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Oceans and fisheries

A study about the underwater noise effects of re-routing a major shipping lane

Re-routing shipping lanes is one way to reduce the effects of underwater noise on the marine environment. 

©Ecoscience, Aarhus University, 2021

However, large-scale diversions happen only rarely and so until now ocean scientists have not been able to study how they might affect noise levels or marine life.

When Denmark and Sweden decided to re-route the main gateway to the Baltic Sea, the Kattegat, used by 40,000 ships annually, researchers at the Eco-science department at Aarhus University in Denmark were quick to spot the opportunity for an important research project. They worked with the Danish Environmental Protection Agency, the Swedish Museum of Natural History in Stockholm, and the Swedish Defence Research Institute (FOI) on a large-scale study of the effects of underwater noise.

Before the route change in 2020, baseline data on underwater noise was collected by another research project. Following the route change, the two-year TANGO project has been collecting data on underwater noise levels and the distribution and behaviour of porpoises that can be compared with the results of the baseline study. To do this, the Aarhus researchers collected a year’s worth of data from sound recording devices to establish noise levels, supplemented by additional instruments called C-PODs. The C-PODs detect porpoises from their underwater calls. All were installed at sea and recovered thanks to their research vessel.


The TANGO project has contributed to increased knowledge on underwater noise in general. More specifically, it has contributed with important new knowledge about the derivative impacts on the marine environment related to re-routing of shipping lanes.

The Baltic Marine Environment Protection Commission (HELCOM) will use knowledge gained from the TANGO project to implement its Baltic Sea Action Plan and the HELCOM Science Agenda. TANGO’s results will also contribute to ongoing discussions about re-routing shipping lanes as a way to protect the marine environment elsewhere.

The preliminary results may be surprising as the number of porpoises is not affected by the shipping activity. Many of the large vessels that have been re-routed emit noise at low frequencies that porpoises cannot hear very well. The study has, however, shown that the porpoises react to individual ships at close range as the animals then can hear the higher frequencies of the ship noise. The result is that the animals may be less disturbed than we might assume from simply looking at the total noise levels across all frequencies.


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