Puerto Rico - September 17, 2025 More than a decade of underwater recordings is providing fresh insight into the reproductive behavior of red hind grouper, a commercially important Caribbean species. Researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute, in collaboration with multiple universities and HJR Reefscaping, studied the fish’s acoustic signals to better monitor and manage populations. Red hind are protogynous hermaphrodites, starting life as females and later becoming males. Each winter, they travel over 30 kilometers offshore to spawn under the full moon in large gatherings. Males use rhythmic, low-frequency sounds to attract mates and defend territory, making them vulnerable to overfishing during these periods. Rather than relying on traditional surveys, the team used passive acoustic monitoring, analyzing over 2,000 hours of recordings from a spawning site off Puerto Rico’s west coast since 2007. The study focused on two types of calls: courtship sounds and territorial signals. Analysis revealed a consistent seasonal spawning pattern aligned with lunar cycles. Between 2011 and 2017, courtship calls were more common. Beginning in 2018, territorial calls nearly tripled, suggesting changes in population structure, sex ratios, or spawning locations. Researchers also observed multiple peaks in sound production in recent years, indicating that spawning may now be spread across more days in each lunar cycle. Dr. Laurent Chérubin, lead author, said: “What’s remarkable is that even a single underwater microphone can reveal so much about fish populations. With consistent long-term monitoring, we can pick up early warning signs – like shifts in spawning behavior or population stress – and give resource managers the information they need to adapt conservation strategies before it’s too late.” Advanced machine-learning tools like FADAR allowed the team to process 12 years of data quickly and accurately, uncovering patterns that would have taken years to detect. This research demonstrates that listening to the ocean can reveal vital insights for reef fish conservation and ecosystem health.