Understanding Bottom Trawling and Its Widespread Use in European Waters

Bottom trawling, a common industrial fishing method, involves dragging large, heavy nets along the ocean floor to capture ground-dwelling species like cod, haddock, and flatfish. These nets, often equipped with metal doors, chains, and rollers, can span widths equivalent to several football fields, exerting immense pressure on delicate seabed habitats. In Europe, encompassing the European Union, United Kingdom, Norway, Iceland, and nearby territories, nearly 5,000 vessels engage in this practice annually, logging over 5.5 million fishing hours across vast expanses of continental shelf. This activity targets demersal fish stocks but inevitably scoops up vast quantities of non-target marine life, known as bycatch, and disrupts sediment layers that store centuries of carbon.

The technique dates back centuries but industrialized in the 20th century with mechanized vessels and synthetic nets. Today, it accounts for a significant portion of Europe's whitefish landings, contributing around 2% of the continent's animal protein supply. However, its footprint extends far beyond catches, overlapping with more than half of Europe's marine protected areas (MPAs), where conservation goals clash with fishing pressures.

The Landmark Study Revealing Hidden Economic Burdens

A groundbreaking peer-reviewed study titled "The Value of Bottom Trawling in Europe," led by marine researcher Katherine D. Millage of National Geographic Pristine Seas and supervised by explorer-in-residence Enric Sala, quantifies the full societal toll for the first time. Published in Ocean & Coastal Management following a March 2025 preprint, it analyzes data from 2016-2021 on nearly 4,900 European-flagged otter trawlers, beam trawlers, and dredgers. The research employs Global Fishing Watch vessel tracking and Sea Around Us catch reconstructions to map effort at high resolution, revealing a net societal loss ranging from €0.33 billion to €10.77 billion annually—up to 90 times the industry's €180 million in private profits.

This comprehensive net value calculation (U = profits + employment value + protein supply - discards - subsidies - carbon costs) shifts the narrative from short-term industry gains to long-term public losses, primarily driven by climate externalities. Sala emphasizes, "Bottom trawling in European waters is not just an environmental disaster, it's an economic failure."

Dissecting the Costs: Subsidies, Discards, and Climate Damage

European governments funnel approximately €1.17-1.34 billion yearly in fuel subsidies to bottom trawlers, rendering fleets in nations like Belgium, Spain, Portugal, and Romania unprofitable without public support. Discards exacerbate waste: up to 75% of catch—valued at €220-230 million—is tossed overboard, including juvenile fish, sharks, rays, and invertebrates, undermining future stocks and food security.

The elephant in the room is carbon emissions. Fuel combustion alone rivals aviation levels globally, but sediment resuspension from gear penetration (up to 20 cm deep) releases stored blue carbon. The study estimates 112 million tonnes of CO2 equivalent annually from European trawlers, monetized via social cost of carbon (SCC) at €43-161 per tonne, yielding €3.57-13.31 billion in damages from sea-level rise, health impacts, and productivity losses. For context, this rivals the emissions of entire small countries.

Seafloor Carbon Dynamics: How Trawling Unlocks Ancient Reserves

Seabeds act as vast carbon sinks, burying organic matter over millennia. Trawling mixes sediments, exposing labile carbon to oxygen and microbes, accelerating mineralization to CO2. The model factors swept volume (gear width × penetration × effort), carbon stock density (Atwood et al.), depletion, labile fraction, degradation rate, and ocean-atmosphere transfer (60%). Sensitivity tests confirm robustness, though high-end SCC drives the €16 billion upper bound.

Read the full study preprint for detailed equations and data sources.

Intrusion into Marine Protected Areas: A Policy Paradox

Europe boasts thousands of MPAs covering 12-20% of waters, yet bottom trawling persists in over 50%, with 12.7-23% of effort occurring there. In the EU, up to 20.3%; the UK caught 1.3 million tonnes in MPAs (2020-2024), 250,000 by trawls. This undermines biodiversity goals, as trawling scars habitats like reefs and seagrass beds essential for fish nurseries. Only 0.07% of European seas ban trawling outright.

Industry Perspective: Profits, Jobs, and Protein Supply

To the private sector, bottom trawling yields €180 million to €0.81 billion in net profits after revenues (€3-4 billion), fuel/labor costs, plus subsidies. It supplies protein valued via human-consumable fractions and substitutable prices, employing under 20,000 directly—far fewer than small-scale fisheries' three-fold job creation. Yet, overcapacity and declining stocks erode viability without aid.

Criticisms of the Study: Debates Over Carbon Accounting

Not all agree. Fishing advocates, like The Skipper editor Niall Duffy, lambast the carbon model as inflated 100-1,000x, recycling flawed degradation rates from Sala et al. (2021), rebutted by Hiddink et al. (2023) in Nature for ignoring natural remineralization and stable buried carbon. Critics argue it neglects effort displacement (trawling provides 20M tonnes globally), innovations (excluders, low-impact gear), and regional variations (e.g., Alaska's efficient fleets). Without carbon costs, some fleets break even. For balance, read the critique.

Success Stories: Where Bans Are Paying Off

Evidence from closures inspires. UK's Lyme Bay MPA ban boosted reef species 95% and juvenile lobsters 400%. Türkiye's Gökova Bay, trawler-free since 2010, revived 73% of fish species, monk seal breeding, and fisher incomes 300% via community rangers. Scotland's Bally Philp switched to creel pots, innovating whale-safe ropes. These show ecosystem recovery enhances fisheries long-term. See Euronews on Gökova.

Pathways Forward: Policy Reforms and Just Transitions

The study advocates phasing trawling from MPAs (EU target 2030), reducing effort 34% for max net benefits, and redirecting subsidies to buyouts, retraining (~20,000 jobs), and sustainable gear. Integrate sediment carbon into EU ETS for credits. Greece/Sweden pledged MPA bans; expand no-trawl zones, enforce via VMS/AIS. Small-scale fisheries offer more jobs, lower emissions.

Challenges: protein displacement risks land-based alternatives' higher footprints; ensure no effort leakage.

Broader Implications for European Marine Research and Policy

This work underscores urgency for holistic valuations in fisheries management, blending ecology, economics, climate science. It fuels EU Common Fisheries Policy reviews, MPA effectiveness audits. Researchers call for refined carbon models, displacement studies. For academics, it highlights interdisciplinary modeling's role in policy—GFW data democratizes analysis. Explore Inside Climate News analysis.

Ultimately, transitioning from bottom trawling could yield € billions in savings, healthier oceans, resilient fisheries—balancing economy and environment.

Photo by Immo Wegmann on Unsplash