Bottom trawling provides around a quarter of the world's seafood, but it is controversial.
The heavy nets and dredges used to catch species like cod, plaice and Norway lobster also disturb the seabed and kill some of the invertebrates that live there. For example, a single pass of the widely used "otter trawl" kills about 6% of seabed animals, while a scallop dredge kills nearly 14%.
EU proposals to limit bottom trawling have sparked controversy between conservationists and shipping lines. Environmental NGOs have called for this practice to be banned, which they consider incompatible with the sustainable management of the seas, while the shipping companies concerned have argued that it is compatible with good management of the environment.
As always, the reality is more complex and the impact of bottom trawling strongly depends on the type of habitat exploited. Although bottom fishing is widespread and intense throughout Europe, even in the most heavily exploited seas, at least 20% of the seabed is unfished. A few years ago I was part of a team that assessed the impact of bottom trawling on continental shelves around the world. In 18 of the 24 regions we assessed, we found that more than two-thirds of the seabed area was unexploited. Most of the time, bottom trawling takes place on muddy, sandy and gravelly bottoms, where animals such as clams, worms and starfish live. the impact of bottom trawling on seabed habitats around the world, and we have ranked each seabed between 1 (totally untrawled) and 0 (totally depleted by trawling). We found that the status varies greatly from region to region, from 0.25 to 0.999, although the worst-off regions are all in Europe. The total bottom trawling area in UK seas was approximately 319,000 km², which is larger than the entire land area of the country.
Only 11% of the North Sea and 18% of the Irish Sea were unaffected, while 10% and 3% were identified as having a full depletion status of 0 . sailors is closely linked to the sustainability of fishing. Regions with depleted seabeds are places where fish stocks are generally overexploited and management regimes are ineffective, while seabeds are healthy where trawl fisheries are managed sustainably. Some bottom trawling also involves more sensitive habitats, such as shallow water oyster reefs and deep water sponge gardens. These vulnerable marine ecosystems have not yet been well mapped on a larger scale, and we do not yet know the impact of bottom trawling on them because few studies have been carried out (for the understandable reason that it is difficult to justify trawling such sensitive habitats for a scientific experiment). We know, however, that even the most resilient of these ecosystems cannot withstand trawling more than once every three years.
Compared to something like agriculture, on the surface it is clear that bottom trawling has a large footprint, but its impact in much of that footprint is nonetheless limited.
Protected areas don't always stop bottom trawling
Of course, this does not resolve the debate over how often bottom trawling is acceptable, or how best to reduce its impact. Marine Protected Areas (MPAs) are a widely used tool for biodiversity conservation, but in the EU and UK most MPAs are not designed to protect the seabed and still allow bottom trawling . The UK and EU are stepping up protection against bottom trawling using MPAs, but conservationists say the measures are not ambitious enough, while the fishing industry feels under pressure and prepares to fight back. If bottom trawling is banned, recovery can be surprisingly fast: just six years on average for sandy, muddy and rocky habitats. In contrast, more vulnerable ecosystems, such as oyster reefs or coral bottoms, take much longer to recover, and the most sensitive deep-sea reefs are unlikely to recover in our lifetime. If the objective of protecting the marine environment is to strike a balance between conservation and the production of seafood, the management of bottom trawl fisheries must give priority to reducing the fishing of overexploited stocks. This will preserve seabed habitats and maximize food production. We should also avoid fishing in the most vulnerable ecosystems. A total ban on bottom trawling would reduce the availability of seafood, as alternative methods of harvesting these fish, such as pots, traps and diving, operate at much smaller scales and mainly in coastal areas.
Jan Geert Hiddink, Professor of Marine Biology Bangor University
The conversation February 28, 2022
