Damaged coral reefs emit chemical cues that repulse young coral and fish, discouraging them from settling in the degraded habitat, researchers found.
    
The study showed for the first time that coral larvae can smell the difference between healthy and damaged reefs when they decide where to settle.
    
The findings suggest that designating overfished coral reefs as marine protected areas may not be enough to help these reefs 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 recognising the degraded area as habitat," said Danielle Dixson, an assistant professor in the School of Biology at the Georgia Institute of Technology in Atlanta, and the study's first author.
    
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.
    
Juveniles of both corals and fishes were repelled by chemical cues from overfished, seaweed-dominated reefs but attracted to cues from coral-dominated areas where fishing is prohibited.
    
Both coral and fish larvae preferred certain chemical cues from species of coral that are indicators of a healthy habitat, and they both avoided certain seaweeds that are indicators of a degraded habitat.
    
The study for the first time tested coral larvae in a method that has been used previously to test fish, and found that young coral have strong preferences for odours from healthy reefs.
    
The study showed that young fish have an overwhelming preference for water from healthy reefs. The 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.
    
The researchers tested the preferences of 20 fish each from 15 different species and 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.
    
The 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.
    
The study was published in the journal Science.