The Science of Sound for Reef Regeneration

Dr Anjani Ganase reviews a recent study where healthy reef sounds promote recovery and regeneration on degraded reefs

 In a previous story, we discussed how noisy a coral reef is. (See  https://wildtobago.blogspot.com/2019/04/the-sounds-of-life-underwater.html) The clicks and snaps of shrimp and crabs, and the grunts and the sighs of the many fish on the reef, may sound like white noise to us, but is easily deciphered by reef residents. Imagine walking through a city and hearing all the different sounds. The same way we differentiate sirens, street signals, chatter, construction, the  fish can distinguish the noises on the reef.

Being able to hear these reef sounds is important for visitors navigating the reef; and very important for the recruitment of new coral and fish life to replenish stocks. Fish and coral larvae are attracted to the sounds that resemble a healthy bustling underwater reef city.  Other areas that are more degraded with fewer reef residents will have limited sound cues to draw the fish to the reef. Other visual and chemical cues may also be lacking, or even act as a deterrent to juvenile fish looking for coral habitats to reside on. 

Fish on the Flying Reef Tobago. Photo by Jonathan Gomez. The Maritime Ocean Collection

In the recent years, coral reefs around the world have experienced unprecedented rates of coral loss because of global warming, which have left many reefs in a depleted state. The fear of many scientists is that the rate of recovery may be too slow, as we predict a future that will consist of more severe and frequent bleaching events. So to encourage recovery and building coral reef resilience to the impacts of climate change, scientists scramble in search of artificial methods that may facilitate more rapid recovery of corals, as we depend on these ecosystems greatly for our livelihoods.

One group of scientists think they have discovered a method that may be able to assist in faster recovery.  On remote Lizard island in the northern Great Barrier Reef, scientists from the University of Exeter and the Australian Institute of Marine Science have experimented with a simple, ingenious method for attracting reef fish to degraded reefs by playing sounds of healthy reefs. Reef fish are key in the recovery process of coral reefs following a disturbance, as they carry out vital  roles, such as grazing and predation that will assist in coral recruitment. Sounds of health reefs tend to be louder; and feature a cacaphony of reef sounds from a wide range of marine organisms – fish, invertebrates, everything that moves and interacts on a reef.

Using the isolated patches of reef around of Lizard Island, many of which have been severely impacted by the major coral bleaching event in 2016, marine biologists inserted underwater speakers on patches of degraded reefs and played sounds of health reefs over a period of forty days. During this time, they observed the number and types of fish within in the vicinity of the speaker. They observed that the fish populations increased and stabilised on the test reefs over forty-day periods. Of the fish communities attracted to the acoustically enhanced reefs, most of them were juveniles and spanned the many different food groups – fish that feed on algae, plankton, invertebrates and even other fish.  The abundance of damselfish, for example, was twice that compared to reef patches without speakers. It is not understood how long the recruits continue to reside on the reef if the speakers are removed, but the extended time of recruitment may improve the sounds of the degraded reef to the point of positive feedback; where the speakers are no longer needed.

What if reef managers can use this to speed up recovery of coral reefs following a disturbance event? How would this be implemented and what are other considerations?

The use of this method may only be advantageous in an already well-established marine park system with well regulated marine fisheries. In the experiment, Lizard Island is part of the larger Great Barrier Reef (GBR) system, which is well connected and over thousands of kilometres in length; it is also well managed and protected. Owing to this management, surrounding healthy reefs can assist in the recovery of areas that have been impacted by disturbance. Lizard Island is also extremely remote and has relatively low impacts of local disturbances – water pollution, illegal and unregulated fishing and dredging etc. Therefore recovery of the patch reefs was always likely, albeit at a slow pace.

Let us have a look at the Caribbean and Tobago’s coral reefs that have much more limited recovery potential, owing to the long history of over-fishing and degradation. This is because of the close proximity of the reefs to our coastal communities and cities and human activities on the water for economical and recreational use. Think about Buccoo Reef and Bon Accord Lagoon on a weekend: jets skis, glass bottom boats, swimming, and kayakers create much traffic in the marine area. How effective will a strategy of using acoustic reef sounds be in a place that already has a lot of noise pollution? Think about where the recruited fish would come from? If surrounding reefs are not doing well either, does this mean that the fish are already adapting to the sounds of degraded reef systems or will there simply not be enough juvenile fish to recruit? While experiments in acoustic rehabilitation seem to be a neat method for improving reef recovery, this must occur in a reef system that already has regulation and management.

Reference

Gordon, T. A. C., Radford, A. N., Davidson, I. K., Barnes, K., McCloskey, K., Nedelec, S. L., ... & Simpson, S. D. (2019). Acoustic enrichment can enhance fish community development on degraded coral reef habitat (dataset).