Ocean trawling can release locked-in carbon

Some scientists say the controversial practice of ocean trawling stirs up buried organic matter, some of which rises to the surface as carbon dioxide.

Environmentalists have long criticized the practice of dragging giant nets across the seafloor to catch fish - a practice known as bottom trawling - as destructive to underwater ecosystems. Research shows it's also bad for the climate.

The study, published in the magazine Frontiers in Marine Science, found that bottom trawling could release up to 370 million tons of carbon dioxide into the atmosphere each year. The International Energy Agency estimates that this is equivalent to approximately half of the emissions produced annually by the entire international shipping industry.

This carbon comes from organic matter long buried at the bottom of the ocean. Without human intervention, these sediments can persist for thousands of years. However, trawling mixes them up and releases the accumulated carbon back into the water column. Some of this material returns to the sea floor and is compressed. However, some rises to the surface as carbon dioxide (a potent greenhouse gas) and diffuses into the air.

According to research, approximately 55 to 60% of CO2 Released into the water column by trawl nets, it ends up being released into the atmosphere. This typically occurs within seven to nine years, meaning the impacts of trawling are felt within a short period of time.

The study's authors say they were able to identify an often-overlooked source of emissions and suggest regulators have the ability to resolve the problem quickly. If policymakers "create a different strategy to manage where and how fishermen fish"said the study's lead author, Tricia Atwood, an aquatic ecologist at Utah State University, then."The impact of these policies on the climate would be almost immediate".


Other opinions on trawling emissions

But these findings may be controversial. Some scientists believe the article overestimates emissions from trawling.

"I am very skeptical about your estimates"said Jan Geert Hiddink, an ocean scientist at Bangor University in the United Kingdom who was not involved in the new study, in an email to E & E News.

He said the paper may have overestimated the total amount of CO2 emitted into the water column by trawl nets, meaning that the amount of CO2 released into the atmosphere would be "overestimated by many orders of magnitude".

The debate has grown in recent years. The new paper builds on previous research published in 2021 that looked at the benefits of marine protected areas. The article was written by Enric Sala, a former academic who later started the Pistino Sea program of the National Geographic Society to protect the world's oceans. Sala and many of the authors of the original paper, including Atwood, also conducted the new study, which was funded in part by Pristine Seas.

A 2021 study found that restricting fishing and other human activities could protect biodiversity, encourage fishing, and protect marine carbon sinks. The 2021 document also estimates that trawling can release more than one billion tons of CO2 to the water column each year.

These are only rough estimates and do not take into account the actual amount of CO2 released into the atmosphere.
However, the article included a series of headlines comparing trawling's impact on the climate to that of other high-carbon industries, such as aviation.

While in the spotlight, he also faced criticism from Hiddink and other researchers, who said his estimates were exaggerated. In 2023, Hiddink and other scientists published a paper saying that a 2021 study had exaggerated the amount of CO2 released into the water by trawling.

When ocean floor sediments break down, bacteria and other marine life eat them and convert them into CO2. But Hiddink and his colleagues say the 2021 study overestimated how much long-trapped carbon actually turns into CO.2.

In contrast, Atwood and her co-authors issued a response arguing that Hiddink's criticisms are based on incorrect assumptions about the disturbed ocean sediments by trawl nets and the rate at which they are removed and converted into CO2. "Our response challenged their assumptions, which we believe are inaccurate and lack quantitative support."the authors said in a recent statement.

Atwood said the new document takes the 2021 results a step further. Estimate the proportion of CO2 that enters the atmosphere from the ocean, in addition to the total amount produced.

The study found that this proportion does not change in relation to the total amount of CO2 in water. No matter how much CO2 beech floating, between 55 and 60 percent always escapes into the air. That means that if future studies continue to reduce the absolute amount of CO2 released by trawling, they can still trust that ratio to be accurate, Atwood said.

Atwood said this is the first study of its kind to perform such an evaluation.

"In reality, it is just helping to provide policymakers with the information they need to make good decisions.", he claimed.


With information of: https://www.scientificamerican.com/