The porpoise is the smallest and most abundant whale in Denmark, but it is extremely shy by nature, which makes monitoring its behaviour difficult.  Danish and German researchers have therefore joined forces in this aerial survey project, which involves transect-based aerial surveying to estimate the population size.

"Porpoises are small, shy and extremely mobile marine mammals, and so they are not easy to count. Their home range covers the territorial waters of several countries, which makes international collaboration hugely important. We are therefore collaborating with researchers from Germany to conduct an aerial count which involves overflying a total of ten sections and observing the animals in their habitats. This is the best method of obtaining a specific and realistic idea of population abundance, although it's still only an estimate," explains Signe Sveegaard, senior adviser at the Department of Bioscience.

A Danish team and two German teams will cover the ten counting areas from three airplanes. The 43,000 km² area includes the recommended management area for the relevant population and thus includes as few porpoises as possible from neighbouring populations in the Baltic Sea and in the North Sea/Skagerrak.

The counts can only be performed during favourable weather conditions, because the small and fast animals are difficult to spot if the waves are too high. The first six areas have already been successfully covered at the end of ?June, and MiniSCANS II is expected to be finished by mid-July.

Two porpoise populations

The Baltic Sea population is divided into two populations, one of which - the Bælthavet (belt seas) population - lives in Kattegat, the Bælthavet and the western part of the Baltic Sea, while the other population lives in the inner Baltic Sea. The populations are genetically separate from each other and vary in number: The latest estimate for the belt seas population is from 2016, when the population was estimated at approx. 42,000 animals.

This is comparable to a previous estimate from 2012 of approx. 40,000 animals, and the population is therefore considered to be fairly stable. In stark contrast to this, the population in the inner Baltic Sea is much smaller: likely totalling fewer than 500 animals and assessed to be 'critically endangered' in the IUCN (International Union for the Conservation of Nature) Red List of endangered species.

Porpoises are particularly vulnerable to anthropogenic disturbances such as underwater noise and marine pollution, but the biggest threat to porpoises in the Baltic Sea is assumed to be the risk of ending up as bycatch in fishing nets. Therefore, accurate and reliable estimates of their population sizes are necessary for effective protection and management.

The count is financed by the three countries surrounding the habitats of this population, namely Denmark, Germany and Sweden, and is carried out in a trilateral partnership between the Department of Bioscience, Aarhus University, the Institute for Terrestrial and Aquatic Wildlife Research (ITAW), University of Veterinary Medicine Hannover, and the Swedish Museum of Natural History in Stockholm.

The results of the count will also be included as part of the three country's obligations to monitor and report on the status of small whales in accordance with the EU Marine Strategy Framework Directive and the Habitats Directive.

Contact:

Signe Sveegaard,
Institute for Bioscience,
Aarhus Universitet,
Telephone: 28951664
Email: ssv@bios.au.dk