Dolphins May Be Listening for Dinner in Florida’s Underwater Soundscape

Every five minutes, for 54 days, a recorder hanging beneath an unmanned wave glider opened its ears for 15 seconds. The craft moved between Fort Pierce and Jacksonville, cutting across the East Florida Shelf while its hydrophones caught whistles, click trains, fish calls and the low noise of human activity. The resulting map did not show dolphins as dots on the sea. It showed the places where their different sounds appeared.

That distinction mattered. Whistles, usually used in social interactions, gathered most strongly in a set of nearshore waters. Echolocation, the fast sequence of clicks dolphins use to inspect their surroundings and find prey, was associated with a different environmental mix, including the presence of soniferous fish, fish that make sounds. The study offers a way to infer whether a stretch of ocean may be more important for dolphin social activity or foraging without first seeing an animal.

One Coast Produced Two Acoustic Maps

Jessica Carvalho of Florida Atlantic University and her colleagues reviewed nearly 62 hours of recordings, divided into 14,974 files. Dolphin sounds occurred in 1,691 files, just over 11 percent. Of those detections, whistles made up 55.4 percent, echolocation 29.5 percent and both signals 15.1 percent. The team then paired the detections with measurements and mapped their probability across the glider’s route.

The whistle model explained substantially more of the variation in the data than the echolocation model, but the two patterns were plainly unlike one another. Whistle detections were linked to location, sound pressure level, temperature and chlorophyll-a, a measure related to microscopic plant-like organisms in the water. Their predicted habitat was especially concentrated near St. Augustine, with other areas near Ponce Inlet and Melbourne Beach.

Echolocation detections were linked to location, depth, salinity, sound level and whether fish calls were present. Their modeled hotspots formed a broad band from Melbourne through Cape Canaveral toward Ponce Inlet, plus an offshore patch near Long John Reef. Dolphins use clicks whose echoes carry information about targets at much shorter distances than whistles can travel, a difference that helps explain why the researchers analyzed clicks on 1-kilometer grids and whistles on 10-kilometer grids.

“What we’re seeing is a fascinating separation between the acoustic behaviors dolphins use for social life and those they use for finding food,” said Greg O’Corry-Crowe, a co-author and research professor at FAU Harbor Branch. “In essence, the same ocean can become a different habitat to a dolphin depending on whether it is looking for a meal or communicating with other dolphins.”

Fish Calls May Advertise a Meal

The association between fish sounds and dolphin echolocation is the study’s most provocative result. The recorder picked up calls from groups that included drums, toadfish and jacks, all among the sound-producing fishes that can occur in dolphin diets. Where those calls were present, echolocation was more likely to be detected. That is consistent with dolphins using prey noise as a cue, rather than needing to find every fish solely by sending out their own sonar clicks.

There is experimental precedent for the other half of that acoustic contest. In a Florida field experiment, playbacks of low-frequency dolphin foraging sounds cut Gulf toadfish calling rates by 50 percent, suggesting prey can alter their own sound production after detecting a predator. The study did not examine the same species or setting, but it makes the Florida glider’s mismatches between fish and dolphin activity biologically plausible.

“Everybody is snooping on everybody else,” O’Corry-Crowe said. “This raises a fascinating question for our future research: Are dolphins listening for fish, and are fish listening for dolphins?”

Still, an association in a moving recorder’s data is not proof of eavesdropping. Fish could be calling because both they and dolphins favor the same reef or water conditions. And where fish calls disappear, the explanation might be silence in response to a predator, a shift in fish location, or simply a limit of the sampling. Reviews of fish communication find evidence that some fish reduce calling when predator sounds are played, but stress that the field evidence remains sparse and species-specific. That uncertainty is central to interpreting the new pattern.

The Recorder Also Heard a Noisy Ocean

Vessel and other human-made noise complicated the map. It can cover, or mask, animal signals, meaning dolphins may find communication harder and researchers may miss calls that are actually present. Some locations with high predicted whistle activity also experience substantial boat traffic. Other research has linked vessel encounters with altered dolphin behavior and whistle structure, though the direction and scale of effects depend on the species and situation. A review of Australian studies found behavioral responses to vessel traffic across multiple cetacean populations, not a direct test of the Florida animals.

The platform was designed to make this kind of long, spatially broad listening possible. A surface float drove the wave glider, while a submerged glider and towfish held the recorder and environmental sensors 4 to 10 meters below the surface. “A wave glider can travel through the ocean and listen continuously, allowing us to detect patterns that would be almost impossible to capture with occasional boat surveys,” said Laurent Chérubin, the study’s senior author and a research professor at FAU Harbor Branch.

Passive-acoustic gliders are useful precisely because they can combine sound recordings with ocean measurements, although the platforms themselves can generate noise and their slow movement complicates estimates of animal abundance. A 2023 review of the technologydescribes both advantages and those constraints.

What the Glider Could Not Identify

The study was observational and based on one deployment from March 8 to April 30, 2019. The team could not visually confirm species, even though several coastal and offshore dolphin taxa may use the region. An acoustic detection also does not necessarily mean an animal was directly beside the glider. Whistles may be detectable from more than 20 kilometers away, while clicks are generally heard over much shorter ranges.

March also had more dolphin detections than April. The authors say seasonal movements may be involved, but their data cannot distinguish changing species or stocks from changing local behavior. The maps therefore mark candidate habitat associations, not fixed boundaries or protected areas.

Repeated deployments, visual observations and species-specific acoustic work could test what the glider heard only indirectly. For now, its route leaves a more modest but useful proposition: a dolphin’s whistle and a dolphin’s click can turn the same stretch of Florida coast into two different kinds of habitat.

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