Protecting the Baltic harbour porpoise: New technology helps WWF find harmful ghost nets
3 March 2025
A Remote Operated Vehicle (ROV) camera inspection was carried out aboard the Baltic Explorer to better understand the seabed habitat. This took place alongside sonar prospecting and ROV dives, enhancing the survey’s effectiveness. © P-dyk AB
The Baltic harbour porpoise is critically endangered, with only a few hundred animals remaining. One threat is ghost nets — fishing nets that have been lost or dumped at sea. Porpoises risk getting entangled in these rogue nets and drowning when they cannot reach the surface to breathe. WWF is working to retrieve as many ghost nets as possible from the Baltic Sea, but locating these nets is like looking for a needle in a haystack. Now, WWF is testing new technologies to make finding the nets more efficient.
New technology tested
Last year, WWF conducted an expedition to map ghost nets in a key area for Baltic harbour porpoises: Natura 2000 area Hoburgs bank and Midsjöbankarna, located south of Gotland, Sweden. Here, porpoises mate and raise their calves in summer months. The expedition utilised Side Scan Sonar, an advanced sonar technology that reads the seabed using sound waves. The sonar signals are then interpreted by a computer to generate detailed images of the underwater environment, revealing what is hiding in the depths.
“Identifying and removing ghost nets in critical breeding and calving habitats is important for the survival of the Baltic Proper harbour porpoise. These efforts help reduce the risk of entanglement, allowing the porpoises to raise their young more safely. This will hopefully support the recovery of this critically endangered population,” says Stina Nyström, Cetaceans Expert at WWF Sweden.
Harbour porpoise (Phocoena phocoena) © Alamy Stock Photo / Buiten-Beeld
The expedition tested the viability of using an Autonomous Underwater Vehicle (AUV) equipped with Side Scan Sonar. Unlike traditional sonar towed behind a boat, AUV sonar is self-driving, covering larger areas at lower costs, improving ghost net detection. While towed sonar is limited to one unit per boat, multiple AUVs can operate simultaneously, greatly increasing survey coverage and efficiency. This project marked the first use of one AUV for ghost gear searches, nearly doubling the planned scan area while highlighting technical challenges for future WWF studies.
Promising results
The new technology has shown promising results in identifying net locations. Although no ghost nets were found specifically within the selected important harbour porpoise areas, this may simply indicate that the problem is not as extensive there. However, the area covered during the expedition was very small.
During the return journey back from the target area, a wreck of a fishing boat was found with a large number of nets attached to it. This highlights the need to continue efforts to locate and remove ghost nets.
Ghost Gear Mapping project
The work took place under the ‘Ghost Gear Mapping in Swedish Waters’ project, run by WWF Sweden together with WWF Germany. WWF Sweden designed the project, while WWF Germany is responsible for the technical aspects of mapping with sonar and AUV, as well as processing the collected data. WWF is also collaborating with partners who are experts in diving and ghost nets, in this case primarily P-dyk, who specialises in salvaging ghost nets.
Ghost net – an old net left behind. © Shutterstock / Luca Vaime / WWF-Sweden
The Ghost Gear Mapping in Swedish Waters project focuses particularly on protected areas for the Baltic harbour porpoise, with the goal of reducing the amount of ghost nets in Swedish waters and thereby also reducing the risk of bycatch in ghost nets. The project also aims to raise awareness around the new technology and the sonar methodology being tested, with the help of the project app – GhostNetZero. The app allows divers to participate in the preparation of ghost net recovery by viewing, documenting and verifying whether reported objects really are ghost nets. WWF also hopes the project will lead to the development of methodology for mapping ghost nets with AUVs, enabling mapping and clearing of larger areas in a more cost-effective way.
Future ambitions
WWF ultimately aims to ensure that the most important habitats for the Baltic harbour porpoise are completely free of ghost nets. Additionally, they hope that the GhostNetZero app will be used in larger areas in the future. WWF Germany is able to launch their GhostNetZero.AI platform, which aims to detect potentially lost fishing gear using machine learning. This will make it possible to analyse pre-existing and new sonar data, collected not only within the project but also by authorities, offshore projects and other research institutes. This will significantly speed up the rate of data processing and expand the areas that can be analysed.
“By providing a platform where external sonar data can be uploaded and analyzed via AI, we will save a lot of resources. It helps us focus on priority areas where targeted action against ghost nets is needed,” says Gabriele Dederer, Project Manager of Derelict Fishing Gear International at WWF Germany.
Hundreds of tons of ghost nets have been retrieved from the Baltic Sea by WWF and other organisations in recent years. Collaboration in both knowledge and technology has proven to be essential for effective ghost gear removal programmes. In a joint effort between WWF France and WWF Germany, five sonar expeditions have been conducted at depths of up to 100 meters, and this March, the first ghost nets at these depths will be targeted for removal using a Remote Operated Vehicle.
This article was originally published in Swedish by WWF-Sweden, and then translated and adapted by WWF Baltic Sea Programme.
Baltic harbour porpoise facts:
The harbour porpoise (Phocoena phocoena) is one of the smallest whales in the world. It is about 1.5m long and weighs around 60kg.
The Baltic harbour porpoise is genetically and ecologically distinct from harbour porpoises in adjacent areas. The population is classified as Critically Endangered (CR), with only a few hundred individuals left. It faces an extremely high risk of extinction in the wild.
Bycatch in fishing gear remains the most significant threat to the harbour porpoise. Bycatch refers to the unintentional capture of marine life in fishing gear, both during active use and as a result of lost or abandoned nets (ghost fishing).
Underwater noise from shipping, construction, and other human activities is another significant threat, as it disrupts the communication, navigation, and foraging of the porpoises. Key prey species are also in decline, due to a combination of factors, including expansion of oxygen-depleted areas due to eutrophication, overexploitation of fish stocks, pollution and climate change.
