Baby fish follow sounds of a healthy reef

Why did the fish go to the coral reef? It sounds like the start of a bad joke, but the answer is: we don’t know.

Research conducted in Maui, Hawaii suggests that fish larvae in the open ocean looking for a home can only hear the sounds of reefs and other fish when they’re in relatively close proximity. This finding contributes to a bank of knowledge that could one day be used to lure fish to damaged parts of the reef in an effort to improve the health of the ecosystem.

“To keep a reef healthy, you need a constant supply of new larvae to repopulate animals that die off,” explains Max Kaplan from the Woods Hole Oceanographic Institution in Massachusetts and lead author of the paper published in Scientific Reports.

“How larvae find reefs has been a big question, though. We think sound may play a role in attracting them, but exactly how far away they can sense those sounds has not yet been accurately measured.”

Among most fish species, the process of “hearing” is actually interpreting movement or vibrations through the water via a bone-like mechanism in the skull called an otolith. 

“Think of it like being at a loud concert: if you’re standing next to a huge speaker, you effectively feel the sound as it vibrates your skin,” Kaplan explains. “Fish and invertebrates sense sound in a similar way.”

The team recorded two components of sound: pressure waves, which are like the ones that push against your eardrum, and particle motion, which is the physical vibration of water molecules as a sound wave travels through.

While studies have previously been conducted into the interpretation of pressure waves underwater, the particle motion side of things – which is generated by adult fish and invertebrates – was relatively unknown.

“Particle motion is really the relevant cue for marine animals,” says co-author Aran Mooney, also at Woods Hole.

“When we’re measuring pressure, we’re measuring the wrong thing. It only gives a ballpark sense of what marine species hear. We think studying particle motion is a big step to figuring out how larvae find their way to a reef.”

To study particle motion, the researchers used an accelerometer – a machine that measures acceleration forces – specially designed for marine research. These were deployed at a few different sites across a Maui reef, and measurements were taken at dawn and mid-morning on three days.

Surprisingly, the level of particle motion in the reef was lower than expected, dropping below levels that most marine creatures could sense at just a few metres from the reef.

Predictably, the results show that reef sound pressure reduces at greater distances from the reef, but the level of reduction was found to vary at different times of the day.

“Given that sound production on many reefs including Hawaiian reefs has been shown to peak at dusk and dawn, it is possible that these times of day represent a reliably higher sound level that larvae may use to orient,” the researchers explain.

While the hearing ability of fish larvae is yet to be tested, the researchers believe there’s more to reef location than these low-level sounds.

“It’s possible that larvae are still able to use chemical signals from other animals to locate the reef, or maybe can read the currents to move towards shore,” says Kaplan.

“Based on this data, it seems unlikely that they’d be able to use sound to find the reef. That was a surprise to us.”

The researchers point to past studies, which showed fish can be lured to parts of the reef by soundscapes played through underwater speakers. Given these findings, volume levels would need to be high for this to work efficiently, Kaplan explains.

“You’d have to boost sound levels by quite a bit to get the response you want, but it could be one solution […] If we can figure out the hearing threshold of each species’ ability to sense particle motion, we might be able to amplify that motion to make it audible to them at a distance.”