Is sustainable seabed trawling possible? A look at the evidence
Bottom trawling provides around a quarter of the world’s seafood, but it is controversial. The heavy nets and dredges that are used to catch species like cod, plaice and langoustines also disturb the seabed and kill some of the invertebrates that live there. For example, a single pass with the widely used “otter trawl” kills about 6% of seabed animals, while a scallop dredge kills almost 14%.
EU proposals to curb bottom trawling have sparked a row between conservationists and industry groups. Environmental NGOs have called for a ban because they consider it incompatible with the sustainable management of the seas, while fishing industry groups have argued that the practice is compatible with good environmental management.
As always, the reality is more complex and the impacts of bottom trawling strongly depend on the type of habitat fished. Although bottom fishing is extensive and intense throughout Europe, even in the most exploited seas, at least 20% of the seabed is unexploited. A few years ago, I was part of a team assessing the impact of bottom trawling on the continental shelves of the world. In 18 of the 24 regions we assessed, we found that more than two-thirds of the seabed surface remained untrawled.
Most bottom trawling occurs on muddy, sandy and gravelly bottoms, with animals such as clams, worms and starfish. Colleagues and I recently collated all the existing evidence on the impact of trawling background on seabed habitats around the world and have rated each seabed between 1 (completely untrawled) and 0 (completely depleted by trawling). We found that the status varied greatly by region, from 0.25 to 0.999, although the worst-performing regions were all in Europe.
The total bottom trawled 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 state of 0 depletion.
The state of the seabed is closely linked to the sustainability of fishing. Regions with depleted seabeds are places where fish stocks are generally overfished and have ineffective management regimes, while seabeds are healthy where trawl fisheries are sustainably managed.
Some bottom trawling also occurs on more sensitive habitats, such as oyster reefs in shallow waters and sponge gardens in deep waters. These vulnerable marine ecosystems are not yet well mapped on a larger scale, and the impact of bottom trawling on them is not yet known because few studies have been conducted (for the understandable reason that it is difficult to justify trawling over 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, above ground it is clear that bottom trawling has a large footprint, but its impact in much of that footprint is nonetheless limited.
Protected areas do not always stop bottom trawling
Of course, this does not resolve the debate over how much 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 increasing protection against bottom trawling using MPAs, but conservationists do not see the plans as ambitious enough, while the fishing industry feels pressured and prepares to retaliate.
If bottom trawling is banned, recovery can be surprisingly quick – just six years on average for sandy, muddy and gravelly habitats. However, more vulnerable ecosystems, such as oyster reefs or coral gardens, take much longer to recover, and the most sensitive deep-sea reefs are unlikely to recover in our lifetimes.
If the goal of marine management is to strike a balance between conservation and seafood production, bottom trawl fisheries management should prioritize reducing fishing of overexploited stocks. This will benefit seabed habitats and maximize food production. We must also avoid fishing on the most vulnerable ecosystems. A total ban on bottom trawling would reduce the availability of seafood because alternative methods of harvesting these fish, such as pots, traps and diving, operate at much smaller scales and mainly in coastal areas.
Report highlights urgent need to end bottom trawling
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