Bottom Trawling and Seabed Carbon

What Bottom Trawling Does to the Seabed

Bottom trawling gear comes in several forms, all sharing the same fundamental principle: heavy gear is towed along the seabed to capture fish and invertebrates. Otter trawls use hydrodynamic boards (otter doors) to hold the net mouth open; beam trawls use a rigid metal beam; dredges use a toothed metal frame to scrape cockles, scallops, and other shellfish from the substrate. All of these gear types exert enormous mechanical force on benthic habitats. A single otter trawl sweep can crush, bury, or displace the biogenic structures — coral, sponge, tube worm colonies — that make up three-dimensional reef habitats and provide nursery and refuge function for many commercially important species. Bottom trawl gear does not distinguish between target species and the habitat complexity upon which those species depend.

Amoroso et al. (2018) conducted the most comprehensive spatial analysis of bottom trawling footprints yet undertaken, using high-resolution Vessel Monitoring System (VMS) data from 24 continental shelf and slope regions across five continents [1]. The results were striking: the proportion of seabed trawled varied more than 200-fold among regions, from 0.4% to 80.7% of the area surveyed to 1,000 metres depth. In the most intensively trawled regions — parts of the North Sea, the Barents Sea, and portions of the Australian shelf — the majority of the seabed area is trawled at least once per year, and some areas are trawled dozens of times annually. At this frequency, there is simply no opportunity for benthic communities to recover between disturbance events.

Benthic Recovery Times: The Hiddink et al. (2017) Meta-Analysis

How long does it take for seabed communities to recover after a trawl pass? Hiddink et al. (2017) addressed this question with the most rigorous global meta-analysis yet conducted, fitting models to empirical data from trawling disturbance studies worldwide [2]. Their headline finding: median community recovery times after a single trawl event were 1.9 years for mobile benthic megafauna and 2.9 years for sessile (attached) species. These figures might appear reassuring — suggesting that benthic communities can recover within a few years — but the picture is more complicated in practice.

Recovery times of 1.9–2.9 years assume that trawling does not recur before the community has recovered. In intensively trawled areas trawled multiple times per year, communities are maintained in a perpetually disturbed early-successional state dominated by opportunistic, small-bodied, fast-reproducing species. The complex, three-dimensional biogenic habitats formed by cold-water corals (*Lophelia pertusa*, *Eunice aphroditois*), deep-sea sponge grounds, and seagrass beds that characterise undisturbed continental shelf and slope environments may take decades to centuries to recover — timescales that are incompatible with any realistic cessation of trawling pressure in areas that have been commercially fished for generations [2]. Furthermore, Hiddink et al. noted substantial uncertainty in recovery estimates for the most vulnerable habitat types, precisely because there are few study sites where trawling has actually ceased long enough to observe full recovery.

Trawling and Carbon: The Sala et al. (2021) Nature Paper

The most consequential recent development in the scientific discussion of bottom trawling is the recognition that disturbed seafloor sediments release stored organic carbon into the water column, with potential implications for atmospheric CO₂ concentrations. Sala et al. (2021) embedded this insight into a landmark *Nature* paper examining the co-benefits of ocean protection for biodiversity, food security, and climate [3]. The paper estimated that trawl-induced sediment disturbance releases carbon at a rate comparable to global aviation — potentially 0.58–1.47 Gt CO₂ equivalent per year — representing a climate impact that had been almost entirely ignored in both fisheries management and climate policy.

The mechanism operates as follows. Seafloor sediments — particularly in continental shelf environments — contain large quantities of particulate organic carbon (POC) derived from the settling of dead plankton, marine snow, and other organic material from surface waters. Under undisturbed conditions, this material is buried in sediment layers where it is sequestered from the atmosphere, potentially for centuries or millennia. Bottom trawl gear mechanically resuspends this sediment, exposing buried organic carbon to oxygenated bottom water where it can be remineralised by bacteria, releasing CO₂ into the water column. A fraction of this dissolved CO₂ eventually reaches the atmosphere, where it contributes to anthropogenic warming. The permanence and magnitude of this pathway is debated among marine biogeochemists — Hiddink et al. (2023) challenged some of the more extreme estimates in a *Nature* Comment — but the directional effect is not in scientific dispute.

The Global Footprint of Trawling: Spatial Scale

Kroodsma et al. (2018) tracked the global fishing fleet via AIS transponder signals and found that trawling and dredging account for a very large share of the total spatial footprint of industrial fishing [5]. The continental shelf — the biologically productive area of seabed from the coast to about 200 metres depth — covers approximately 26 million square kilometres globally. Estimates from Amoroso et al. (2018) and other sources suggest that trawling disturbs roughly 14–15 million square kilometres of seabed per year — more than the combined land area under annual crop production globally [1]. This comparison underscores the scale of the physical impact: no comparable area of terrestrial habitat is subjected to annual mechanical disturbance at this intensity.

Depth matters enormously in assessing trawling impacts. Shallow shelf environments fished continuously for decades are often described as being in a 'trawling equilibrium' — degraded relative to their pre-fishing baseline but no longer declining rapidly because the most vulnerable species and structures have already been removed. It is in deep-water environments — beyond 200 metres, on seamounts, slopes, and deep-sea coral reefs — that the most catastrophic single-pass impacts occur, because these habitats have never evolved under any historical fishing pressure and their communities are dominated by slow-growing, fragile species utterly unprepared for mechanical disturbance.

What Is Being Destroyed: Cold-Water Corals and Sponge Grounds

The cold-water coral (*Lophelia pertusa* and related species) reef systems found along the margins of the northeast Atlantic, Norwegian fjords, and deep seamounts worldwide represent some of the most biodiverse benthic habitats on the planet outside of tropical coral reefs. These structures take hundreds to thousands of years to develop and host hundreds of associated species including commercial fish that use them as nursery habitat. A single trawl pass can destroy centuries of coral framework in minutes. Surveys of cold-water coral reefs in the northeast Atlantic have documented extensive destruction corresponding to known historical trawl tracks, with some reefs showing no signs of recovery decades after protection was implemented.

Deep-sea sponge grounds present similar concerns. Glass sponges and demosponges in boreal and polar shelf environments form extensive biological structures that filter vast quantities of seawater, cycle silicon, and provide habitat for numerous associated species. Sponges can live for centuries; their destruction by trawling removes centuries of biological capital in an instant. The OSPAR Commission (protecting the northeast Atlantic) and the CCAMLR (governing Antarctic waters) have established closures protecting key coral and sponge habitats, but the coverage of effective closures is small relative to the known extent of these habitats.

Protected Areas and the Sala et al. Case for Reform

Sala et al. (2021) made the explicit case that protecting areas of ocean from bottom trawling produces triple dividends: biodiversity recovery, increased fisheries yields (through spillover from protected areas to adjacent fished zones), and carbon protection [3]. Their modelling found that protecting approximately 28% of the ocean, selected to maximise overlap of biodiversity value, carbon storage, and food production potential, could achieve these triple benefits simultaneously. Crucially, the areas identified as delivering the greatest carbon protection were overwhelmingly continental shelf environments — precisely the areas most heavily trawled — rather than the abyssal plain, which stores more total carbon but where fishing pressure is lower.

The policy implications are significant. Several nations have moved toward trawling bans in marine protected areas — the UK announced a ban on bottom trawling in all 40 of its 'highly protected marine areas' in 2023; the EU is debating restrictions on trawling within Natura 2000 marine protected areas. However, the fishing industry in many countries remains a powerful political constituency, and trawling bans face sustained opposition from commercial fishing interests who argue that spatial closures impose disproportionate economic costs on fishing communities dependent on demersal species. The scientific evidence for the environmental harm of intensive bottom trawling is, at this point, overwhelming; the challenge is the political translation of that evidence into binding management action at the scale the crisis demands.

Alternatives and Gear Innovation

Not all demersal fishing is equally destructive. Trap and pot fishing for lobster, crab, and some fin fish causes minimal seabed disturbance and can be highly selective. Demersal longlining for groundfish is less spatially extensive in its physical impact than trawling, though it creates bycatch issues. Handline and jig fishing for demersal species are inherently low-impact but limited in the scale of catch they can deliver economically. Some trawl gear modifications — including roller gear that allows nets to pass over rough substrate, reducing the need to tow through the most biologically complex areas — have reduced habitat damage relative to older designs, though the improvement is relative rather than absolute. The fundamental challenge is that bottom trawling's efficiency advantage over alternative methods is so large that transitioning fishing effort away from it requires either strong regulation or a dramatic shift in the economics of seafood markets — perhaps through ecolabelling schemes that price seabed-destructive fishing at its true environmental cost.

"Protecting the ocean from fishing and other human pressures can help restore marine biodiversity and provide additional benefits — including higher long-term fisheries yields and carbon sequestration." — Sala et al., Nature, 2021 [3]