Researchers discovered that young fish and corals can detect good and bad reefs through "chemical" smells released by seaweed that invade dying reefs.

Research involved doing tests on two water samples in the lab, one which reeked of seaweed, while the other had the "scent" of healthy coral.

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The samples were collected from heavily-fished areas with a lot of seaweed and marine-protected reefs that contain healthy coral, said Dr Danielle Dixson from the Georgia Institute of Technology who led the said Fiji-based study.

First, young fish were tested. Fifteen species of fish were placed in special chambers that allowed them to choose between the water samples.

What scientists found was that the fish turned away from the water that smelled of seaweed, while they were drawn to the "healthier smelling" water.

The fish spent 80 percent of their time on the water sample which "smelled" of healthy coral.

Scientists, however, were surprised to find out that doing the same test on young coral yielded similar results. Corals swam to healthier water by propelling themselves with their little hairs or cilia.

"For fish, people weren't too surprised that it happened, even as babies. They have tails, they have a nose. But for corals... they thought we were nuts," said Prof. Mark Hay, the study's senior author.

Researchers then concluded that in order to repopulate reefs that were once depleted, controlling the presence of seaweed is a must. It seems that simply reviving a reef is not enough if seaweed has invaded.

Prof Steve Widdicombe, head of science in Marine Life Support Systems at the Plymouth Marine Laboratory, said the groundbreaking study will prove to be vital in the protection and rehabilitation of coral reefs.

"It was very interesting to see this idea that if we're going to bring these habitats back, just leaving them alone might not be enough. We might have to make some active steps," he noted.