SeaGen produces underwater noise during operation that contains both tonal and broadband components which cover a frequency range similar to the most sensitive hearing range of several marine species. Underwater sound propagation models were used to predict noise levels around SeaGen. Within one kilometre of SeaGen, simple transmission loss models fit the data relatively well; however received levels were found to increase at distances of more than 3km. The source level of SeaGen was back-calculated using the field measurements and the sound propagation models; source level (rms) was estimated to be 174 dB re 1μPa. Although this is lower than some anthropogenic noise sources (e.g. seismic airguns), levels are comparable to a large vessel underway and it is important to highlight that SeaGen produces noise more or less continuously requiring longer exposure times to be taken into account when predicting hearing damage. The potential effects on fish and marine mammals of underwater noise from SeaGen were predicted using current information on hearing abilities and observed responses from previous studies, and are presented as a series of influence zones (e.g. zone of audibility, zone of behavioural responses) around SeaGen. It is important to highlight that predictions of influence zones must be viewed alongside their associated caveats. Specifically, empirical noise data were only available out to 3.5km from SeaGen; predictions of influence zones at ranges greater than this are therefore based solely on transmission loss models and must therefore be treated with caution. This series contains the assessment report.