Power extracted from fast-moving tidal currents has been identified as a potential commercial-scale source of renewable energy. Marine and hydrokinetic (MHK) device developers and utilities are pursuing deployment of prototype tidal turbines to assess technology viability, site feasibility, and environmental interactions. Deployment of prototype turbines requires environmental review and permits from a range of regulatory authorities. Ensuring the safety of marine animals, particularly those under protection of the Endangered Species Act of 1973 and the Marine Mammal Protection Act of 1972, has emerged as a key regulatory challenge for initial MHK deployments. The greatest perceived risk to marine animals is from strike by the rotating blades of tidal turbines. Development of the marine animal alert system (MAAS) was undertaken to support monitoring and mitigation requirements for tidal turbine deployments. The prototype system development focused on the Southern Resident killer whale (SRKW), an endangered population that frequents Puget Sound, Washington, and is seasonally present in the part of the sound where deployment of prototype tidal turbines is being considered. Passive acoustics were selected as the primary means to detect the SRKWs because of the vocal nature of these animals. The MAAS passive acoustic system consists of a two-stage process involving the use of an energy detector and a spectrogram-based classifier to distinguish between SKRW calls and background noise. A prototype consisting of two 2D symmetrical star arrays separated by 20 m center to center was built and evaluated successfully in the waters of Sequim Bay, Washington, using whale-call playback.