Sound Could Be the Key to Saving Coral Reefs
- Heather West
- 1 day ago
- 8 min read
Hello beautiful readers and welcome to Ocean Simplified, your place for ocean science. Grab a coffee, tea, or snack of choice, and let’s uncover the secret to saving coral reefs. Our journey begins on Lizard Island, a remote island on the northern Great Barrier Reef, where scientists placed underwater speakers playing healthy reef sounds at different underwater sites. The results? Read below to find out more.
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Why do coral reefs need saving?
Climate change. The words have weaseled their way into our news feeds for years, but what exactly is it? And why exactly does it matter? The answer isn’t so simple, but I’ll do my best to make it so. First, let me point out the difference between weather and climate. Weather is what you wake up to everyday, while climate is an average long term predictable pattern of weather. Over the last several decades, the influx of greenhouse gases have been warming the earth’s climate past natural temperatures, stressing coral reefs.
You may be wondering how atmospheric gases have any sort of effect on coral. Don’t worry, I was confused by this too. Atmospheric gases impact coral reefs in two main ways.
Ocean Acidification
Ocean acidification is an overlooked piece of climate change, but an important one. But how exactly does the ocean become acidic? The answer lies in Henry’s law. Henry’s law basically says that the atmosphere and the ocean must maintain a constant ratio of CO2 with each other. So, as we pump more CO2 into the atmosphere, a portion will dissolve into the ocean to maintain the ratio. When CO2 dissolves into seawater, it forms carbonic acid, which breaks down to bicarbonate and releases excess hydrogen ions. These hydrogen ions are positively charged particles. Charged particles always want to become neutral, so they bind with carbonate. In the ocean, carbonate makes up shells and coral skeletons. These excess hydrogen ions are known culprits for breaking down shells and corals, effectively disintegrating hard structures in the sea.
Coral Bleaching
As CO2 crowds the atmosphere, the temperature rises. This is because CO2 and other greenhouse gases trap heat leaving the earth. This trapped heat was supposed to be reflected back into space, but instead got stuck in our atmosphere like a fly in a spider’s web. Greenhouse gases are like the filaments to that web. The more filaments, the more flies get trapped, and the more heat gets trapped. Because the ocean is in direct contact with the air, the heat from the air moves naturally into the water, warming it up. In fact, the upper ocean has absorbed over 90% of atmospheric heat leading to changes of 0.9-1.45 degrees Celsius.
While these may seem like minor changes in temperature, I invite you to recall the last time you had a fever. The average human body temperature is 98.6 degrees Fahrenheit or 37 degrees Celsius. A fever of 104 degrees Fahrenheit is only 38.8 degrees Celsius, a change of only 1.8 degrees Celsius. Remember how poorly you felt. That is the new normal for our oceans.
This “ocean fever” causes stress to corals. Major stress. So much stress that they kick out their cooks and starve. When the water becomes too warm, corals expel zooxanthellae algae from their skeleton. This expulsion, called coral bleaching, leaves behind a stark white skeleton. These white corals are not dead, but they are starving.
Where Sound Comes in
The paper we’re covering today examines how sound can help rebound reefs that underwent significant bleaching in 2015-2016. During this bleaching event, over 60% of Lizard Island’s coral bleached resulting in massive casualties. This particular site remains near and dear to my heart, as I visited here in early 2020, right before COVID.
Sounds of a Coral Reef
If you’ve swam on a healthy reef, you’ve probably noticed it’s loud. In fact, the reef is full of noises we can't even hear. Check out this video if you're curious about the secret sounds of coral reefs!
I first saw this video in my fish biology class, and it truly changed my perception of coral reefs. Our ears only pick up the snapping shrimp, but hydrophones have captured more sounds than humans can hear. The reef sings with life.
Why Sound Matters
What you may not know before reading this paper and what I didn’t know, is that the sound of a healthy reef actually recruits more juvenile fish. Larval fish typically ride the currents until they reach a settlement phase and find a reef to call home. As fish are settling, they want to pick a healthy reef just like we want to pick a house with a proper structure and a safe neighborhood. While humans and many land creatures use vision as their primary sense, fish use chemicals and sounds. Healthy reefs with strong sounds and smells attract more fish.
But okay. Let’s slow down. Healthy reef sounds attract more fish, but how does attracting fish affect coral?
This study cites two additional studies pointing to the fact that degraded reefs with more fish have a higher chance of recovery. So essentially, the thought process of these scientists goes like this:
If healthy reef sounds = more fish and more fish = higher chance of recovery, then healthy reef sounds = higher change of recovery
Honestly, brilliant.
But did their hypothesis work? Can sound really help save coral?
Yes! Let’s dive deeper. The study focused on damselfish recruitment. The authors noted that they chose damselfish because they are highly abundant, easy to spot, and can be visually surveyed accurately with minimal disturbance to the developing fish community. After 40 days, twice as many juvenile damselfishes settled on reefs that played sound than on reefs with no sound.
Diving deeper: The Core Results
As a result of placing underwater speakers playing healthy reef sounds for 40 days at various sites, the results showed significantly more herbivores, omnivores, planktivores, invertivores, and piscivores on reefs with sound than on reefs without. The results showed no significant differences between the control groups (one control group had no speakers with no sound, another had speakers placed but did not play sound). Scientists noted the increase in multiple tropic guilds is meaningful in establishing fish communities needed to make colonization sustainable, boosting the reef long term.
Not only was there a 2x increase in juvenile fish in reefs with sound, these reefs also carried a 50% greater species richness than both control groups. Also, reefs with sound had greater Shannon diversity than those with no speakers or dummy speakers. There was no significant differences between the groups with the dummy speakers or no speakers, but groups with sound showed a significant difference from both other groups.
What's the best sound to play?
This study was an introduction to whether sound would even be useful to recovering reefs. The scientists first needed to find out if sound works before they find out which sound works best. Based on these results alone, scientists are not sure which sound is best to play on the reef, but they do say that healthy reef noises are better for recruitment than degraded reef sounds or white noise. This also suggests that specific sounds pull fish to the reef, not just any sound in general.
Need the full details? Here’s how the study happened
For the science method gurus, allow me to set the scene for you. In the northern great barrier reef lies Lizard Island. Two years before this study, over 60% of the surrounding corals had bleached, putting this reef in the perfect window for recovery. Scientists seized this unfortunate opportunity and set up three different groups: (1) sites with speakers playing healthy reef sounds, (2) sites with dummy speakers not playing sounds, and (3) sites with no speakers and no sound. Group 2 is especially important because an increase in settlement due to the presence of an object is ruled out when sites with a silent object show no difference in results from sites with no object. This means that the sound is making the difference, not the presence of an object.
If you’re thinking that these test sites are too close to each other, you’d be right on track. After all, wouldn’t the sound from acoustic sites travel to silent sites and impact the study? The scientists thought of that too, so they found the distance at which the sound volume significantly decreased and placed the test sites according to that. Check this out:
According to this figure, scientists measured the sound pressure level and particle acceleration coming from the active speakers. By the time scientists were 50 meters away from the speakers, there was hardly any sound level or even particle movement. Measuring sound pressure level and particle acceleration is important because fish perceive sound slightly differently than humans, so the simple act of listening underwater would not have been enough. Using this data, scientists concluded that the sites needed to be at least 100 meters apart from their nearest neighbor in order to be truly independent.
The sound played by the scientists:
The sounds used in this study were collected at night from a healthy reef in the middle of the study site in November 2015, before the mass bleaching occurred. The sounds were collected overnight because reef fish settlement is mostly a nocturnal activity (something I did not know before reading this paper).
Setting up the sites:
Scientists in this study gathered coral rubble (skeletons) from a designated site. They collected an even volume of branching, tabular, and spherical rubble types to mimic the natural scapes of reef fish. Surveys were visually conducted throughout the 40 days for damselfish, but at the end of the study, the entire rubble site was dismantled and every fish was sampled.
Is this a credible study? My critiques
If you’re new here, ocean lovers, first welcome. Second, this is a question we’ll always ask and walk through together because some studies simply are not well done. Here we'll go through some of the methodology and critiques about this study, the figures, or the scientific objective as a whole.
I personally love this paper. It’s been one of my favorites for years which is why I chose it as my first writing. I believe it's easy to follow, relevant, and the figures are well done.
My only critique is dismantling the test sites. If the point is to truly help coral reefs recover, why increase the amount of fish to help the reef and then remove them? The fish were released alive onto neighboring reefs, but I’d be curious to find a paper about what this means for fish. Do they survive? Do they assimilate or are they pushed out until preyed on? I find that disassembling the sites is rather unethical and unhelpful to an already struggling reef. My thoughts overall, though, remain positive.
Final Thoughts
Sound could be the key to saving coral reefs! By using underwater speakers to attract fish, scientists have found an innovative way to boost reef restoration. Coral reefs continue to face threats from climate change and pollution so minimally invasive efforts like this could offer serious hope for coral. I hope to see speakers integrated into more reef restoration projects, helping reefs thrive once again. Thanks for reading, and I'll see you next week!
About this paper:
Year published: 2019
Journal: Nature Communications
Authors: Timothy A.C. Gordon, Andrew N. Radford, Isla K. Davidson, Kasey Barnes, Kieran McCloskey, Sophie L. Nedelec, Mark G. Meekan, Mark I. McCormick, & Stephen D. Simpson
Place Studied: Lizard Island, Australia
READ THE FULL PAPER HERE
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