Pacific Corals and Fish Can Smell Good and Bad Reefs

Pacific corals and fish can both smell good and bad reefs, according to a study based in Fiji.

Damaged coral reefs emit chemical cues that repulse baby coral and fish, discouraging them from settling in the degraded habitat. The study shows for the first time that coral larvae can small the difference between healthy and damaged reefs when they decide where to settle, according to Georgia Institute of Technology press release.

Coral reefs are declining all over the world, according to the study. Overfishing is one of the reasons, which depletes the herbivores fish that removes the seaweed that forms in damaged reefs.

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

Researchers were able to show how chemical signals from seaweed repel young coral from settling in an area dominated by seaweed.

Once a coral reef has decayed and seaweed takes over, stopping fishing in the area may not be enough to bring the coral back, according to the release.

"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," said Dr. Danielle Dixson from the Georgia Institute of Technology, the study's first author, according to the release.

Dixson's research was published in the journal Science. The research was sponsored by the National Science Foundation (NSF), the National Institutes of Health (NIH), and the Teasley Endowment to Georgia Tech.

For the study, three marine areas were examined in Fiji that had adjacent fished spots. The country has established no-fishing areas to protect healthy habitats and also to allow damaged reefs to recover, according to the release.

The study, for the first time ever, tested coral larvae in a method that has been used previously to test fish. Researchers found that young coral have strong preferences for odors from healthy reefs.

"Not only are coral smelling good areas versus bad areas, but they're nuanced about it," said Mark Hay, a professor in the School of Biology at Georgia Tech and the study's senior author, according to the release. "They're making careful decisions and can say, 'settle or don't settle.'"

The study proved that young fish have a preference for water from healthy reefs.

Researchers put water from healthy and degraded habitats into a flume that allowed fish to choose to swim in one stream of water or the other. Preferences of 20 fish each from 15 different species were tested and researchers found that regardless of species, family or trophic group, each of the 15 species showed up to an eight times greater preference for water from healthy areas.

Researchers then tested coral larvae from three different species and found that they preferred water from the healthy habitat five-to-one over water from the degraded habitat.

Chemical cues from corals also swayed the fishes' preferences, the study found.

Researchers also soaked seaweed in water and tested fish and coral preferences. Cues from the seaweed Sargassum polycystum reduced the attractiveness of water to fish by up to 86 percent compared to water without the seaweed chemical cues, according to the study.

 Chemical cues from the seaweed decreased coral larval attraction by 81 percent.

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

A small amount of intervention at the right time and place could jump start the recovery of overfished reefs, Hay said. This 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, according to the release.

"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 said.