25/08/2014: Seaweed vs coral on the sea floor

Unhealthy coral reefs give off chemical cues that repulse young coral and fish, discouraging them from moving into the neighborhood.

Coral reefs are declining around the world. Overfishing is one cause of coral collapse, depleting the herbivorous fish that remove the seaweed that sprouts in damaged reefs. 

Once seaweed takes hold of a reef, a tipping point can occur where coral growth is choked and new corals rarely settle.

A new study shows how chemical signals repel young coral from settling in a seaweed-dominated area. The smell of water from these damaged reefs also scares away young fish, researchers say.

The findings suggest that designating overfished coral reefs as marine protected areas may not be enough to help them recover because chemical signals continue to drive away new fish and coral long after overfishing has stopped.

“If you’re setting up a marine protected area to seed recruitment into a degraded habitat, that recruitment may not happen if young fish and coral are not recognizing the degraded area as habitat,” says first author Danielle Dixson, assistant professor in the School of Biology at Georgia Institute of Technology (Georgia Tech).

Published in the journal Science, the study examined three marine areas in Fiji that had adjacent fished areas. The country has established no-fishing areas to protect its healthy habitats and also to allow damaged reefs to recover over time.

Negative seaweed
The researchers also soaked seaweed in water and tested fish and coral preferences in that water. Cues from the common seaweed Sargassum polycystum, which can bloom and take over a coral reef, reduced the attractiveness of water to fish by up to 86 percent compared to water without the seaweed chemical cues. Chemical cues from the seaweed decreased coral larval attraction by 81 percent.

“Corals avoided that smell more than even algae that’s chemically toxic to coral but doesn’t bloom,” Dixson says.

Future work will involve removing plots of seaweed from damaged reefs and studying how that impacts reef recovery.

A minimum amount of intervention at the right time and the right place could jump start the recovery of overfished reefs, Hay says. That could bring fish back to the area so they settle and eat the seaweed around the corals. The corals would then get bigger because the seaweed is not overgrown. Bigger corals would then be more attractive to more fish.

“What this means is we probably need to manage these reefs in ways that help remove the most negative seaweeds and then help promote the most positive corals,” Hay says.
The National Science Foundation, the National Institutes of Health, and the Teasley Endowment to Georgia Tech supported the research.

Read more HERE.

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