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Reef halos and soundscapes may provide a window into reef health

Reef halos and soundscapes may provide a window into reef health

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Coral reef halos, clearly visible from satellite imagery, may provide a window into reef health around the world, and listening to the sound of reefs may help track the state of their health.

Coral reef patches surrounded by halos of sand produced by herbivore members of a patch assemblage (fish, mollusks, and sea urchins) who remove sea grass and algae
Coral reef patches surrounded by halos of sand produced by herbivore members of a patch assemblage (fish, mollusks, and sea urchins) who remove sea grass and algae

Coral reef halos, also known as grazing halos or sand halos, are bands of bare, sandy seafloor that surround coral patch reefs. Fish and invertebrates hide in a patch of coral and venture out to eat algae and seagrass that cover the seabed.

They only dare stray a certain distance from cover in any direction for fear of predators, which explains why the cleared area in the midst of the lagoon’s algal meadows is circular.

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Fish and invertebrates that eat seagrass and algae may not venture far out from the coral providing them with cover. Photo: Scott Bennett

But many marine scientists wondered how some halos could grow so large if herbivores were scared to venture very far from the protection of the reef.  Elizabeth Madin, assistant research professor at the Hawai'i Institute of Marine Biology and her colleagues filmed the behaviour of fish around reefs with halos, they found herbivore behaviour alone failed to account for the barren sand. 

When researchers monitored fish activity at night they found an explanation: fishes that root through the sand, turning up and eating invertebrate prey. All that rooting around dislodges the algae growing on the sand, expanding the halo.

Indicators of reef health

Understanding the full suite of mechanisms governing halo formation and maintenance may enable us to use them as proxies for species interactions. Given the global extent of halos, their role in affecting sedimentary carbon storage, and their relationship with marine reserve existence and maturity, they may ultimately serve as globally relevant indicators of coral reef ecosystem functioning and health.

 

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Juvenile three-spot damselfish.Damselfish makes croaking sounds

Listening to corals

Monitoring the sounds of coral reefs can also serve as a non-intrusive, inexpensive and efficient method for tracking the state of their health – and for planning better conservation interventions in the long run.

The crackle and clicking sounds we hear underwater are from life unfolding. Fish chat with each other or graze, and hard-shelled creatures scrape against the surfaces.

New research shows that degraded coral communities do not sound as crackling and vibrant as healthy ones because the reduced biodiversity means less activity, so you can in effect judge the health of a reef by its decibel level.

Audio file
Audio file

While healthy reefs give readings 0f 2-8 kilohertz, less diverse reefs tend to be above 12kHz, as they become overpopulated with macroalgae, which create bubbles of oxygen that float to the surface and pop, making a specific high-frequency sound.

This type of data collection has the potential to be highly cost-effective, cover a large area and be a constant source of information, as opposed to intermittent diving surveys.

Katey Lesneski, director of restoration science at Coral Vita,

Sources
The University of Chicago Press Journals
Web Ecology
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Press releases from Divers Alert Network (DAN)