Harbour porpoise monitoring

Why do we monitor harbour porpoises?

Under EU legislation (the Habitats Directive and the Marine Strategy Framework Directive), all Member States are required to complete monitoring and reporting on the status of harbour porpoises. The harbour porpoise is the only resident species of cetacean in Swedish waters, yet cetaceans play an important role in ecosystem health and functioning.

Therefore, understanding possible changes in the abundance and distribution of harbour porpoise populations is important to understand other ecosystem-level impacts. The Swedish Museum of Natural History is responsible for completing the Swedish National Monitoring program on harbour porpoises on behalf of the Swedish Agency for Marine and Water Management (SwAM).

Three harbor porpoise populations are present in Swedish waters: the critically endangered Baltic Proper population, the Belt Sea population, and the North Sea population.

How do we monitor harbour porpoises?

The Swedish Museum of Natural History carry out passive acoustic monitoring (PAM) of harbour porpoises, that is we listen for them, by anchoring acoustic data loggers (C-PODs) in specific locations. PAM is particularly useful for harbour porpoises since they produce narrow-band high-frequency (NBHF) clicks almost continuously in order to find prey, communicate, and navigate, but spend little time close to the surface and can be difficult to observe. The harbour porpoise clicks are easily identified from other underwater sounds, which makes it possible for us to measure the relative occurrence of harbour porpoises at each monitoring station and analyse changes over time. In the majority of the Baltic Sea, only PAM is possible due to the very low density of harbour porpoises.

We also collaborate with other countries that carry out visual line-transect surveys of harbour porpoises from airplanes or boats. This method can be used where the density of harbour porpoises is higher.

Where do we monitor harbour porpoises?

In the Baltic Sea, we have carried out continuous PAM of harbour porpoises at 11 stations since 2017. In the Kattegat Sea, we carry out PAM periodically, so far at 14 stations during 2019-2023. Several stations are located within Natura 2000 sites that have been designated to protect harbour porpoises.

In addition to the harbour porpoise monitoring stations run by the Swedish Museum of Natural History, we collaborate with the Swedish Defence Research Agency (FOI) who monitors underwater noise at one station close to Sundsvall, one station at the Northern Midsea Bank in the Baltic Sea and one station close to the island of Hönö west of Gothenburg. At these stations, underwater noise and harbour porpoises are monitored jointly.

In the southwestern Baltic, the Sound, Kattegat and Skagerrak Seas, we participate in visual line transect surveys from airplanes. The Belt Sea population was surveyed in 2020 (MiniSCANS-II) and both the Belt Sea and North Sea populations in 2022 (SCANS-IV).

What do we do with the results?

All data collected as a part of the national monitoring program are delivered to the Swedish Meteorological and Hydrological Institute (SMHI) and HELCOM. They are made publicly available via Sharkweb External link. and HELCOM's Biodiversity database External link..

We have completed an analysis comparing the detection rates at the stations in the Baltic Sea during the national monitoring program (2017-2020) to the detection rates at the same stations collected during a previous research project called SAMBAH (2011-2013). This enabled us to demonstrate that at the monitored stations, the detection frequency of this Critically Endangered population has actually increased over the last decade, with an average rise of 2.4% per year.

Although this is positive news, the rate of increase is well below what is possible for a healthy harbour porpoise population in the absence of threats. Additionally, as harbour porpoises are highly mobile, it is not possible to know if an increase at a smaller number of stations truly reflects an actual increase in the entire population size. The results strongly point to the need for population-wide surveys, and that measures to protect the population need to be implemented to allow it to recover.

Based on the survey results of the Belt Sea population, we have estimated the limit for human induced mortality that is viable in the long term, and the trend in population abundance. The results show that the known bycatch is several times higher than the sustainable limit, and that the population likely is in decline.

Together with national monitoring data from other countries, the data are used for developing indicators to assess the status of harbour porpoise populations, and how they change over time. The results of the national monitoring program are also presented at conferences, such as Vattenmiljöseminariet.

Other associated work

The Swedish Museum of Natural History also acts as experts in working groups or by giving advice to national representatives in international organisations such as HELCOM, OSPAR, ASCOBANS and ICES. We also collaborate with the County Administrative Boards in their development of the regional monitoring program of harbour porpoises.

Project details

Period

2017 – ongoing

Funding

The Swedish Agency for Marine and Water Management External link. (SwAM)