Seabed Sentinels: Trawling Leaves Detectable Fingerprints in North Sea Protected Areas

The seabed of the North Sea is one of the most heavily trawled seafloors on Earth, and a new baseline study has now captured, with unusual precision, exactly what that means for the animals living beneath the sand. Researchers from the German Marine Research Alliance examined three marine protected areas in the German Exclusive Economic Zone — Borkum Reef Ground, Sylt Outer Reef, and Dogger Bank — and found that mobile bottom-contact fishing leaves measurable fingerprints on the infauna communities even after decades of sustained disturbance. The findings, published in Nature Conservation, arrive at a pivotal moment: the European Union has banned mobile bottom-contact fishing in parts of Borkum Reef Ground and Sylt Outer Reef since March 2023, and in parts of Dogger Bank since November 2025, creating a rare natural experiment in seabed recovery.

The three protected areas belong to the European NATURA 2000 network and were designated to achieve Good Environmental Status under the Marine Strategy Framework Directive. Yet assessments have shown that these sites consistently fail to meet that status, largely because of the sheer volume of human activity taking place within them. Mobile bottom-contact fishing — gears such as beam trawls that penetrate the sediment to depths of two centimetres or more — is among the most widespread of these pressures. More than 60 percent of the southern North Sea seafloor has been trawled at least once per year, and although fishing effort has generally declined since 2000, it remains concentrated in the shallower eastern and south-eastern parts of the basin.

To disentangle the effects of fishing from those of natural environmental drivers, the team sampled along gradients of decreasing fishing intensity within each protected area. They collected infauna samples with a 0.1 square metre van Veen grab at stations spaced along transects, sieving the sediment over one millimetre mesh and identifying every retained organism to the lowest possible taxonomic level. In parallel, they analysed sediment composition — mud, sand, gravel, and shell contents — and recorded water depth at each station. Fishing intensity data came from the International Council for the Exploration of the Sea, derived from vessel monitoring system and logbook records at a resolution of one square nautical mile, a scale fine enough to resolve the small focus areas.

The three sites proved strikingly different in their physical character. Borkum Reef Ground, the smallest at 625 square kilometres, sits roughly 40 kilometres from the East Frisian coast and features a median sand content of 98 percent, with small amounts of gravel found nowhere else in the study. Dogger Bank, a shallow sandbank in the central North Sea where northern and southern North Sea species meet, also showed 98 percent median sand but with increasing mud content at greater depths. Sylt Outer Reef, the largest site at 5,603 square kilometres, was the most heterogeneous, with median sand content of just 78 percent, the highest mud content at around 17 percent, and the richest shell content at 6 percent. Median depths ranged from about 30 metres at Borkum Reef Ground to 43 metres at the other two sites.

Fishing intensity varied dramatically across the three areas. Borkum Reef Ground recorded the lowest median intensity, with the sediment trawled roughly once per period in the subsurface layer. Dogger Bank recorded the highest, at a median value of 16, meaning parts of the bank were penetrated 16 times over the aggregated period. Sylt Outer Reef showed intermediate values with clear gradients, increasing from west to east in the eastern German Bight and from north to south along the western transects. These gradients were the analytical backbone of the study, allowing the researchers to test whether communities changed predictably as fishing pressure declined.

In total, the team identified 215 infauna taxa across the three focus areas. Only about a quarter of these — 25.58 percent — occurred in all three areas, exemplified by the bivalve Abra alba, the polychaete Lagis koreni, and the amphipod Tryphosa nana. Nearly half — 47.91 percent — were found in just one area: the amphipod Harpinia antennaria appeared only at Sylt Outer Reef, the bivalve Spisula elliptica only at Borkum Reef Ground, and the polychaete Clymenura lankesteri only at Dogger Bank. Multivariate analyses confirmed that the communities of the three sites were significantly different, separating at a 35 percent similarity level with an ANOSIM statistic of 0.98.

Within each site, the analyses revealed two significantly distinct communities. At Borkum Reef Ground, one community was characterised by high abundances of the lancelet Branchiostoma lanceolatum and the polychaete Lanice conchilega, while the other was dominated by the polychaete Spiophanes bombyx and juvenile echinoids. At Dogger Bank, one community featured the brittlestar Amphiura filiformis alongside Spiophanes bombyx and Bathyporeia amphipods, while the other was marked by the small echinoid Echinocyamus pusillus. At Sylt Outer Reef, one community was defined by the bivalves Nucula nitidosa and Thyasira flexuosa, the other by the bivalve Varicorbula gibba and the gastropod Turritellinella tricarinata. Notably, the researchers also recorded high abundances of the non-native polychaete Loimia ramzega at Borkum Reef Ground, a novel observation for this region.

The statistical disentangling of drivers produced the study’s most consequential result. Distance-based linear models showed that sediment composition — sand, mud, gravel, and shell contents — correlated significantly with community structure at all three sites, but with strikingly different explanatory power. At Borkum Reef Ground, cumulative sediment fractions accounted for 88.1 percent of community variation, reflecting the site’s strong physical heterogeneity. At Dogger Bank the figure was 34.68 percent, and at Sylt Outer Reef just 20.38 percent. Fishing intensity, by contrast, explained 9 percent of variation at Borkum Reef Ground — where it was not statistically significant — but 16 percent at Dogger Bank and 12.7 percent at Sylt Outer Reef, both highly significant. Distance-based redundancy analysis confirmed that stations with high fishing intensity plotted close to the fishing vector, which correlated strongly with the first ordination axis at both Dogger Bank and Sylt Outer Reef.

Why would fishing effects emerge clearly at two sites but not at the third? The researchers point to the interplay between natural and anthropogenic disturbance. Benthic communities exposed to high natural disturbance from currents, waves, and storms develop species adapted to that variability, and their response to trawling becomes smaller or undetectable. Borkum Reef Ground, with its dynamic reef and sandbank structures, may fall into this category. The team also noted the prevalence of opportunistic, small-bodied, r-strategic species in the communities at Dogger Bank and Sylt Outer Reef — a classic signature of continuous disturbance that favours short-lived organisms over longer-living ones. Decades of trawling have already shifted North Sea species composition in this direction, exemplified by the historical loss of extensive bivalve patches of Spisula subtruncata and Mactra stultorum from the north-eastern and central Dogger Bank.

The bans now in force transform this baseline into a reference point for recovery. Parts of Sylt Outer Reef and Borkum Reef Ground have been closed to commercial and scientific mobile bottom-contact fishing since March 2023, and parts of Dogger Bank since November 2025. Recovery has already been documented for infauna communities in a protected area in the Kattegat and for epifauna communities at Sylt Outer Reef, suggesting the German sites may follow suit. The German Federal Agency for Nature Conservation will carry out long-term monitoring over years to decades, indispensable for tracking change and assessing responses across human activities. The researchers caution that shipping, pelagic fishing, and pollution will continue within the protected areas and may hinder conservation measures. Still, with the baseline now established, scientists can finally measure whether the seabed communities of the German North Sea — after decades of trawling — can rebuild toward the Good Environmental Status their designation promised.

Subject of Research: Marine Conservation

Article Title: Effects of natural environmental drivers and different mobile bottom-contact fishing intensity on infauna communities in NATURA 2000 sites in the German Exclusive Economic Zone of the North Sea

Article References: Ohde, R., & Kröncke, I. (2026). Effects of natural environmental drivers and different mobile bottom-contact fishing intensity on infauna communities in NATURA 2000 sites in the German Exclusive Economic Zone of the North Sea. Nature Conservation, 66, 1-17. https://doi.org/10.3897/natureconservation.66.193151

Image Credits: AI Generated

DOI: 10.3897/natureconservation.66.193151

Keywords: marine biology, North Sea, bottom trawling, marine protected areas, NATURA 2000, benthic ecology, conservation, Effects, natural, environmental, drivers, different