Understanding the effect of marine energy development on the underwater noise levels in a proposed installation area forms a crucial part of any Environmental Impact Assessment. This paper uses boat-based hydrophone survey data taken from an area subject to tidal currents up to 3 m s-1 and deter-mines how the background noise levels change with tidal ﬂow and ﬂow direction. It was found that a more meaningful expression of background noise was gained by reference to “Power level” as measured by the Root Mean Square of the spectrum. Hydrophone measurements were taken during low season (with respect to other maritime activity) and in high season over the entire tidal cycle. Back-ground noise levels ranged between 72 and 108 dB re 1 µPaRMS with higher sound pressure levels occurring at frequencies below 100 Hz. Background noise levels increased with increasing tidal ﬂow and were higher on ebb tides than ﬂood tides.