A red hind grouper, which produces sounds associated with courtship, territoriality or mating.

More than a decade of acoustic recordings of grouper grunts are providing new insight into how sound can be used to monitor and manage vulnerable fish populations. The research by Florida Atlantic University’s Harbor Branch Oceanographic Institute focused on the red hind (Epinephelus guttatus), a commercially important Caribbean grouper species.  

Red hind are protogynous hermaphrodites, starting life as females and later becoming males. Each winter, they travel more than 30 kilometers to offshore sites to spawn under the full moon in large gatherings. Males use rhythmic, low-frequency sounds to attract mates and to defend territory. This predictable behavior makes them especially vulnerable to overfishing during spawning season.

Rather than traditional survey methods, FAU Harbor Branch and FAU College of Engineering and Computer Science researchers, in collaboration with HJR Reefscaping, the University of the Virgin Islands, and the University of Puerto Rico, turned to passive acoustic monitoring. This technique enables continuous, long-term monitoring of reproductive behavior, even in remote or hard-to-access areas, without disrupting the animals or their habitat.

To examine how red hind reproductive behavior has changed over time, researchers analyzed more than 2,000 hours of underwater recordings from a single spawning site off Puerto Rico’s west coast, continuously monitored since 2007.

Unlike most acoustic studies that track general sound levels, this research focused on specific mating calls tied to distinct behaviors. Red hind produce two primary sound types – one for courtship and another for territorial defense. Tracking these sounds over time allowed researchers to detect subtle shifts in spawning behavior and population dynamics.

The result, published in the ICES Journal of Marine Science, is one of the most extensive and uninterrupted acoustic datasets ever assembled for a reef fish species.

The analysis confirmed a consistent seasonal pattern in red hind spawning activity, closely aligned with lunar cycles. However, one of the most striking findings was a notable shift in the balance of call types over the 12-year period. Between 2011 and 2017, calls linked to courtship were more common. But starting in 2018, calls linked to competition and territorial behavior became dominant – nearly tripling over the study period.

“This shift could indicate changes in the population, such as an increase in the number of older or more dominant males, changes in sex ratios, or even a shift in the core spawning area,” said Laurent Chérubin, Ph.D., lead author and a research professor at FAU Harbor Branch.    

Researchers also observed more frequent and multiple peaks in sound production in recent years, suggesting that spawning may now be spread over more days in each lunar cycle than in the past. These changes could be responses to environmental or population shifts.

Importantly, the study shows that passive acoustic monitoring can be a powerful tool for tracking reproductive behavior over time and detecting early signs of population or behavioral change – information that is critical for managing and conserving red hind and similar reef fish species.

“What’s remarkable is that even a single underwater microphone can reveal so much about fish populations,” said Chérubin. “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.”

At the center of the analysis was an advanced machine-learning tool called FADAR (fish acoustic detection algorithm research). This custom-built acoustic classifier allowed the team to detect and distinguish between different types of mating calls with extraordinary speed and accuracy.

“This study shows how much we can learn simply by listening,” said Chérubin. “Thanks to FADAR, we processed 12 years of acoustic data in weeks – uncovering patterns that would have taken years to find. It’s a game changer for monitoring and managing reef fish like red hind.”

By using advanced acoustic technology at a single site, researchers can detect shifts in reproductive behavior and population dynamics remotely, providing crucial early warning signs of stress. This kind of data is essential for resource managers developing strategies to protect spawning grounds and sustain fisheries.

“As passive acoustics advances, it’s transforming our understanding of the ocean,” said Chérubin. “By tuning in to underwater soundscapes, we’re discovering not just when and where fish spawn, but how those patterns change over time – offering vital insights into the health of marine ecosystems.”

Study co-authors are Caroline Woodward, United States Geological Survey; Michelle Schärer-Umpierre, Ph.D., associate researcher, HJR Reefscaping and the University of Puerto Rico-Mayaguez; Richard S. Nemeth, Ph.D., research professor, Center for Marine and Environmental Studies, University of the Virgin Islands; Richard Appeldoorn, Ph.D., a faculty member; and Evan Tuhoy, a Ph.D. candidate, both within the Department of Marine Sciences, University of Puerto Rico; and Ali K. Ibrahim, Ph.D., a research engineer in FAU’s Department of Electrical Engineering and Computer Science.

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Each winter, red hind groupers gather under the full moon, grunting low-frequency calls to attract mates and defend territory. Researchers used acoustic passive monitoring to detect their grunts by deploying specialized underwater microphones via the autonomous wave glider, a device they developed themselves.

-FAU-