Maritime authorities in Sweden and Denmark have proposed a rerouting of the main shipping routes into the Baltic Sea, scheduled to be implemented in 2020. This creates a unique opportunity to study effects of heavy ship traffic in a shallow sea. The IQOE-endorsed project "Rerouting shipping lanes in the Kattegat – effects on soundscape and ecosystem (TANGO)" will measure a range of parameters in the existing shipping lane, the new shipping lane and reference areas away from the shipping lanes. This will be done for at least a full year prior to rerouting, to establish a baseline, and at least one full year after the change. The primary goal of the project will be to describe changes to the soundscape (by sound recorders) in ecological important areas and quantify effects on harbour porpoise abundance (by passive acoustic monitoring). Secondary goals will be to study other ecosystem components: birds (by aerial surveys), fish (by eDNA) and benthic invertebrates (by surveys and sampling). This is an example of the opportunistic approach employed by IQOE-related projects to study the effects of sound on marine organisms without adding sound to the ocean.