The harbour porpoise ( Phocoena phocoena ) is the most abundant cetacean in the Belgian part of the North Sea. We developed a mooring system for static passive acoustic monitoring (PAM) of this species using c-PoDs at locations of opportunity. Data of moorings between 2010 and 2015 at two locations were analysed. They revealed a significant seasonal trend in detections, assessed by month, with peaks in late winter - early spring and late summer, consistent with the results of aerial surveys and with strandings data. At one location there were significant differences in detections between years, with higher detection rates in every year between 2011 and 2014, and the highest detection rates in 2013 and 2014. The experiences gained are used to design a subsequent study strategy to monitor harbour porpoise presence in Belgian waters, including possible effects on their presence due to the construction of offshore windfarms.