The Marine Animal Alert System (MAAS) was developed by the Pacific Northwest National Laboratory as an element of compliance monitoring to support deployment of marine hydrokinetic energy devices. An initial focus is prototype tidal turbines to be deployed in Puget Sound in Washington State. The MAAS will help manage the risk of injury or mortality to marine animals from blade strike or contact with tidal turbines. In particular, development has focused on using both active and passive acoustic approaches to detect, classify, and localize listed Southern Resident killer whales within 200 m of prototype turbines. A passive acoustic system consisting of a pair of four-element symmetrical star arrays and parallel processing of eight channels of acoustic receptions has been designed, built, and evaluated under field conditions. Field evaluation of the performance of the system for detection and localization of killer whale calls and of the acoustic beacons was conducted in Sequim Bay, Washington, during the last calendar quarter of 2011. The probability of detection of whale calls with source levels of 150 dB referenced to a micropascal was found to be on the order of 0.75 when the whale playback transducer was at a range of approximately 190 m and increased to 1.0 at a range of approximately 130 m. The whale call classifier correctly classified essentially all of the calls detected. The location of sound sources was estimated by computing a two-dimensional (2D) bearing to the sound source from each star array and then computing the 2D location at which the 2D bearings crossed. Except for locations where the sound source was very close to the line passing through the centers of both star arrays, bearing errors were on the order of 1°, and range errors were less than 30 m at the maximum ranges tested. Inclusion of an active acoustic component into the system was not pursued following findings that the 200-kHz sonars selected based on whale target strength and acoustic propagation considerations were emitting sound within the hearing range of killer whales at levels that could possibly affect their behavior. The results of measurements of sideband sound made for the selected echo sounders are presented.