Music to our ears: noisy reefs indicate good health

By Danielle Moloney 

Edited by Sara Gagliardi 

Introduction

Millions of people around the globe rely on coral reefs for food, income, recreation, and a host of other services. As climate change and other anthropogenically influenced factors continue to batter sensitive coral reefs, environmentalists strive to prevent and undo damage by conserving these natural ecosystems. Conservation strategies often focus on increasing both coral growth and coral cover, but often do not take into account the other measures that indicate ecosystem health. For example, countless other marine fishes and invertebrates need to be present in order for the entire ecosystem to be considered “healthy”. With this in mind, researchers from the University of Exeter set out to compare how acoustics differ between healthy and degraded reefs. The findings from their study help provide another means through which reef health (and conservation success) can be measured. 

Methods 

In order to test the hypothesis that reef soundscapes increase in richness and diversity as a reef recovery progresses, scientists monitored underwater acoustics in three distinct reef categories: healthy, degraded, and restored. Subject reefs were categorized using prior research conducted by the Mars Coral Reef Restoration Project where evidence of blast fishing (an extremely harmful fishing method through which dynamite is used on reefs in order to access fish) is highly linked to reef health. Healthy reefs showed no signs of current or former blast fishing, degraded reefs showed significant signs of blast fishing, and restored reefs were between one and three years out from the start of restoration efforts post-blast fishing. 

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Figure 1. A map depicting the study sites used for this project, all of which are located in Indonesia. The color-coded key in the top right section of this figure shows which of the reefs were considered healthy, restored, or degraded. (Figure courtesy of Lamont et. al 2021). 

Two study sites from each of the three categories were recorded. Each site was recorded in one hour periods, on several occasions. The authors made sure to record from the same spot under varied circumstances, such as time of day or lunar phase, to ensure an accurate measure (and randomization) of bioacoustics in the area. They then used a combination of manual and automated analysis to break down their findings. The study was based in Indonesia for two reasons: 1) reefs in this area tend to be heavily impacted by anthropogenic stressors, and 2) one of the largest reef restoration projects occurs in this region, allowing the research to test whether or not the restoration efforts has yielded any positive results. 

In order to obtain the recordings, a snorkeler placed a hydrophone with a built-in recording device 0.5m above the seafloor, centered in a 10x10m grid (which was also observed for coral cover). A piece of tape was affixed to the location of the hydrophone to ensure that it was always placed in the same spot for subsequent recordings. The snorkeler placed the hydrophone at least 10 minutes before the start of the recording and swam at least 500m away from the recording device for the hour-long recording period in order to maintain the natural soundscape of the habitat. After one hour was up, the hydrophone was retrieved so that the recording could be further analyzed. 

What did they find? 

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Figure 2. Spectrograms depicting the sounds recorded from various samples. Different calls were placed into categories based on their resemblances. Did you know that a fish could knock, whoop, or laugh? The x axis shows time over which a call was recorded (in seconds) and the y axis shows the frequency of the call (kHz) as well as the amplitude. (Figure courtesy of Lamont et. al 2021). 

Results indicated a clear difference between healthy, restored, and degraded reefs. While healthy and restored study sites exhibited similar levels of phonic richness, degraded reefs showed a significantly lower amount of rich sound production. This trend held up when different time of day samples were compared, suggesting that the pattern observed is not simply due to external factors. These factors could be due to lunar cycles, time of day, or even an error in the equipment picking up anthropogenic sound such as a motor boat passing or a low flying plane. This suggests that the observed pattern of reef health correlating to noisiness is truly based on ecosystem health. 

Conclusions 

An acoustic study (as compared to a visual study related to fish community composition or coral cover) allows for a much broader range of observation. Acoustic findings can be used to strengthen other measures of conservation success. The authors go on to point out some of the other benefits that these findings provide. For example, certain marine species may partially decide on where to settle based on soundscapes, so restoring acoustic functionality is an important part of improving degraded reefs. The higher amount of phonic richness correlates to a higher amount of species diversity (as different species will produce different sounds and contribute to a more varied soundscape), hence acoustic recordings can be used as a proxy measurement for species richness following restoration efforts. Acoustic recordings also have the ability to fill gaps that other research cannot, such as recording nocturnal species, allowing observation over extended periods of time (especially pre-, mid-, and post-restoration to assess how successful the efforts have been). 

Overall, this research highlights the importance of considering all facets of ecosystem health when attempting to restore coral reefs. The continued conservation of these vulnerable ecosystems is an important part of our future. 

Read the full study from Lamont et. al here

Please contact the author with any questions: dmoloney@fandm.edu

Featured image courtesy of Popular Science.

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