Aarhus University Seal / Aarhus Universitets segl


of Harbour porpoises

Harbour porpoises are shy animals and can be difficult to track. The only really effective way to study the life of a porpoise is by tagging it with a tracking system. These different tags can measure the behavior and movement of the animal and then transmit this information either by satellite or by saving the data on the unit. It is difficult to capture a porpoise, and it is therefore important to collaborate with the local gill net fishermen. It happens now and then that porpoises are trapped in the nets in which they are captured and we are able to tag them before releasing them back into the wild. Since 1997, we have monitored the movement and diving behavior of harbour porpoises together with The University of Southern Denmark, The University of Kiel, Technical University and Denmark and Fjord&Bælt in Kerteminde.

The porpoise is tagged on board the ship by mounting a tag on its dorsal fin. This process takes up to 30 minutes, and while mounting the tag we take blood samples for further health investigations. The satellite tags start sending information immediately after release, and when the tag falls of, it is often lost for good. However, we sometimes find the tags, and if you ever find one, we are very interested in getting it back and will pay a finders’ fee.

Why do we tag porpoises?

To be able to study:

  • Seasonal movement patterns
  • Their preferred habitat areas
  • Diving behavior
  • Differences between gender and age
  • Healthstatus
  • Echolocation and foraging
  • Bycatch

Project 1997-2001

From the data provided to us from the satellite tags, we can se that some of the porpoises travel far, some of the animals even go from the inner danish waters, along the coast of Norway and then crossing the North sea to England, but the majority of the seksual mature porpoises are staying within a more limited area like the inner danish waters. 

Maximum daily diving depth collected from 16 tagged animals between 1997-2001
The map is showing the movement of porpoise no. 4. It was tagged on the 8.th of november 2000, in Kerteminde. The arrows point in the porpoise swimming direktion.
Showing the mean porpoise diving time based on information from 43.000 dives.

Diving behaviour

The harbour porpoise often dive to the seabed which is also the location of many fish species.


  • The harbour porpoise prefer to be at a depth less than 40 meters.
  • The maximum depth was measured in Skagerak at 200 meters


  • No change in diving activity between night and day.

Diving time:

  • Maximum time is 10 minutes
  • Most frequent dive length is 1-2 minutes

Due to a higher amount of red blood cells in the blood and muscles of the harbour porpoises, it can hold it's breath longer than humans. But it still have to surface for air frequently. These blood cells trap the oxygen when it breathes, and storage it for later use. This storage of oxygen exceeds the one of humans, and this is why the porpoise can be submerged for a longer period of time. Also the porpoise can shut down the use of some organs by contracting arteries and limit blood flow, then only the heart, brain and muscles get new oxygen during a long dive.


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The harbour porpoise is released after being tagged with a transmitter. The video is recorded by Florian Graner, Sealife production.

A harbour porpoise is tagged, in Boring Vig 2018, with a VHF transmitter that can measure heart rate and more. Photos by Esteban Iglesias Rivas