Spotting ghost nets from above - Airborne imaging to detect ALD fishing nets in shallow water in Lillebælt, Denmark

Ghost nets - abandoned, lost or discarded (ALD) fishing nets – account for a significant anthropogenic pressure on coastal geomorphic systems, negatively affecting benthic habitats and marine life. This study investigates the potential for efficient detection of ghost nets in shallow water coastal environments using high-resolution orthophotos in Lillebælt, Denmark from 2021 to 2025 (Andersen et al., 2026).

The primary method applied was visual interpretation of annual orthophotos generated by the Danish Agency for Climate Data based on aerial photos recorded in spring and autumn from 2021 to 2025 with a spatial resolution of approximately 10–12 cm. Morphometric analyses on airborne lidar bathymetry (ALB) data were integrated to support the identification of ghost nets. Ground-truth data for validation consisted of remotely operated vehicle (ROV) video footage and removal of nets in selected areas.

A total of 73 ghost nets were identified and assigned a confidence level ranging from one (uncertain) to three (certain). Approximately 30% of the detected nets were classified with high confidence (level three), while 46% were assigned confidence level two. In total, 64 of the 73 appointed nets were verified through ROV observations and net removal campaigns (North Nature, 2026). Further analyses revealed that ghost nets affect local vegetation coverage as well as local seabed erosion and deposition.

The results demonstrate that high-resolution orthophotos provide a cost-effective and time-efficient tool for detecting ghost nets in shallow coastal waters. A relatively high temporal resolution of imagery enhances detection capabilities and may support targeted mitigation efforts, contributing to reducing negative impacts on coastal ecosystems.

This project highlights the potential of integrating airborne remote sensing data (orthophotos and bathymetry) and ROV validation data to assess human-induced pressures on coastal environments at local to regional scale.

Acknowledgements

The project was funded by the Danish Environmental Protection Agency. The verification and removal of ghost nets were carried out by Nature North for the Danish Environmental Protection Agency in November 2025.

References

Andersen MS, Larsen IR, Nielsen NLJ, Hansen LØ, Larsen M, Andersen SB, Rödel L-G, Al-Hamdani Z & Ernstsen VB (2026). Near coastal seabed mapping in Lillebælt 2024-2025 (in Danish). GEUS Report for the Danish Environmental Protection Agency.

North Nature (2025). Removal of ghost nets in Lillebælt (in Danish). North Nature Report for the Danish Environmental Protection Agency.