Each winter, they travel over 30 kilometers to offshore sites where hundreds gather under the full moon to spawn in what marine biologists call fish spawning aggregations. Passive acoustic monitoring can operate continuously in remote locations without disturbing the animals. The research team combined acoustic monitoring with underwater visual surveys to confirm that increases in calls corresponded with increases in fish density. As passive acoustic monitoring becomes more sophisticated, it’s opening new possibilities for fisheries management. For red hind groupers, the changing chorus of underwater calls tells a story still being deciphered.