For over a decade, scientists have turned their attention underwater, listening intently to the vocalizations of the red hind (Epinephelus guttatus), a charismatic grouper species native to the Caribbean. This species’ distinct grunts, critical components of their reproductive behavior, have become a window into understanding complex population dynamics, spawning habits, and responses to environmental pressures. Now, a pioneering research effort by Florida Atlantic University’s Harbor Branch Oceanographic Institute (HBOI), combined with interdisciplinary collaborators, harnesses advanced acoustic monitoring and machine learning to decode these sonic signals. Their work not only charts a new course in marine biology research but also offers a powerful tool for the conservation and management of vulnerable reef fish.
The red hind is a protogynous hermaphrodite, meaning individuals start life as females before transitioning to males later in their development. Such life history traits already make the species’ population structure particularly complex. Each winter, these groupers embark on a majestic migration spanning over 30 kilometers, congregating in large aggregations at offshore sites to spawn. This synchronization with lunar cycles ensures increased reproductive success but also exposes the fish to heightened risk during spawning seasons, when they gather conspicuously and are therefore more susceptible to overfishing.
What sets the FAU team’s research apart is their adoption of passive acoustic monitoring — an innovative technique that captures the subtle vocalizations emitted during fish interactions without disrupting natural behaviors or habitats. While traditional survey methods often rely on visual counts or catch data, these approaches can be invasive and limited, especially for species inhabiting challenging environments. By deploying underwater microphones at a single key spawning site off Puerto Rico’s west coast, the scientists collected an unprecedented 2,000-plus hours of continuous data spanning more than a decade, allowing for a longitudinal study rarely seen in marine biology.
These auditory recordings provided a novel lens through which researchers could distinguish between two primary categories of red hind sounds: those linked with courtship and those used in territorial defense. Each call type encodes specific behavioral contexts; courtship grunts facilitate mate attraction, whereas territorial calls are central to male competition and dominance assertion. By applying a custom-built fish acoustic detection algorithm, known as FADAR (Fish Acoustic Detection Algorithm Research), the team efficiently classified these sound types across years of data with remarkable accuracy and speed—turning a previously mammoth task into an attainable objective.
Findings published recently in the ICES Journal of Marine Science reveal a compelling temporal trend in the balance of these vocalizations. Between 2011 and 2017, courtship calls predominated during the spawning season, signaling active reproductive efforts within the population. However, starting in 2018, the acoustic landscape shifted dramatically. Territorial defense calls surged, nearly tripling in frequency by the end of the study. Such a pronounced change suggests potential alterations in the population’s social structure, possibly reflecting an increase in older or more dominant males, shifting sex ratios, or even a reconfiguration of the primary spawning grounds.
Moreover, the study uncovered an increase in more frequent and multiple peaks in sound production across lunar cycles in recent years. This extension of spawning activity over a broader temporal window could be an adaptive response to changing environmental conditions or population pressures. It might also indicate behavioral plasticity among red hind, allowing them to optimize reproductive success amid evolving challenges. These acoustic signatures are invaluable early indicators of shifts in reproductive ecology that may precede observable changes in population numbers.
Dr. Laurent Chérubin, the lead author of the study and a research professor at HBOI, emphasizes the transformative power of this acoustic approach: “The sheer volume and quality of data we’ve obtained from a single underwater microphone over such an extended period is astonishing. This continuous acoustic record enables us to detect subtle but telling behavioral changes that traditional survey methods might miss entirely.” His team’s work exemplifies how non-invasive technologies can unlock new insights into marine life.
The integration of machine learning through FADAR revolutionizes the speed at which such complex datasets can be processed. The algorithm’s ability to recognize and classify nuanced variations in call types not only accelerates data processing but also enhances precision, reducing human bias and error. This advancement holds promise for extending similar monitoring frameworks to other acoustically active reef fish species, thereby broadening the scope and scale of marine conservation monitoring.
Ecologically, the use of passive acoustic monitoring supports improved fisheries management and conservation strategies by providing detailed behavioral data in near real-time. Understanding when and how fish aggregate, compete, and reproduce enables managers to identify critical spawning habitats that require protection, adjust fishing seasons to minimize disruption, and detect early signs of stress or population decline before they manifest overtly. The red hind’s vulnerability during its spawning aggregations means timely management actions informed by such data are essential to ensure sustainable fisheries.
This research also reflects a broader evolution in oceanographic science—a shift from snapshots in time to continuous, long-term observation. Such datasets are crucial for discerning patterns shaped by climate change, ocean acidification, and anthropogenic impacts, which often occur over multi-year timescales. By listening to the ocean’s acoustic environment—its soundscape—scientists gain a richer, more dynamic picture of ecosystem health.
The study’s collaboration across multiple institutions, including the University of Puerto Rico, the University of the Virgin Islands, HJR Reefscaping, and the College of Engineering and Computer Science at FAU, underscores the interdisciplinary nature of modern marine ecology. Engineers, ecologists, and computer scientists working together facilitate innovative technological applications that directly address ecological questions and conservation needs.
Looking forward, the successful deployment of passive acoustic monitoring paired with machine learning heralds a new era of marine research methodologies. As technology continues to evolve, and data processing tools become more sophisticated, we are witnessing a profound transformation in how scientists study the ocean’s inhabitants—from tracking population dynamics to unveiling behavioral subtleties. The red hind case study stands as a testament to the power of listening in the depths and learning from the language of fish to better protect our fragile marine ecosystems.
In sum, this extensive acoustic dataset, coupled with groundbreaking analytical tools, provides an unparalleled understanding of red hind spawning behavior over an extended period. It exemplifies how integrating bioacoustics with cutting-edge technology can yield critical insights into reproductive biology and population health, delivering actionable knowledge for conservation. The transition from courtship to territorial dominance in call types is more than a behavioral curiosity; it may be a vital signal prompting adjusted management strategies to safeguard these vital fish populations in a rapidly changing world.
Subject of Research: Animals
Article Title: Assessing red hind (Epinephelus guttatus) spawning aggregation changes from long-term relative variations in call types associated with reproductive behaviors
News Publication Date: 14-Aug-2025
Web References: http://dx.doi.org/10.1093/icesjms/fsaf138
References: ICES Journal of Marine Science publication (doi: 10.1093/icesjms/fsaf138)
Image Credits: FAU Harbor Branch
Keywords: Marine fishes, Endangered species, Mating behavior, Behavioral ecology, Animal communication, Sound, Underwater acoustics, Applied acoustics, Animal sounds, Vocalization, Mating success, Foraging behavior, Aggression, Population, Sex ratios, Applied ecology, Conservation ecology, Wildlife management
