If you dragged a heavy net weighted with steel chains across a forest, flattening every tree, pulling up every root, and scraping the topsoil down to bare earth, you’d call it environmental destruction. Nobody would debate whether it was damaging. It would be obvious.

Bottom trawling does essentially this to the seafloor. Heavy nets weighted with steel doors, chains, and rollers are dragged across the ocean bottom, scooping up everything in their path: target species, non-target species, reef structures, sponge gardens, sediment communities, and anything else that happens to be there.

It is the most widespread physically destructive human activity affecting the ocean floor. And despite decades of scientific evidence documenting its impacts, it remains one of the most commonly used commercial fishing methods globally.

How It Works

Bottom trawling uses a large cone-shaped net that’s pulled along the seafloor behind a fishing vessel. The mouth of the net is held open by two heavy metal “otter boards” or “trawl doors” that can weigh several tonnes each. These doors dig into the sediment as they’re dragged, ploughing furrows in the seafloor.

Between the doors, the bottom of the net (the “ground rope”) is weighted with chains, rollers, or rubber discs that bounce over or scrape along the bottom. The purpose is to disturb the sediment and flush out fish and crustaceans so they swim upward into the net.

The width of a typical trawl path is 25-50 metres. A single vessel can trawl tens of square kilometres per day. Commercial trawling fleets operating in the same area may pass over the same ground repeatedly throughout the fishing season.

What Happens to the Seafloor

The scientific evidence on trawling impacts is extensive and largely unambiguous.

A landmark 2024 meta-analysis published in PNAS reviewed over 200 studies on the effects of bottom trawling on seafloor habitats globally. The findings were consistent across regions and habitat types:

Physical structure is destroyed. Rocky reefs, sponge gardens, cold-water coral mounds, bryozoan beds, and other three-dimensional habitat structures are flattened or removed by trawling. These structures provide shelter, feeding sites, and nursery habitat for hundreds of species. Once destroyed, they can take decades to centuries to recover - if they recover at all.

Sediment communities are disrupted. The organisms living in and on soft sediment - worms, bivalves, crustaceans, small fish - are killed or displaced by each trawl pass. Studies consistently show 20-50% reductions in benthic (bottom-dwelling) organism abundance after a single trawl pass, with cumulative impacts from repeated trawling creating permanently degraded communities.

Sediment is resuspended. Trawling stirs up fine sediment that clouds the water, reducing light penetration and affecting filter-feeding organisms. The resuspended sediment can travel kilometres from the trawl site, impacting habitats that weren’t directly trawled. In nutrient-rich sediments, resuspension can trigger localised algal blooms.

Carbon is released. This is a relatively recent finding that has attracted significant attention. Seafloor sediments store enormous quantities of organic carbon, accumulated over thousands of years. When trawling disturbs these sediments, stored carbon is exposed to oxygen and released as CO2. A 2021 study in Nature estimated that bottom trawling releases approximately 1 billion tonnes of CO2 annually - comparable to the aviation industry’s emissions.

The Australian Context

Australia is not immune to these impacts. Bottom trawling operates in Commonwealth-managed fisheries across the continental shelf and slope, as well as in several state-managed fisheries.

The most significant trawl fisheries in Australian waters include:

Northern Prawn Fishery - the largest Commonwealth trawl fishery, operating across northern Australia from Western Australia to Queensland. It targets banana prawns, tiger prawns, and endeavour prawns. The fishery has implemented significant management reforms since the 2000s, including area closures, seasonal restrictions, and bycatch reduction devices, but trawling still occurs across large areas of the northern seabed.

South East Trawl Fishery - operating across southern NSW, Victoria, and Tasmania, targeting various demersal fish species. This fishery operates in areas that include sensitive cold-water habitat types.

Queensland East Coast Trawl Fishery - a state-managed fishery targeting prawns along the Queensland coast, including areas adjacent to the Great Barrier Reef Marine Park.

Research by CSIRO and AIMS has documented trawling impacts on Australian seafloor habitats. Studies in the Great Barrier Reef inter-reef areas found that trawled areas had significantly lower diversity and biomass of benthic organisms compared to untrawled reference sites. Sponge gardens and soft coral communities, which provide important habitat for fish species, were particularly affected.

The Australian Fisheries Management Authority (AFMA) has implemented spatial closures that protect some sensitive habitats from trawling, and the system of marine parks provides varying degrees of protection. However, trawling continues in substantial areas, and the cumulative impacts are significant.

Recovery Times

One of the most concerning aspects of trawling damage is how long recovery takes.

Soft-sediment communities can show partial recovery within 1-5 years after trawling ceases, depending on the species involved and the frequency of prior disturbance. But “recovery” in scientific studies often means a return to some level of abundance, not necessarily a return to the original community composition.

Structured habitats - rocky reefs, sponge gardens, cold-water coral - are a different story entirely. Cold-water corals grow at rates measured in millimetres per year. A coral mound destroyed by a single trawl pass may take hundreds of years to regrow. Deep-sea sponges, some of which can live for centuries, are similarly slow to re-establish.

In some heavily trawled areas, the seafloor has been trawled so frequently for so many decades that the original habitat is essentially gone. The seafloor has been converted from a complex, structured environment to a flat, homogeneous plain dominated by fast-growing, disturbance-tolerant species. This is sometimes called a “trawled seascape” - a permanently altered state rather than a temporarily disturbed one.

Alternatives

Several fishing methods catch the same target species as bottom trawling with much less seafloor impact.

Mid-water trawling uses nets that don’t touch the bottom, targeting fish that swim in the water column. This avoids seafloor contact entirely but is only suitable for pelagic species that school mid-water.

Trap and pot fishing uses baited cages placed on the seafloor. The impact is localised to the pot’s footprint and is minimal compared to trawling. It’s widely used for crustaceans and is being trialled for some demersal fish species.

Hook and line methods - longline and handline - catch fish without seafloor contact. They’re more selective and produce higher-quality fish that commands better market prices.

Danish seining uses a lighter net than bottom trawling and relies on herding fish with ropes rather than scraping the bottom with heavy gear. Its seafloor impact is significantly lower than otter trawling.

The economic argument against switching methods is that trawling is efficient - it catches large volumes at relatively low cost per kilogram. But this calculation doesn’t account for the external costs: habitat destruction, bycatch, carbon release, and the long-term decline of fish populations that depend on the habitat trawling destroys.

Technology is helping make some of these alternatives more viable. Some enterprise AI groups are working on predictive modelling that helps fishing operators find fish concentrations more accurately, reducing the need for wide-area bottom trawling by targeting specific areas with more selective methods. Acoustic monitoring, satellite data, and oceanographic modelling can identify where target species are likely to aggregate, which makes methods like Danish seining or pot fishing more productive.

Where Things Stand

The global conversation about bottom trawling is shifting. In 2023, the EU considered (and ultimately weakened) a proposal to ban trawling in marine protected areas. Several countries, including Palau, Belize, and Hong Kong, have banned bottom trawling entirely. In 2024, the European Parliament passed measures restricting bottom trawling in some marine protected areas, though implementation has been slow.

In Australia, there’s ongoing tension between the fishing industry (which employs thousands of people and contributes billions to the economy) and conservation science (which consistently documents that trawling causes serious environmental harm). The current management approach - using spatial closures, gear modifications, and catch limits to mitigate impacts - reduces damage compared to unmanaged trawling but doesn’t eliminate it.

The seafloor is the least visible part of the ocean and the most physically abused. Most people will never see what the bottom of the continental shelf looks like, which makes it easy to ignore. But the evidence is clear: bottom trawling is causing persistent, large-scale damage to marine ecosystems, and the current management framework isn’t enough to prevent it.