Written By Sofia Perez

One of the main ingredients needed to easily annoy someone is noise, such as a fly buzzing in your ear. 

However, noise can also have the opposite effect, whether through the soothing energy of a Bob Marely song or the increased concentration and focus associated with ‘white’ noise. 

But we are not the only ones with an affinity for sound. In fact, across the animal kingdom there is a whole range of animals who share this love-hate relationship. Less widely known is the value of sound underwater, where it travels quicker and farther than on land. In fact, marine animals rely on sound heavily for sensorial signaling, which influences all facets of aquatic behavior and ecology. 

Like the sounds of a city, the sounds of the oceanscape are diverse and change on a daily as well as seasonal basis. In 2018, a team of researchers led by Simon & Lauren Freeman set out to unmask the source of a mysterious ringing noise coming from reefs. During the study, researchers recreated the conditions for the ringing noise in a tank using a species of Hawaiian algae called Salicornia gracilaria and discovered that the noise came from the release of oxygen & nitrogen gas bubbles produced during photosynthesis. The sound came from the miniscule movements of the bubbles as they relaxed into their spherical shape. 

These sounds and other sounds of the reef aren’t unique to S. gracilaria and could therefore be an important indicator in monitoring the decline of reef ecosystems worldwide. Much like a city, the more silent a reef, the fewer fish and other animals present. Just as fewer people walking around cities during COVID has caused the economy to suffer, fewer fish in a reef causes the exchange of energy, oxygen, and carbon dioxide to come to a staggering halt. 

Scientists from this 2018 study believed the sound produced by coral’s symbiotic algae could be used to estimate algal abundance(a proxy for photosynthetic activity) in a less expensive and time-consuming fashion. Without algae to carry out photosynthesis, coral polyps do not receive the energy needed to grow and reproduce, causing them to bleach and eventually die. Therefore, with a way to efficiently measure photosynthetic activity, sound began to be part of the solution to marine habitat loss. 
But fast-forward three years to 2021 and the eerie silence of ghost-town coral reefs is a common occurrence. (Listen to a healthy vs. bleached reef.) While reef sounds vary with the changing phases of the moon and seasons, human sounds are also becoming increasingly regular among the ocean soundscape. Ever since the Industrial Revolution, sounds from fishing, shipping, infrastructure, and development have resulted in the silencing or drowning-out of sounds from marine animals. In 2021, a global team of researchers found strong evidence that marine life is negatively impacted by noise, as it disrupts the behavior, physiology, and reproduction of ecosystems as well as causing an increase in mortality.

Thus, the problem of irregular noise is not only annoying to people who are trying to concentrate, but also to animals all over the ocean who rely on sound to guide their behaviors. The larval phase of many fish and invertebrates rely on information about habitat quality from the acoustic signature of the habitat, combining visual and chemical cues that allow them to recognize the optimal reef habitat to settle on. 

On the other hand, degraded reefs are quiet, meaning fewer fish are attracted to them.This sets off a negative feedback loop in which degraded reefs stay degraded due to their silence. To address this, a team of scientists from UK & Australia used underwater loudspeakers to try and entice fish back to dead coral reefs in a process called “acoustic enrichment”. The results were jaw-dropping: twice as many fish arrived and stayed compared to patches where no sound was played. It’s like a restaurant with live music- you walk in because of the sound, but stay for the food. 

Nonetheless, the complex struggle for a healthy soundscape in the ocean does not stop here. There is still much room for growth within science and policy-making alike. Most people do not realize the effects of human sounds on marine ecosystems or the intricate link between acoustic cues and behavior. Therefore, policies are needed on a global scale to mitigate the effects of too much noise, and more research must be done to understand how sound-enrichment can be used to help. 

Similar to following a balanced diet, it is important not only to pay heed to what sounds are in the sea, but the proportions in which they are heard. We must not silence the trail of sounds fish larvae use to settle. We must not allow the reefs to fall silent. 

Bibliography

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Cover Photo: https://www.goodnewsnetwork.org/reviving-damaged-coral-reefs-with-underwater-speakers/