Data from Green Balkans’ acoustic monitoring reveal a persistent presence of harbour porpoises and varying dolphin activity in four areas along the Bulgarian Black Sea coast. What happens beneath the surface of the Black Sea when we cannot see it? This is the question our team is exploring through acoustic monitoring of marine mammals in Bulgarian waters.
Since 2020, the association has been involved in the BlackCeTrends project, which uses specialised underwater acoustic devices known as F-PODs, developed by the British company Chelonia. The devices record sounds produced by marine mammals, making it possible to monitor their presence and activity in the marine environment. Four such devices are currently deployed in Bulgarian Black Sea waters, in the areas off Kavarna, Galata, Ravda and Sozopol. They use hydrophones - underwater “microphones” that detect the acoustic signals of marine mammals passing nearby.
A Strong Presence of Harbour Porpoises
The data collected during the first four months of 2026 show a particularly strong presence of harbour porpoises in the monitored areas. The harbour porpoise is one of the three cetacean species inhabiting the Black Sea. Unlike surface-based observations, where an animal must be visually detected in the field, acoustic monitoring makes it possible to record its presence through its characteristic high-frequency acoustic signals, without requiring observers to be physically present or relying on favourable weather conditions.
The data indicate that harbour porpoises actively use the monitored marine areas, with acoustic devices detecting their presence throughout the different months. The picture is more variable for dolphins as in some months they are completely absent. Recorded activity fluctuated between months, with the lowest values recorded in February, while April stood out with the highest recorded activity during the period analysed.
It is important to note that the technology used can identify harbour porpoises based on their characteristic acoustic signals, but the two dolphin species occurring in the Black Sea cannot be distinguished from one another using F-PODs. For this reason, their data are presented collectively as “dolphin” detections.
The Underwater World We Can Hear
The sounds produced by marine mammals are one of the most fascinating tools for studying life beneath the water’s surface. Harbour porpoises produce extremely high-frequency signals that are beyond the range of human hearing. This means that even when an animal is active in the monitored area, we would not be able to hear its vocalisations without specialised equipment.
To allow people to experience this otherwise hidden acoustic world, our team has prepared an audio file featuring recorded harbour porpoise sounds. The original high-frequency signals have been technically modified to bring them into a frequency range that can be perceived by the human ear. In this way, the recording does more than simply present the results of scientific monitoring – it allows the wider public to literally hear one of the inhabitants of the Black Sea.
The Advantages of Acoustic Monitoring
Monitoring marine mammals is traditionally associated with visual observations from boats or from the shore. This approach is valuable, but it has its limitations. Animals need to surface and come within the observer’s field of view, while the effectiveness of observations depends to a large extent on weather conditions, sea state and visibility.
Acoustic monitoring offers a different approach. F-POD devices can detect the acoustic activity of marine mammals continuously and automatically, without requiring people to be present at the monitoring site. This allows monitoring to continue regardless of the time of day and under conditions in which visual observations would be difficult or impossible.
This approach generates large volumes of data that can be analysed over time and can help identify patterns in the presence and activity of marine mammals. Combining acoustic monitoring with other observation methods provides an opportunity for a more comprehensive understanding of how marine mammals use Bulgarian Black Sea waters.
Protecting What We Cannot See
The sea can often appear empty when viewed from the shore. Beneath its surface, however, there is a dynamic world that remains largely invisible to us. That is why collecting systematic data on the presence of marine mammals is important not only from a scientific perspective. Understanding where and when animals use particular areas provides an important basis for assessing the state of the marine ecosystem and taking appropriate conservation measures.
The BlackCeTrends project contributes to building knowledge about marine mammals in the Black Sea and to developing a better understanding of their presence and behaviour in Bulgarian waters. And sometimes, to understand what is happening in the sea, we do not need to look more closely. We simply need to listen.
The Green Balkans team would like to express its special thanks to the “Friends of the Sea” Association – Burgas for their assistance in deploying and servicing the acoustic devices, as well as to Aqua Food Ltd., Morska Zvezda 76 Ltd., Black Sea Shells Ltd. and Petroceltic Bulgaria for providing platforms for the deployment of the devices.
This Project is possible thanks to support by Chelonia Ltd. and OceanCare.
Since 2020, the association has been involved in the BlackCeTrends project, which uses specialised underwater acoustic devices known as F-PODs, developed by the British company Chelonia. The devices record sounds produced by marine mammals, making it possible to monitor their presence and activity in the marine environment. Four such devices are currently deployed in Bulgarian Black Sea waters, in the areas off Kavarna, Galata, Ravda and Sozopol. They use hydrophones - underwater “microphones” that detect the acoustic signals of marine mammals passing nearby.
A Strong Presence of Harbour Porpoises
The data collected during the first four months of 2026 show a particularly strong presence of harbour porpoises in the monitored areas. The harbour porpoise is one of the three cetacean species inhabiting the Black Sea. Unlike surface-based observations, where an animal must be visually detected in the field, acoustic monitoring makes it possible to record its presence through its characteristic high-frequency acoustic signals, without requiring observers to be physically present or relying on favourable weather conditions.
The data indicate that harbour porpoises actively use the monitored marine areas, with acoustic devices detecting their presence throughout the different months. The picture is more variable for dolphins as in some months they are completely absent. Recorded activity fluctuated between months, with the lowest values recorded in February, while April stood out with the highest recorded activity during the period analysed.
It is important to note that the technology used can identify harbour porpoises based on their characteristic acoustic signals, but the two dolphin species occurring in the Black Sea cannot be distinguished from one another using F-PODs. For this reason, their data are presented collectively as “dolphin” detections.
The Underwater World We Can Hear
The sounds produced by marine mammals are one of the most fascinating tools for studying life beneath the water’s surface. Harbour porpoises produce extremely high-frequency signals that are beyond the range of human hearing. This means that even when an animal is active in the monitored area, we would not be able to hear its vocalisations without specialised equipment.
To allow people to experience this otherwise hidden acoustic world, our team has prepared an audio file featuring recorded harbour porpoise sounds. The original high-frequency signals have been technically modified to bring them into a frequency range that can be perceived by the human ear. In this way, the recording does more than simply present the results of scientific monitoring – it allows the wider public to literally hear one of the inhabitants of the Black Sea.
The Advantages of Acoustic Monitoring
Monitoring marine mammals is traditionally associated with visual observations from boats or from the shore. This approach is valuable, but it has its limitations. Animals need to surface and come within the observer’s field of view, while the effectiveness of observations depends to a large extent on weather conditions, sea state and visibility.
Acoustic monitoring offers a different approach. F-POD devices can detect the acoustic activity of marine mammals continuously and automatically, without requiring people to be present at the monitoring site. This allows monitoring to continue regardless of the time of day and under conditions in which visual observations would be difficult or impossible.
This approach generates large volumes of data that can be analysed over time and can help identify patterns in the presence and activity of marine mammals. Combining acoustic monitoring with other observation methods provides an opportunity for a more comprehensive understanding of how marine mammals use Bulgarian Black Sea waters.
Protecting What We Cannot See
The sea can often appear empty when viewed from the shore. Beneath its surface, however, there is a dynamic world that remains largely invisible to us. That is why collecting systematic data on the presence of marine mammals is important not only from a scientific perspective. Understanding where and when animals use particular areas provides an important basis for assessing the state of the marine ecosystem and taking appropriate conservation measures.
The BlackCeTrends project contributes to building knowledge about marine mammals in the Black Sea and to developing a better understanding of their presence and behaviour in Bulgarian waters. And sometimes, to understand what is happening in the sea, we do not need to look more closely. We simply need to listen.
The Green Balkans team would like to express its special thanks to the “Friends of the Sea” Association – Burgas for their assistance in deploying and servicing the acoustic devices, as well as to Aqua Food Ltd., Morska Zvezda 76 Ltd., Black Sea Shells Ltd. and Petroceltic Bulgaria for providing platforms for the deployment of the devices.
This Project is possible thanks to support by Chelonia Ltd. and OceanCare.
