Echolocation activity of harbour porpoises, Phocoena phocoena, shows seasonal artificial reef attraction despite elevated noise levels close to oil and gas platforms.
Harbour porpoises frequently alter their behaviour in response to underwater sound from shipping, seismic surveys, drilling and marine renewables. Less well understood is the response of porpoises to sounds emitted from oil and gas (O&G) platforms during routine operations. The responses are not easily predicted as platforms can act simultaneously and to varying degree as a source of disturbance through noise and attraction through an artificial reef effect with increased prey abundance and diversity. To investigate the presence and feeding behaviour of harbour porpoises around platforms, autonomous acoustic loggers were placed for up to 2 years, at 21 stations 0-25.6 km from the largest platform in the Danish North Sea. Harbour porpoises were detected at all distances year round in two distinct seasonal activity patterns. During July-January, porpoises were attracted to the platform as indicated by high foraging activity within 800 m of the platform. Echolocation activity levels were up to twofold higher than those observed at 3.2-9.6 km from the platform. Similar high echolocation activity was observed 200 m from neighbouring offshore installations located within 15 km, regardless of their size, during May-July. This study shows that porpoises may be attracted to offshore O&G platforms despite confirmed elevated underwater noise and are likely exploiting higher prey abundance in the vicinity of such structures. This is possibly due to increased prey availability created by the combined effect of the artificial reef formed by the underwater structure and the local protected area around all platforms where fishery is banned. Hard substrate and untouched seabed are rare and valuable habitats to many organisms in heavily trawled waters like the North Sea, and the ecological importance of these structures should be considered in the development of decommissioning strategies.