The sound emitted by tidal turbines overlaps with the generalized hearing ranges for marine mammals and may, consequently, affect marine mammal behavior. Because of the strong environmental protections for marine mammals, these effects are of interest to resource agencies and project developers. The primary goal of this project is to improve the understanding of how sound emitted by tidal turbines may affect marine mammal behavior, with the intent that such understanding may allow this environmental risk to be “retired” for initial demonstrations and arrays. A secondary project goal is to evaluate the efficacy of marine mammal observations from a shore-based vantage point as a method to detect change as a consequence of marine energy stressors. Three, two-week trials were conducted across spring, summer and autumn of 2017 during which vantage-point observations of marine mammals were made from land. We refer to these two-week periods as ‘trials’ as they represent different periods of field data collection. These observations consisted of scan samples every 10 minutes with a set of binoculars and a DSLR camera. When pinnipeds were sighted, still images were taken. When harbor porpoise were sighted, focal follows using video were initiated. These data were later analyzed to estimate the location of animals sighted using photogrammetric techniques. For approximately half of each trial period, a boat was moored in the study area from which an underwater projector was used to play back tidal turbine sounds at a broadband source level of 158 dB re µPa @ 1 m. The other half of each trial period had no playbacks and was used as a control period. During the control and playback periods, ancillary data were collected in the study area near the sea floor (ambient noise, tidal velocity, and porpoise clicks) while weather and vessel tracks were recorded near the vantage-point station. Harbor seals did not show a significant response to the simulated turbine noise, nor did harbor porpoise change their closest point of approach or change the direction/deviation of the paths during periods when the playback was occurring. While porpoise sightings and porpoise acoustic detections were significantly reduced during playback periods in the first two trials, the number of detections during the playback periods increased during the third trial. Similarly, the calculated distance from porpoises to the playback location suggests that harbor porpoises avoided an area around the playback location of approximately 300 m during Trial 1, which decreased to approximately 100 m in Trial 2, and disappeared in Trial 3. This could be an indication of habituation or increased tolerance, either to the playback sound or the vessel presence. Our finding of no significant effect of turbine noise on harbor seals contrast with the findings in a previous study by Hastie et al. (2017) conducted in United Kingdom waters. However, due to substantive differences in source levels and propagation loss, seals in our study would need to have been within 10 m of the playback location to experience similar received levels to those in the Hastie et al. study. Consequently, the two studies may actually be in agreement. Our findings of significant effect of turbine noise on harbor porpoise are in line with previous studies suggesting the harbor porpoise are sensitive to noise. However, we note that, due to cost and logistical constraints, the playback vessel was only mooring during the playback periods. Consequently, we cannot definitively ascribe the changes in porpoise behavior to the simulated turbine noise, playback vessel presence, or a combination of both factors. This methodological limitation and other lessons learned are discussed to benefit future studies that explore the effects of acoustic stressors on marine mammals.