The Pomeranian Bay in the Baltic Sea is a transition zone between two harbour porpoise populations: the western Belt Sea population and the critically endangered Baltic Proper population of the central Baltic Sea. Data from twelve years of passive acoustic monitoring in this area and adjacent waters east of Rügen were analysed. Stationary monitoring devices recorded the high-frequency clicks that harbour porpoises use to orientate themselves and search for food.
A clear upward trend was observed in acoustic detection rates from 2010/11 to 2021, particularly during the seasonal period from May to October. In autumn, the increase was more moderate over the years. In winter, when detections were generally low, no consistent pattern emerged.
The data indicate an increase in acoustic presence. Whilst this is an indication, it does not constitute direct evidence of an increase in population size. Overall, the results are consistent with greater seasonal use of eastern areas by harbour porpoises from the Belt Sea population.
It can therefore be assumed that harbour porpoises from this population have been making increasing use of the Pomeranian Bay during the warmer months over recent years, venturing further east than previously thought. In winter, individuals from the endangered Baltic Proper population in the central Baltic Sea may also be found in this area.
At the same time, it remains unclear what role environmental changes, prey availability, the animals’ behaviour or the conditions under which they were detected play. The study thus underlines the importance of long-term monitoring programmes in a region that is subject to heavy pressure from shipping, fishing, offshore wind energy and other human activities.
