Stony reefs in shallow water support abundant and species-rich animal communities, and may thus attract top predators such as the harbour porpoise. In summer 2008, the nature restoration project Blue Reef re-established 45 000 m(2) of cavernous stony reef at L so Trindel in the northern Kattegat, Denmark. To investigate whether the re-established reef attracts harbour porpoises, the acoustic activity of porpoises was monitored by static acoustic data loggers, T-PODs, before and after the restoration project. T-PODs were placed at the L so Trindel reef and at a reference station 10 km away between June and August from 2006 to 2012. Results showed that porpoise activity increased significantly at L so Trindel reef after the reconstruction in 2008. The number of minutes with porpoise recordings (PPM) increased on average from 9.5 PPM per day in 2006 to a maximum in 2010 (15.2 PPM per day) followed by a small decrease (12.75 PPM per day in 2012). An increase in mean encounter duration from 3.0 min in 2006 to 4.7 min in 2010 (3.7 min in 2012) showed that porpoises not only appeared more often, but also stayed longer at L so Trindel. Furthermore, there was a striking diel pattern in porpoise activity at L so Trindel, with significantly higher activity during the night. This pattern became increasingly apparent over the study period. At the reference station, in contrast, most activity took place during the day throughout the study. The results suggest that these changes reflect a new food source which occurs at night on the re-established stony reef and is exploited by the porpoises.