Over a decade’s worth of acoustic recordings of grouper grunts is shedding light on how sound can be leveraged to monitor and manage at-risk fish populations. This research, conducted by Florida Atlantic University’s Harbor Branch Oceanographic Institute, focuses on the red hind (Epinephelus guttatus), a vital Caribbean grouper.
Red hind are unique as protogynous hermaphrodites; they begin their lives as females and transition to males later on. Each winter, these fish travel over 30 kilometers to offshore locations to spawn during the full moon, gathering in large groups. During this time, males use rhythmic, low-frequency sounds to attract mates and defend their territory. This predictable behavior makes them particularly vulnerable to overfishing during spawning seasons.
Unlike traditional methods, researchers from FAU Harbor Branch, FAU’s College of Engineering and Computer Science, HJR Reefscaping, the University of the Virgin Islands, and the University of Puerto Rico opted for passive acoustic monitoring. This approach allows for continuous, long-term tracking of reproductive behaviors, even in remote locations, without disturbing the fish or their environment.
To investigate changes in red hind reproductive behavior over time, researchers analyzed more than 2,000 hours of underwater recordings from a continuous monitoring site off Puerto Rico’s west coast since 2007.
This study stands apart from most acoustic research that typically measures general sound levels; it instead focuses on specific mating calls tied to distinct behaviors. Red hind produce two main types of sounds: one for courtship and another for territorial defense. By tracking these calls over time, researchers could identify subtle changes in spawning behavior and population dynamics.
The findings, published in the ICES Journal of Marine Science, represent one of the most extensive and uninterrupted datasets of acoustic information ever collected for a reef fish species.
The analysis confirmed a predictable seasonal pattern of red hind spawning activity in sync with lunar cycles. However, a remarkable finding was the shift in call types over the 12-year period. Between 2011 and 2017, courtship calls were more prevalent. But starting in 2018, competition and territorial calls surged, nearly tripling during the study.
“This shift could signal changes in the population, like an increase in dominant males, shifts in sex ratios, or even changes in core spawning areas,” explained Laurent Chérubin, Ph.D., the lead author and research professor at FAU Harbor Branch.
Researchers also noted an increase in sound production frequency and multiple peaks in recent years, indicating that spawning may now occur over more days during each lunar cycle compared to the past. These changes could reflect environmental or population dynamics.
Importantly, the study highlights that passive acoustic monitoring can effectively track reproductive behavior over time and spot early signs of changes in population or behavior—critical information for managing and conserving red hind and similar reef fish species.
“It’s incredible how much a single underwater microphone can reveal about fish populations,” said Chérubin. “With consistent long-term monitoring, we can detect early warning signs—like shifts in spawning behavior or population stress—and equip resource managers with the intelligence needed to adapt conservation strategies in time.”
Central to this analysis was an advanced machine-learning tool known as FADAR (fish acoustic detection algorithm research). This custom-built acoustic classifier enabled the team to quickly and accurately identify various mating calls.
“This study illustrates how much we can learn simply by listening,” Chérubin noted. “Thanks to FADAR, we processed 12 years of acoustic data in mere weeks—uncovering patterns that would have taken years to find. It’s a transformative approach for monitoring and managing reef fish like red hind.”
By employing advanced acoustic technologies at a single site, researchers can remotely detect shifts in reproductive behavior and population dynamics, providing vital early warning signs of stress. This data is crucial for resource managers to formulate strategies that protect spawning grounds and sustain fisheries.
“As passive acoustics evolve, they’re redefining our understanding of the ocean,” Chérubin emphasized. “By tuning into underwater soundscapes, we’re uncovering not only when and where fish spawn, but how those patterns change over time—offering essential insights into the health of marine ecosystems.”
The study also involved co-authors Caroline Woodward from the United States Geological Survey; Michelle Schärer-Umpierre, Ph.D., associate researcher with 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., faculty member; Evan Tuhoy, a Ph.D. candidate with the University of Puerto Rico; and Ali K. Ibrahim, Ph.D., research engineer at FAU’s Department of Electrical Engineering and Computer Science.
Summary: Research from Florida Atlantic University reveals how acoustic recordings of red hind grouper are utilized to monitor fish populations effectively. This innovative approach focuses on passive acoustic monitoring, enabling scientists to track spawning behaviors and detect changes over time. Findings indicate shifts in mating calls, which could reflect important population dynamics and environmental changes, underscoring the value of sound monitoring in fish conservation efforts.
