The industrialisation of the seabed: Estimating century-scale impacts of bottom trawling on shelf sea sediments

Technical Summary: The Industrialisation of the Seabed

Problem Statement
Bottom trawling is the most widespread anthropogenic pressure on continental shelves globally, yet its historical footprint and long-term intensity remain poorly resolved. Systematic marine observation and data collection generally commenced only after bottom trawling had already become widespread, resulting in a "shifted baseline" where modern scientific understanding of seabed conditions is derived from an environment already heavily altered by human activity. Consequently, the magnitude and spatial extent of historical seabed disturbance, particularly regarding physical properties, biological communities, and biogeochemical functions like carbon sequestration, are largely unknown. This study addresses the gap in understanding the century-scale evolution of trawling impacts in the Northeast Atlantic, the birthplace of industrial fishing.

Methodology
The authors reconstructed over five centuries (1377–1925) of British and Irish trawling expansion using a multi-source historical approach.

• Data Compilation: The study integrated over 5,000 spatial records from diverse sources, including 19th and 20th-century government inquiries, Royal Commission testimonies, academic syntheses, grey literature, and digitised nautical charts.
• Fleet Reconstruction: Historical fleet capacity (vessel counts, tonnage, propulsion type) was synthesized for regional fleets (English, Welsh, Scottish, Irish) and distant-water fleets. Temporal gaps were addressed via linear interpolation.
• Spatial Reconstruction: Trawling grounds were georeferenced using modern admiralty charts and validated against bathymetric and habitat data (EMODnet). Pre-1900 grounds were derived from textual descriptions, while post-1900 grounds were digitised from charts. Rough ground areas were excluded for sail-era estimates.
• Intensity Quantification: The study calculated the Swept Area Ratio (SAR)—the ratio of total annual swept area to the total area of a fishing ground—to quantify trawling intensity. Effort allocation for specific years (1825, 1850, 1875, 1900, 1916, 1925) utilized proxies such as "ground importance" scores derived from testimonies and monthly Admiralty charts.
• Sediment Mobilisation Modeling: A probabilistic Monte Carlo framework was employed to estimate sediment resuspension and mixing.
  • Resuspension: Calculated using an empirically derived relationship linking hydrodynamic drag coefficients of specific gear types (beam trawls, otter trawls, dredges) with sediment silt fractions.
  • Mixing: Estimated based on gear penetration depth, sediment density, and porosity.
  • Uncertainty: Key parameters were sampled 1,000 times within ranges of ±5% or 95% confidence intervals to provide mean values with 25th and 75th percentiles.

Key Contributions
This study provides the first spatially resolved, multi-century reconstruction of the industrial expansion of bottom trawling in the Northeast Atlantic. It quantifies the transition from localised inshore activity to a continental-scale industrial footprint driven by fleet expansion and the shift from sail to steam power. Crucially, it establishes a quantitative baseline for sediment mobilisation decades before the advent of systematic data collection, challenging the assumption that modern baselines represent a pre-industrial state.

Results

• Spatial and Temporal Expansion: Trawling evolved from clustered activity in the Thames Estuary and English Channel (14th–18th centuries) to a widespread industrial operation covering the continental shelf by the early 20th century. By 1925, trawlers swept 309,540 km² of seabed annually.
• Intensity Trends: The total swept area increased 13-fold between 1825 and 1925. By 1875, substantial grounds were being swept more than five times annually. High-intensity zones (SAR > 5) were concentrated in sheltered inshore environments, with some sites experiencing cumulative pressures of over 2,000 sweeps between 1825 and 1900.
• Sediment Mobilisation:
  • In 1825, total mobilisation was 1.13 Gt yr⁻¹. By 1925, this peaked at 11.75 Gt yr⁻¹.
  • Resuspension: Trawling surpassed natural sediment resuspension rates in the North Sea by approximately 1880. By 1925, trawling resuspended >1 Gt of sediment annually, a mass comparable to the annual sediment flux of the Amazon River (~1.2 Gt yr⁻¹).
  • Mixing: Sediment mixing remained the dominant component of mobilisation, reaching 10.74 Gt yr⁻¹ by 1925.
  • Cumulative Impact: Over the century from 1825 to 1925, fleets cumulatively mobilised an estimated 602 Gt of sediment (33 Gt resuspension; 569 Gt mixing).
• Technological Drivers: The transition from sail to steam propulsion (late 19th century) was a critical inflection point, enabling the use of heavier gear, higher towing speeds, and access to deeper, silt-rich grounds, which significantly increased sediment resuspension rates.

Significance and Claims
The paper claims that the intensification of bottom trawling fundamentally altered the ecological and biogeochemical state of the Northeast Atlantic shelves well before the earliest large-scale benthic surveys of the early 20th century.

• Redefining Baselines: The study argues that "baseline" conditions described by modern science are not natural or pre-industrial but are instead the product of multi-century industrial activity. The physical and ecological baselines for nearshore environments were likely fundamentally altered more than a century ago.
• Ecosystem Implications: The chronic disturbance likely eliminated disturbance-sensitive taxa (e.g., oysters, mussels, tube worms) and biogenic structures, leading to cascading effects on sediment stability, bioturbation depth, and organic matter retention.
• Carbon Storage: The authors suggest that the most significant release of sedimentary carbon as CO₂ likely occurred during the initial industrial expansion (post-1880) when reactive carbon fractions were mobilised. Contemporary observations may therefore capture a depleted legacy pool rather than current rates of loss.
• Policy Relevance: Acknowledging the timing and magnitude of this transformation necessitates a redefinition of applied baseline conditions in marine management. Failure to account for the scale of historical seabed disturbance risks underestimating long-term ecosystem degradation and the potential for recovery.