News•December 16, 2014

Acidic Oceans Could Quiet Coral Reefs

Doug E. Fresh may have some competition in the beatboxing arena from unlikely source. It’s not from some underground phenom but rather an underwater rising star, or well, fish.

Take a listen to this beat laid down by a croaker fish off the coast of Indonesia. A rhythmic thumping provides the beat for an otherwise ambient ocean noise track.

“This one has just stuck with me,” said Julius Piercy, a PhD candidate studying underwater acoustics at the University of Essex, who discovered this particular virtuoso.

Piercy has been recording the sounds of fish and crustaceans at tropical coral reefs around the world. The thumps, whistles, grunts and snaps of those reef inhabitants are more than just fodder for multi-platinum recordings. They give Piercy and other marine scientists a snapshot of reef health and biodiversity that can be done at a fraction of the cost of traditional reef monitoring.

Piercy’s recent research shows that the future ocean may be a lot quieter than our current if overfishing and ocean acidification continue to take a toll on the reefs that support the symphony of life.

The research looked at, well listened to, a dozen tropical reefs across the Pacific and Indian Oceans with varying degrees of health. Healthier reefs were louder and sound propagated further from them into the open ocean.

Listening to the ocean isn’t a new concept. Scientists have been monitoring underwater sounds for decades, in part because sound propagates so efficiently underwater. But in the past 10 years, scientists have started exploring how sonic cues influence fish behavior.

“The overall sound level was significantly lower in the impacted reefs. So much so that the reef detection zone for these little larvae, fish larvae and crustacean larvae is reduced by tenfold,” Piercy said.

In other words, fish that migrate away from reefs during adolescence might have a harder time finding their way home. If they stay lost in the open ocean, they’re more likely to die, further impacting reef health.

Credit: U.S. Fish and Wildlife Service/flickr

While the degraded reefs Piercy looked at were mostly ravaged by overfishing, climate change poses a global long-term problem for reefs. Warmer and more acidic waters can eat away at reefs and change the habitats that support specific species of fish, crustaceans and corals. That, in turn, can take away tourism dollars and food security for communities that rely on reefs. Recent research also shows that barrier reefs can provide cheap and effect storm surge protection.

“Warming and ocean acidification don’t happen overnight and it may be that some of the ecosystem shifts they facilitate will take years to become visually apparent,” Simon Freeman, a postdoctoral fellow with the American Society of Engineering Education who has also done a series of underwater recordings, said.

While scientists have recently created a baseline on ocean acidification, monitoring systems for impacts on reefs is inadequate. The current best way to assess reefs is flippers in the water with scientists scuba diving around reefs with clipboards in hand to take a reef census. It’s a great way to get information but painstakingly slow and costly.

Google Street View technology is also helping build a new reef database, but listening to reefs — which Freeman stressed was a completely unobtrusive method — offers a complementary method to keep track of reef health on a continuous basis, including at night or rough weather when dives aren’t possible.

“By no means should sound monitoring be a substitute for other methods. Visual surveys are by far the most accurate method,” Piercy said. “But with sound, we can assess a large number of reefs in a really rapid time. You only need to drop a mic over the side of a boat for five minutes and go onto the next place.”

Freeman also said the price made sound recordings a particularly appealing complement to visual surveys.

“They’re collecting a lot of data. Flash memory and lithium ion batteries have become very cheap. Five years ago, the cost of a memory card was very high. Now it’s practically nothing,” he said.

By building a sonic database, scientists can track long-term changes to reefs and respond to any sudden shifts, such as big coral bleaching events that can occur when ocean waters suddenly warm. Piercy also said future research could look into whether changes in the underground chorus might actually repel certain fish, further altering the way the ocean looks. And sounds.