Tracking the impact of bottom trawling on benthic habitat status

Bottom trawling is the most widespread physical disturbance to marine benthic habitats, yet broadscale assessments of its impact remain limited. We developed a quantitative framework to evaluate trawling effects on benthic habitats at broad spatial scales based on the sentinels of seabed (SoS) indicator. We applied this approach across five broad benthic habitat types in the Bay of Biscay and the Gulf of Cádiz, two regions that are subject to substantial trawling pressure. Using data from over 2000 monitoring locations, we assessed habitat sensitivity to trawling through generalized additive models (GAMs) and the SoS indicator, which quantifies benthic habitat quality based on the proportion of sentinel species (i.e., species typical of the habitat and sensitive to the pressure). We also sought to establish scientifically grounded quality thresholds to assess bottom-trawling impacts by applying three complementary approaches: distance to degradation, natural variation, and detectable change. We conducted a temporal analysis to evaluate changes in trawling impacts over time. Impact assessments varied depending on the method used to determine thresholds, although the natural variation and distance to degradation approaches yielded consistent results. Despite the methodological differences, all approaches identified offshore circalittoral mud and offshore circalittoral mixed sediments as the most affected habitats. The Gulf of Cádiz had more overall impacts than northern Spain. Impacts decreased significantly across all benthic habitats in association with declining trawling intensity, although extensive areas remained at risk. Our approach provides a scalable and robust way to assess seabed condition at broad spatial scales. By establishing critical benchmarks for habitat quality and pressure, it offers a scientifically sound tool to inform national and regional marine management strategies. The approach supports the implementation of environmental policies, such as the European Union Marine Strategy Framework Directive, and contributes to achieving good environmental status.