These receivers monitor the presence of fish tagged with acoustic transmitters that broadcast a unique identification code and other information (e.g., depth). Receivers are installed on all thirteen offshore FADs surrounding Oʻahu (see above) to monitor movements of tunas. An additional array of acoustic receivers is operational around Oʻahu in depths of 30 meters or less to monitor the movements of large coastal sharks. This array monitors movements of ecologically important top predators, and the data collected from this system are used to estimate transfer rates between areas, describe patterns of residency at FADs and associative behavior of groups of fish, to improve estimates of the population size, and to better inform local agencies regarding public safety issues with respect to sharks.