The harbour porpoise is the only resident cetacean species in the German Baltic Sea. Within the last several decades this harbour porpoise stock declined drastically, causing deep concern about its status. Plans of the German government for proposing Marine Protected Areas (MPAs) to implement Natura 2000 and for assessing the impact of offshore windmill constructions on the marine environment led to an increased research effort on the harbour porpoise in German waters. For the first time, long-term passive acoustic monitoring has been conducted in the German Baltic Sea from the Kiel Bight to the Pomeranian Bay from August 2002 to December 2005. Porpoise detectors (T-PODs) have been installed five to seven metres below the water surface at up to 42 measuring positions throughout the investigated area, registering the exact times of echolocation signals of passing harbour porpoises. The proportion of monitored days with porpoise detection in each quarter of the years has been analysed. A correlation of the results with the longitude of the measuring position revealed a significant decrease from west to east in the percentage of days with porpoise detections. Comparison of data gathered in the first quarters with the third quarters of the monitoring years displayed a seasonal variation with fewer days of porpoise detections in winter time than in summer time. Nevertheless, harbour porpoises have been detected year-round at most of the measuring positions in the German Baltic Sea. The present study clearly indicates a regular use of the German Baltic Sea by harbour porpoises with a geographical and seasonal variation in the usage of the German Baltic Sea. The larger numbers of harbour porpoise detections in spring to autumn compared with winter suggests that the German Baltic Sea is an important breeding and mating area for these animals.

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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 ( 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.

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