. 

“None of them are perfect, but they offer options to limit the damage from trawling and let fished areas recover,” Brogan said.

Bottom trawling is the most common human-caused physical disturbance to the world’s seabed. Those environmental impacts are crucially important to seafood sustainability certification and rating organizations, such as the Marine Stewardship Council (MSC).

MSC does not pre-judge sustainability based on different fishing gears, and examines the actual impact of a gear on a certain fishery, not the assumed impact, according to David Agnew, science and standards director at the Marine Stewardship Council. MSC has certified multiple types of bottom-trawl fisheries  – from otter trawls to beam trawls.

“Bottom trawling does cause some impact, but it can be sustainable if affected habitats are not compromised to the point that they can’t recover, and if very sensitive habitats such as Vulnerable Marine Ecosystems are avoided,” Agnew told SeafoodSource.

About half the fisheries that are either certified or going through assessment with MSC are bottom trawls, Agnew said.

Additionally, a recent tabulation by MSC of efforts by fisheries to reduce the impact on habitat found that the majority of actions to taken to improve habitat status, management and information were taken by bottom-contact fisheries, such as trawls, dredges, and longlines, Agnew said.

Though the new study should offer policymakers and regulators some perspective on trawl impacts, more information is on the way.

Hilborn said that he’s collaborating on an upcoming study of the global geographic footprint of trawling – a study that he said will give a reality check to policymakers who think that trawling destroys ecosystems.

“Trawling at some levels is compatible with benthic ecosystems,” he said.