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Bottom Trawling Harms More Than Just Our Oceans

By: Julia Slabe


Bycatch and emissions are critical environmental concerns, representing unintended consequences of human activities that pose significant threats to marine ecosystems and contribute to the broader challenges of climate change. Bottom trawling is a fishing technique involving the towing of a net along the ocean floor to capture target species like groundfish and crabs. This method utilizes floats on the head rope and weights on the footrope to keep the net open as it moves through the water. Different types of sweeps, such as chain sweeps for smooth surfaces and rockhoppers for rocky bottoms, are attached to the net to collect marine animals. The design of the trawl gear varies based on the target species and the type of bottom surface.


Bycatch, the unintentional capture of non-target species, has been a major concern for a long time. Sea turtles, which often rest and forage on the ocean floor, face significant risks from bottom trawling and are prone to becoming bycatch. Capture in a trawl can lead to drowning, injuries from the weight of the catch, and stress. To address this, turtle excluder devices (TEDs) have been introduced to certain trawl fisheries, greatly reducing sea turtle mortality. TEDs are currently only mandatory in trawl fisheries targeting shrimp and summer flounder.


Marine mammals can become entangled in trawl gear while swimming to forage or migrate. Species foraging near the sea floor, such as pilot whales and common dolphins in the Atlantic, are particularly susceptible to being caught in bottom trawls. Some voluntary mitigation measures to reduce marine mammal bycatch include reducing the number of turns per tow at night, shortening the duration of each tow, and promoting frequent radio communications between captains to raise awareness of nearby animals.

Turtle excluder devices have significantly reduced sea turtle mortality in trawl fisheries, and they may also aid in allowing small cetaceans to escape. Voluntary measures for reducing marine mammal bycatch include modifying towing practices and enhancing communication between fishing vessels to minimize the impact on non-target species.


In addition to bycatch, bottom trawling has also contributed massively to greenhouse gas emissions. A groundbreaking study recently published in the journal Frontiers in Marine Science reveals that bottom trawling is accountable for releasing up to 370 million metric tons of carbon dioxide annually, highlighting the destructive impact of this practice on both marine ecosystems and the climate. This number has been found by combining past carbon storage potential research, fishing data tracking trawling locations, and ocean circulation models. Modeling future scenarios, the study considered the impact of stopping trawling by 2030 and 2070. Between 55-60% of emissions are released into the atmosphere within an average of nine years, while the rest contributes to ocean acidification. This yearly fraction is nearly double the annual emissions from fuel combustion for the entire global fishing fleet. The study highlights the environmental consequences of bottom trawling, emphasizing its devastating effects on sensitive ecosystems and its unexpected "neighbor" effect, where CO2 emitted in one area may impact another. The researchers advocate for improved fishing regulations to mitigate these emissions and protect marine environments.


The recent findings on the environmental impact of bottom trawling have sparked discussions about taking assertive action. Critics, including experts like Camilo Mora, a data analyst at the University of Hawaii who has studied the impacts of climate change on marine and terrestrial ecosystems, argue that the practice can not be justified considering the limited food it provides compared to its significant ecological and moral repercussions. Mora emphasizes the need for informed consumer choices, stating that individuals should be aware of the consequences of consuming certain types of fish. While the European Union mandates labeling of unprocessed fish with the type of fishing gear used, compliance varies. In the U.S., where bottom trawling is prohibited in over half of federal waters, there are no specific regulations on labeling. Some argue that putting the responsibility on consumers may be a tactic to deflect industry accountability. However, others, like Enric Sala, a marine ecologist, National Geographic Explorer-in-Residence and one of the authors of the new study, advocate for a ban on bottom trawling in marine protected areas as a more effective measure to address its severe ecological and climate impacts. Sala describes bottom trawling as the most damaging way to obtain food from the ocean, highlighting its disastrous ecological consequences and its newly identified global warming impact.





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