During recent decades, the 2 distinct harbour porpoise populations of the Baltic Sea have decreased sharply in abundance. The Baltic Proper population is down to a few hundred individuals and is regarded as ‘Critically Endangered’ by IUCN; the more abundant Belt Sea population also appears to have experienced a severe decline. We summarize the results of extensive static acoustic monitoring in the German part of the Baltic Sea and compare them to published results of aerial and acoustic surveys. Acoustic monitoring confirmed seasonal changes in detection rates consistent with proposed east–west migrations. Detection rates, and thus presumably porpoise density, decrease from west to east from a long-term mean (2002 to 2012) of 94% detection-positive days per month (DPD mo−1) around the island of Fehmarn and 71% DPD mo−1 in Kadet Trench to 4.4% DPD mo−1 in Pomeranian Bay as one crosses the putative population boundary. Acoustic monitoring results show a recent increase in porpoise registration rates in the Kadet Trench and in Pomeranian Bay, although this does not necessarily indicate a population increase. This large dataset supports the previously suggested proposition that each population uses the boundary waters in Pomeranian Bay alternately, leading to the presence of registration peaks in (late) summer by Belt Sea porpoises, and in winter by Baltic Proper animals. The critical status of porpoises in the Baltic Sea highlights the urgent need for protective measures which still await national and international implementation.