Pacific corals and fish possess the ability to smell a bad neighborhood and therefore, avoid settling in damaged reefs, according to a study by the Georgia Institute of Technology. The researchers said that damaged coral reefs release chemical signals that repel young coral and fish from settling in degraded habitats.
This is the first study to determine coral larvae's ability to differentiate between healthy and damaged reefs. The findings suggest that defining over-fished coral reefs as marine-protected areas may not be enough to help these reefs recover because chemical signals continue to drive away fish and coral.
"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 Danielle Dixson, an assistant professor in the School of Biology and the study's first author, in a press 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," said Mark Hay, a professor in the School of Biology and the study's senior author.
For the study, the researchers examined three marine areas in Fiji that had adjacent fished areas.
The researchers found that juveniles of both corals and fish were repelled by chemical cues from over-fished, seaweed-dominated reefs but were attracted to signals from coral-dominated areas. Both coral and fish larvae favoured chemical cues that indicated a healthy habitat and avoided those that signalled a degraded habitat.
"Not only are coral smelling good areas versus bad areas, but they're nuanced about it," said Hay. "They're making careful decisions and can say, 'settle or don't settle.'"
In the first experiment, the researchers then subjected 20 young fish of 15 different species to water from healthy and degraded habitats. They found that each of the 15 species showed up to an eight times preference for water from healthy areas. In the case of coral larvae from three different species, they too likened water from healthy habitat five-to-one over water from the degraded habitat.
In the second experiment, the researchers soaked different corals in water and examined the behavior of fish in that water. Both fish and coral larvae were greatly attracted to the cues of the common coral Acropora nasuta that those from hardy corals. This is because Acropora nasuta is primarily found in healthy habitats.
In the third experiment, the researchers soaked seaweed in water and observed the behaviour of fish and coral in that water. Cues from the common seaweed Sargassum polycystum lowered the attractiveness of water to fish and coral larvae by up to 86 percent and 81 percent, respectively.
"Corals avoided that smell more than even algae that's chemically toxic to coral but doesn't bloom," Dixson said.
The finding is published in the journal Science.
