Scientists decode grouper fish 'language' and unlock spawning secrets

The ocean doesn’t stay silent. Beyond the crash of waves and the songs of whales, reef fish keep their own soundscape alive. Grunts, pulses, and rumbles tell stories of courtship, rivalry, and survival.

For years, scientists mostly watched fish. Now they are listening, and what they hear is changing how we understand the ocean.

Red hind fish life

The spotlight is on the red hind, a Caribbean grouper. This species begins life as female and later shifts to male. Every winter, thousands travel offshore to gather under the full moon.

Males announce themselves with low, rhythmic calls. Those sounds invite mates and warn rivals. The gatherings are spectacular but risky. When spawning time is predictable, fishing pressure rises.

Tracking fish by sound

Instead of depending only on diver surveys or catch records, scientists used passive acoustic monitoring. An underwater microphone records continuously, capturing calls without disturbing the fish.

Since 2007, one spawning site off Puerto Rico has been monitored this way, creating more than 2,000 hours of data. This approach offers a clear view of behavior that often goes unseen.

Fish calls and spawn

Red hind calls fall into four groups, but two matter most. One signals courtship, the other territorial defense. By sorting the sounds, researchers tracked not only abundance but also social behavior.

Unlike studies that measure background noise, this work tied specific calls to mating and competition. That made it possible to see changes over time with surprising clarity.

Patterns across years

The analysis showed a strong seasonal rhythm. Calls peaked around the full moon, especially at dusk, matching spawning activity. Yet the balance shifted. Between 2011 and 2017, courtship calls dominated. From 2018 onward, territorial calls surged, almost tripling in number.

“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 research professor at FAU Harbor Branch.

Shifts in daily rhythms

Earlier in the study, courtship sounds peaked right at sunset. After 2018, the timing spread out. Territorial calls, meanwhile, stayed steady and strong.

These changes may signal that spawning shifted slightly away from the recorder or that social structure itself changed. Small details in sound reveal shifts that divers or catch data might miss.

Density and competition

Diver surveys confirmed that higher fish density meant more sound. But the type of sound mattered even more. Courtship calls pointed to active pairings. Territorial calls showed competition heating up.

This meant sound could reveal both numbers and the balance between males and females. Acoustic monitoring gave a sharper picture of social dynamics than head counts alone.

Environment affects fish spawning

Spawning happens where currents stay weak, helping eggs and larvae survive. Changing circulation patterns, possibly linked to climate change, may be forcing fish to adjust when and where they spawn.

The data also showed that spawning now stretches across more days in each lunar cycle. That spread may reflect an attempt to adapt to shifting environmental conditions.

Early fish spawn signals

The findings highlight how long-term listening can act as an early warning system. Shifts in sex ratios, competition levels, or spawning sites can be detected well before they appear in fishery records.

“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.”

Technology at work

Behind the study was a machine-learning tool called FADAR. It can distinguish between call types quickly and with high accuracy. Instead of taking years, researchers processed 12 years of recordings in weeks.

“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.”

What listening brings

By focusing on sound, scientists uncovered details about reproduction, density, and social dynamics that visual surveys could not match. Managers now have another way to spot problems before they grow serious.

“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.”

Future research on fish spawning

The voices of fish tell a hidden story. With long-term monitoring, advanced technology, and collaboration across institutions, scientists are building a clearer picture of reef health.

For species like the red hind, that knowledge could decide whether future generations will still hear the chorus of grouper calls beneath the Caribbean waves.

The team included Caroline Woodward from the United States Geological Survey; Michelle Schärer-Umpierre, Ph.D., from HJR Reefscaping and the University of Puerto Rico-Mayaguez; Richard S. Nemeth, Ph.D., from the University of the Virgin Islands; Richard Appeldoorn, Ph.D., and Ph.D. candidate Evan Tuhoy, both at the University of Puerto Rico; and Ali K. Ibrahim, Ph.D., from Florida Atlantic University’s Department of Electrical Engineering and Computer Science.

The study is published in the ICES Journal of Marine Science.

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