Seafloor scars expose trawling inside a protected Baltic zone

In a strip of protected seabed between Germany and Denmark called the Fehmarn Belt, bottom trawling was banned at the end of 2024. A year on, researchers at the Leibniz Institute for Baltic Sea Research Warnemünde report that the proportion of the reserve's seafloor scarred by trawl marks did not fall. It rose, more than doubling in a single year — evidence, the institute says, that trawlers kept working inside a zone where the practice is supposed to be off-limits.

The finding comes from two companion studies describing a new way to measure trawling's footprint on the Baltic seabed: not by tracking where fishing vessels travel, but by reading the furrows their gear leaves behind. The approach, the researchers argue, catches activity that satellite-based ship tracking misses entirely.

Furrows instead of flags

Bottom trawlers tow weighted "otter boards" that hold a net open as it drags along the sea floor, cutting shallow trenches into soft sediment. Regulators have mostly relied on the Automatic Identification System, or AIS, the shipboard transponder that broadcasts a vessel's position, to estimate where and how much trawling occurs. But AIS has gaps: smaller boats are not always required to carry it, signals can be patchy near the coast, and a transponder reveals nothing about how hard a net actually struck the seabed.

The IOW team instead mapped the seabed itself. One study, led by researcher Mischa Schönke and published in the journal Ecological Informatics, used annual high-resolution multibeam echo-sounder surveys of the Fehmarn Belt between 2020 and 2024 to build what the authors call a "Trawling Index," calculated from the total volume of detectable furrows across the surveyed seabed. A second study, led by geophysicist Peter Feldens and appearing in the journal Anthropocene, trained a neural network to spot trawl marks in digital depth maps covering roughly 1,069 square kilometers across four Baltic areas, using mostly public survey data collected between 2016 and 2025.

A fourfold swing, then a reserve breached

The furrow data showed how quickly trawling pressure can change, and how little of that change AIS registered. In the Fehmarn Belt, total furrow volume across roughly 3.4 square kilometers of seabed fell from about 5,067 cubic meters in 2021 to 1,108 cubic meters in 2022 — a drop of nearly 80 percent in a single year. The share of seabed with no detectable trawl marks climbed from 14 percent in 2021 to as much as 65 percent in later survey years. Vessel-tracking data, by contrast, showed only modest shifts in fishing activity over the same period, missing the scale of the change the seafloor itself recorded.

The more striking number concerns enforcement. Mobile bottom trawling was formally excluded from part of the Fehmarn Belt protected area starting in December 2024. Instead of healing, the share of that zone's seabed carrying visible trawl marks rose from 9 percent in 2024 to 22 percent in 2025, the researchers found — and AIS data from 2025 showed an uptick in fishing activity in the same area, consistent with vessels continuing to trawl there after the ban took effect.

Scars that persist across the wider Baltic

The regional survey, covering waters beyond the Fehmarn Belt, found trawl-mark evidence across more than 80 percent of grid squares examined in Mecklenburg Bay and over 90 percent in Kiel Bay, two heavily fished stretches of German coastal water. About 40 percent of the central Arkona Basin showed no detectable marks. In Mecklenburg Bay, the team also identified circular patches more than 500 meters wide, left untouched by trawling, that coincide with seafloor depressions linked to escaping gas or fluid — natural hazards that fishers likely steer around rather than respect as protected habitat.

"Both methods can help to identify hotspots of damage and better assess the effectiveness of conservation measures," Schönke and Feldens said of combining seabed mapping with vessel-tracking data.

Neither study claims the seabed has recovered biologically just because furrows fade. The researchers found that measures of benthic species diversity correlated with trawling intensity but cautioned that other factors, including sediment type and depth, also shape which invertebrates live where — meaning a smoothed-over seafloor is not necessarily a healthy one.

A fishery already under strain

The Western Baltic has been a difficult place to fish for years. The European Union closed the directed commercial fishery for western Baltic cod in 2022 after the stock collapsed, leaving boats that once targeted cod to redirect their effort toward species such as plaice, sole, and flounder. The IOW researchers say that shift likely explains some of the sharp year-to-year swings in trawling pressure their furrow data picked up in the Fehmarn Belt, changes that would otherwise look inexplicable.

That backdrop matters for who stands to be affected by better seabed monitoring. Commercial fishers already operating under tight quotas face a tool that can document, furrow by furrow, where their gear has been — a prospect fisheries associations are likely to view warily. Conservation groups and the institute itself frame the method as a check on whether marine protected areas, a cornerstone of the EU's Natura 2000 network in the Baltic, are delivering what their designation promises. German and Danish fisheries regulators, who oversee enforcement in the Fehmarn Belt, now have seabed evidence that is harder to dismiss than a ship's absence from a tracking map.

What comes next

The IOW team says the furrow-mapping method, because it relies increasingly on publicly available bathymetric survey data and automated detection rather than dedicated research cruises, could be extended to other Baltic protected areas where enforcement is difficult to verify. A summary of the findings notes that some trawl marks vanish quickly in sandy sediment and others persist for years, so the technique works unevenly across the Baltic's varied seabed, and the researchers stop short of calling their Fehmarn Belt findings definitive proof of illegal fishing in every surveyed square. Still, the combination of rising furrow coverage and accompanying AIS activity inside a zone meant to be off-limits is, in their telling, a strong indication that the ban is not being fully observed — a conclusion regulators will now need to act on, or explain.