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The Devastating Impact of Ocean Floor Trawling

Updated: Jul 30, 2021

Annie Grey reports on the negative impact of the fishing industry on our oceans.

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Photo by David Clode

At a time when the world faces an unprecedented biodiversity crisis, methods of fishing that cause damage to such slow-growing ecosystems can no longer be afforded. Scientists have concluded that a third of the world’s oceans must be protected by 2030 if there is any chance of saving them from collapse. Currently, only 2.7% of the ocean is under some kind of protection.

As coastal and open-water fisheries have become ever more depleted and overexploited, industrial fishing fleets have turned to deep-sea species, with many using highly destructive bottom trawling techniques in their fishing practices. To capture a handful of ‘target’ species, deep-sea bottom trawl fishing vessels drag huge nets armed with steel plates and heavy rollers across the seabed, annihilating sea life in their path.

A large portion of deep-sea catches are taken in the North Atlantic where vessels will typically target a range of fish species such as ling (Molva dypterygia), Greenland halibut (Reinhardtius hippoglossoides), round nose grenadier (Coryphaenoides rupestris), black scabbardfish (Aphanopus carbo), a few species of sharks and more recently exploited species such as Baird’s slick head (Alepocephalus bairdii) and deep-sea red crab (Chaceon affinis).

Fleets worldwide are now fishing on seamounts, deep-sea canyons and the rough seafloor – areas that were once avoided for fear of damaging nets. They plough through biologically rich and diverse seamount ecosystems, crushing corals, sponges, marine life and habitats as they go. Many species of unwanted fish are caught as ‘bycatch’ and thrown back dead into the ocean.

In a matter of weeks, bottom trawl fishing can destroy what took nature many thousands of years to create.

The End of Carbon Sinks

Alongside being critical for biodiversity, bottom trawling produces a gigaton of carbon every year, as reported in Nature. Marine sediments are the largest pool of carbon storage in the world and ocean phytoplankton absorb about 30% of man-made carbon dioxide emissions, but by releasing more CO2 underwater, the amount the ocean can absorb from the atmosphere is drastically reduced.

For example, when carbon is released from the seabed sediment into the water, it can increase ocean acidification, which may well dissolve the calcium carbonate that makes up mussel shells and coral skeleton, as well as interrupt processes like fish breathing. This is alongside adversely affecting productivity and biodiversity.

A group of scientists and economists are pushing for bottom-trawling fishing emissions to be added to nations’ greenhouse gas inventories. Although these underwater emissions cannot be translated directly to atmospheric emissions, Nature study co-author Trisha Atwood said a “substantial” amount ends up in the atmosphere. The researchers aim to estimate this amount by the end of 2021, making it possible for bottom-trawling emissions to be included in countries’ greenhouse gas inventories.

For some coastal countries, counting this released carbon would drastically alter their emissions figures. For example, the study estimates that Croatia’s bottom-trawling CO2 emissions stand at 23 million tons-a-year; if this was attributed to Croatia’s official greenhouse gas inventory, it would more than double. China is also thought to be responsible for 769 million tonnes of underwater CO2 a year, and the EU is estimated to have released 274 million tonnes.

Tip of the Iceberg

Earlier this year, it was announced that two of the UK’s most sensitive fishing sites were set to receive better protection, with the Marine Management Organisation (MMO) reporting that it plans to safeguard fishing areas in Dogger Bank and South Dorset by completely banning bottom-trawling. The sites are already designated as protected areas, but in reality, they are not patrolled and are both over-fished.

“Many of these species have declined so far that they are on the endangered species list. But, so far, marine conservation efforts in the UK have completely neglected doing anything to bring about their recovery.” – Professor Callum Roberts of Exeter University

Dogger Bank is the largest sandbank in UK waters, and underpins the North Sea’s ecosystem. It provides a vital habitat for a wide range of species which live on and within the seabed, including flatfish, starfish, sand eels, crabs, clams, worms, scallops and more. These species in turn provide a vital food source for predators such as porpoises, dolphins and seabirds. Other ‘protected sites’ off the coast of Land’s End and Lincolnshire are under government consideration to move from ‘full’ to ‘partial’ bottom trawling practices.

As Chris Thorne from Greenpeace UK explains:

“Action in these four sites is only the tip of the iceberg in terms of the scale needed to solve the crisis facing our oceans.”

Protecting the Oceans

In the blueprint provided by Nature, outlines are given to which areas of the ocean should be protected to safeguard marine life, boost seafood production and reduce greenhouse emissions.

Scientists argue that by identifying strategic areas for stewardship – for example, regions with large-scale industrial fishing and major economic exclusions zones or marine territories – nations could reap “significant benefits” for climate, food, and biodiversity. They predict that protecting “strategic” ocean areas could produce 8 million tons of seafood.

These protected areas should be where bottom trawling is common and vast amounts of carbon are present. Some examples include the South China Sea, Europe’s North Sea, the US Aleutian Islands and the West and South-West Coast of Africa, where trawling is carried out by local, European and Chinese-owned fishing fleets.

If we want to prevent more marine species from going extinct or joining the growing IUCN red list, we must take decisive action to restrict activities that are known to destroy and disturb vital ecosystems that support life on Earth.


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