Tagging harbour porpoises to gain insight into their habitat use and behaviour

Credits: Annemieke Podt – Delta Porpoise.

Where in the North Sea are harbour porpoises primarily found? Do they utilise the entire North Sea? How far do they swim? Which locations do they prefer? Does the behaviour of individual harbour porpoises vary? How do harbour porpoises use human structures in the sea, such as wind farms? By tagging harbour porpoises with satellite tags, also known as telemetry, researchers can find answers to these and many other questions.
As the North Sea becomes more intensively used by humans, the importance of such knowledge development grows. That is why the government has launched a study on this. The research is being conducted by Wageningen Marine Research (WMR) and TNO on behalf of the Ministry of Agriculture, Fisheries, Food Security and Nature and the Department of Waterways and the Offshore Wind Ecological Programme (Wozep) of Rijkswaterstaat (on behalf of the Ministry of Climate and Green Growth).

To answer all outstanding questions about the harbour porpoise's habitat, long-term research is needed. This project serves as the foundation for potential follow-up studies. In the initial phase, WMR and TNO are examining technical aspects such as which tags to use and how to attach them to the animals, where to tag them, and in which seasons. If these aspects are clear and meet the guidelines of the Animal Experiments Committee, the next phase can begin in 2024: actually tagging harbour porpoises. If tagging proves to be successful and provides the desired information, the last phase can commence: a larger-scale tagging programme.

The project is structured in such a way that decisions are made during each phase about whether it is worthwhile to proceed to the next phase. For example, it is possible that tagging harbour porpoises in the Netherlands may not be feasible at this time. If that's the case, the project will be terminated.

Wageningen Marine Research and TNO are working closely with a specialised veterinarian and researchers from Aarhus University-Department of Ecoscience, who have extensive experience in tagging harbour porpoises in Danish waters (see figure 1).

9 October 2025: Another porpoise tagged in the Wadden Sea
During the third tagging operation in September 2025, the project team again tagged a harbour porpoise in the Wadden Sea. An adult male animal was equipped with a satellite transmitter southwest of Schiermonnikoog. Preliminary data show that up till now the animal primarily resides in the eastern part of the Dutch Wadden Sea (figure 1). 

28 July 2025: Evaluation completed and tagged porpoise Eastern Scheldt resighted
The male harbour porpoise that was tagged in the Eastern Scheldt in May 2024 has been resighted multiple times there this spring/summer by Delta Bruinvis. Since August 2nd, 2024, we had not received any location data from this animal. Up until that point, the animal ranged throughout the entire Eastern Scheldt (figure 1). It was not resighted until April 2025. The transmitter has detached, and based on the photos, the remaining holes appear to have healed well (figure 2). The porpoise seems to be in good condition.

The evaluation of the porpoise incident on March 12th in the Wadden Sea has been completed. An evaluation report was prepared by the project team, incorporating input from two independent international experts (Evaluation Report). The management of Wageningen Marine Research and TNO subsequently conducted an assessment, and the Porpoise Advisory Committee issued an advice on behalf of the Ministry of Agriculture, Nature and Food Quality (LVVN) and RWS-Wozep.

Based on these documents and the discussions with the involved parties and the Harbour Porpoise Advisory Committee, the teams from Wageningen Marine Research and TNO, together with the commissioning authorities, have concluded that it is responsible to continue with the pilot project, provided that several adjustments are made during the fieldwork. These adjustments include, among others, a clearer division of responsibilities and stricter criteria for initiating a capture attempt. Additionally, quicker retrieval and closer monitoring of the net will be given higher priority.

The Harbour Porpoise Advisory Committee emphasizes the added value of this pilot project. Data from the porpoise tagged on the same day show that the animal is swimming from the Wadden Sea toward the Dogger Bank (Figure 3). This is valuable information.

17 March 2025: Second harbour porpoise tagging campaign Wadden Sea
During the second tagging campaign in March 2025, the project team tagged a harbour porpoise in the Wadden Sea to study the species’ behaviour and habitat. The animal was an adult female. A second animal that was caught, a smaller porpoise, was released without a tag according to protocol, because the animal showed signs of stress. A third (female) porpoise was also found in the net; this porpoise unfortunately drowned. The animal was transferred to Utrecht University's Faculty of Veterinary Medicine for further investigation.

This incident is being treated with utmost care by the project team. For this reason, it has been decided to stop the second tagging campaign immediately. A thorough evaluation will now follow to find out what happened. This will involve independent internal and external experts, including the Animal Experiments Committee (DEC) and the Animal Welfare Body of Wageningen Research. Only after the evaluation is completed, a decision will be made on how to proceed with this study. The researchers will continue to monitor the data from the tagged porpoise. 

30 July 2024: Results of post-mortem examination of porpoise found dead
The porpoise, which was found dead on 25 June 2024, was examined by pathologists of the Faculty of Veterinary Medicine at Utrecht University. As the animal was in an advanced state of decomposition, no cause of death could be determined. The post-mortem examination did reveal that the animal was probably in moderate to poor nutritional condition, at the time of death. 

The pathologists paid extra attention to the tag on the dorsal fin. The tag was still well attached to the fin. Microscopic examination of the tissue around the tag attachment holes showed no inflammatory reactions. However, interpretation was complicated by the state of decomposition of the porpoise. 

Furthermore, no link between the death of the porpoise and the transmitter could be demonstrated. The stored data on the tag gave no indication of a possible cause of death either. 

Parasites were present in the ears, stomach and lungs, which is not unusual for porpoises. Microscopic examination could not determine whether there was severe inflammation due to these parasites. No infections were found in other organs either. The stomach contents were examined at Wageningen Marine Research and consisted of 15 prey remains, equivalent to 16 grams of small fish and squid.

Additional information from Stichting Rugvin and Stichting Delta Bruinvis shows that the animal lived to be at least 11 years old; she was first seen in 2017. Because she had a calf at the time, it is likely that she was at least four years old in 2017. Most porpoises do not get older than 12 years.

28 June 2024: One of the two tagged porpoises was found dead
On June 25th, 2024, one of the two tagged porpoises was found dead on the North Sea beach near Oostkapelle, on the south-west side of the Eastern Scheldt storm surge barrier. It was the female animal. Members of Marine Animal Rescue Team (RTZ) took the stranded animal to Utrecht University's Faculty of Veterinary Medicine, where pathologists will examine the animal. Although the animal was already in an advanced state of decomposition, the examination may potentially provide information on the cause of death. Depending on the laboratory tests and additional investigations needed, it may take several weeks before the results are known. 

The animal's tag stopped sending locations some time ago. The available location data will be reviewed in detail. The tag of the other porpoise is still transmitting new locations daily from the Eastern Scheldt.

28 May 2024: Two harbour porpoises tagged in the Eastern Scheldt
During a first fieldwork period, two harbour porpoises were tagged in the Eastern Scheldt. Two adult animals received a tag with transmitter near Kats harbour, on 18 and 21 May, respectively.

Two types of tags were used for this trial; each animal received a different tag. The female - the first tagged animal - was seen again near Kats on May 19th (see figure 2). The tag of this animal is expected to provide detailed data over a period of two to three months. Initial data from the tag show that this harbour porpoise uses the entire Oosterschelde, but seems to prefer locations around Kats and Wemeldinge. The male's tag transmits fewer locations per day. Since the tag’s battery will last longer, it is expected that this porpoise can be tracked for up to a year. In due time, the tags will fall off.

This is the first time porpoises have been successfully tagged in the Netherlands. The research team will decide this summer where, when and how the second fieldwork period will take place.