More than a decade of acoustic recordings of grouper grunts is shedding new light on how sound can be harnessed to monitor and manage vulnerable fish populations. This groundbreaking research, conducted by Florida Atlantic University’s Harbor Branch Oceanographic Institute, focuses on the red hind (Epinephelus guttatus), a commercially significant Caribbean grouper species.
The red hind is a protogynous hermaphrodite, beginning life as a female and later transitioning to male. Each winter, these fish embark on a journey of over 30 kilometers to offshore sites, gathering in large numbers to spawn under the full moon. Male groupers emit rhythmic, low-frequency sounds to attract mates and defend their territory, a predictable behavior that makes them particularly susceptible to overfishing during the spawning season.
Revolutionizing Monitoring with Passive Acoustics
In a departure from traditional survey methods, researchers from FAU’s Harbor Branch, FAU College of Engineering and Computer Science, HJR Reefscaping, the University of the Virgin Islands, and the University of Puerto Rico have embraced passive acoustic monitoring. This innovative technique allows for continuous, long-term observation of reproductive behaviors in remote or challenging environments without disturbing the fish or their habitats.
Since 2007, more than 2,000 hours of underwater recordings from a single spawning site off Puerto Rico’s west coast have been analyzed to assess changes in red hind reproductive behavior over time. Unlike most acoustic studies that track general sound levels, this research zeroes in on specific mating calls linked to distinct behaviors. Red hind produce two primary sound types: one for courtship and another for territorial defense. Tracking these sounds over time has enabled researchers to detect subtle shifts in spawning behavior and population dynamics.
Findings and Implications
The study, 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, a significant finding was the shift in the balance of call types over the 12-year period. Between 2011 and 2017, calls related to courtship were more prevalent. But starting in 2018, calls associated with 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 noted 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.
Technological Advancements in Acoustic Monitoring
At the heart of the analysis was an advanced machine-learning tool known as 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 remarkable 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 employing advanced acoustic technology at a single site, researchers can remotely detect shifts in reproductive behavior and population dynamics, 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.
Future Directions and Conservation Strategies
Importantly, the study demonstrates 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.”
As passive acoustics continues to advance, it is transforming our understanding of the ocean. By tuning in to underwater soundscapes, researchers are discovering not just when and where fish spawn, but how those patterns change over time, offering vital insights into the health of marine ecosystems.
Collaborative Efforts and Broader Implications
The study’s co-authors include Caroline Woodward from the United States Geological Survey; Michelle Schärer-Umpierre, Ph.D., associate researcher at HJR Reefscaping and the University of Puerto Rico-Mayaguez; Richard S. Nemeth, Ph.D., research professor at the 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.
As this research continues to unfold, it underscores the importance of innovative technologies in conservation efforts and highlights the need for collaborative approaches to safeguard the future of vulnerable fish populations.
