Marks left by fishing gear on the Baltic Sea floor can reveal the effects of bottom trawling more clearly than vessel tracking data alone, according to two studies from the Leibniz Institute for Baltic Sea Research Warnemünde (IOW). The researchers used high-resolution acoustic surveys and AI to chart trawl furrows, monitor how they change and assess fishing impacts across a wider area. The findings were published in Ecological Informatics and Anthropocene.
Bottom trawling drags nets across the seabed, while heavy devices that hold the nets open can carve deep furrows. The activity disturbs sediment and can damage habitats and organisms living on the seafloor. Monitoring based on Automatic Identification System (AIS) signals can miss some fishing activity, particularly from smaller vessels.
For a five-year study in the Fehmarn Belt, the team used a multibeam echo sounder to survey up to 4.7 square kilometres annually between 2020 and 2024. The instrument measures the seabed’s shape in detail, allowing researchers to calculate a Trawling Index based on the volume of visible furrows.
The recorded furrow volume fell from about 5,067 cubic metres in 2021 to 1,108 cubic metres in 2022, a drop of almost 80%. Over the same period, the proportion of surveyed seabed with no detectable trawl marks rose from 14% to as much as 65% in subsequent years. These changes were not apparent in the AIS data, the researchers reported.
Repeated surveys also showed that furrows can largely disappear within a year. However, sampling of seabed invertebrates—including mussels, worms and crustaceans—did not establish a consistent pattern of biological recovery. Species diversity was correlated with trawling intensity, but the researchers said other environmental factors also affect these communities.
In a second study, an AI-trained neural network identified trawl marks in digital depth maps covering about 1,069 square kilometres across the Mecklenburg Bay, Kiel Bay, Arkona Basin and Fehmarn Belt. The data, collected from 2016 to 2025, came mainly from surveys by Germany’s Federal Maritime and Hydrographic Agency.
Trawl evidence appeared in more than 80% of surveyed grid squares in Mecklenburg Bay and more than 90% in Kiel Bay. In the central Arkona Basin, nearly 40% of the surveyed area had no discernible marks. The researchers also identified large, circular mark-free areas in Mecklenburg Bay, some more than 500 metres across, which coincide with seabed depressions formed by rising gases or other fluids.
A comparison of surveys in the Fehmarn Belt Marine Protected Area found that the average share of the area covered by trawl marks increased from 9% in 2024 to 22% in 2025. Mobile bottom trawling has been banned there since December 2024, and AIS data also indicated increased fishing activity in 2025. Because furrows can remain visible for months and cannot always be dated precisely, the results point to possible illegal fishing after the ban but do not prove every individual incident.
The studies show how acoustic mapping and AI can help locate areas of seabed damage and assess conservation measures. The researchers caution that furrows disappear especially quickly in sandy areas, and that their disappearance does not necessarily mean the seabed’s ecological functions have recovered. They recommend combining the mapping methods with biological and geochemical measurements.
Sources: Phys.org.