The Royal Canadian Airforce (RCAF) is proud of its well-deserved reputation for environmental stewardship. With the advent of in-service low-frequency active sonar sources the RCAF is reviewing and updating policies and procedures to limit the potential impacts of active sonar on whales. During active sonar activities, passive sonar sonobuoys can be used to acoustically monitor for the presence of whales so that sonar operations can be shut down if an animal is too close to the active sonar source. Defence Research and Development Canada (DRDC) is assisting the RCAF to assess the effectiveness of in-service systems for marine mammal monitoring and mitigation. Case studies demonstrating the effectiveness of using sonobuoys for passive acoustic monitoring of marine mammals are presented using data collected in realistic scenarios.