Australasian Science: Australia's authority on science since 1938

Dead coral spells trouble: fish ‘don’t smell danger’

By Lydia Hales

Dead coral is affecting chemical alarm cues, which could be the death of fish too, a new study shows.

An international team of reef scientists found damselfish have stopped responding to chemical alarm cues in areas of dead coral on the Great Barrier Reef, off eastern Australia.

Those alarm cues are critical to survival: they are the smells fish normally rely on to know danger lurks nearby.

Olfaction is a key tool fish use to learn to recognise predators, according to reef scientist Mark McCormick of James Cook University (JCU) and the Australian Research Council Centre of Excellence for Coral Reef Studies.

McCormick, a world expert on the early life history of fish, says usually fish snared by a predator release a chemical alarm cue specific to the species. Fish downstream pick up the scent, linking it to the smell of the predator.

Through this learning mechanism, nearly every species tested in the marine environment has developed a catalogue of “danger signatures”, he says. But the study found alarming change among damselfish.

“They no longer responded to chemical alarm cues, which is just ludicrous, because that’s so critically important to them learning what’s going on and how they can tell whether they’re at risk,” says McCormick.

The world-first study, published in the journal Ecology and Evolution, suggests a dire link to dead coral.

“There is some alteration – maybe a combination of the chemicals, or a degradation of the alarm cue due to the chemicals given off by the rubble – that is knocking out the chemical activity of the alarm cue,” says McCormick.

His colleague at JCU, eminent reef scientist Philip Munday, says the study’s implications are serious, and worries dead habitat could have the same effect on other chemical cues, such as those used to attract mates or locate food.

Worse, Munday fears there’s a limit to fish adaptation in a dead-coral environment.

“If you’ve got what normally is quite an important ecological alarm cue, and the background habitat effectively renders it inactivated, then there’s not a great deal that adaptation can do about that if the fish just don’t respond to it,” he says.

But as coral degradation around the world becomes more common, Munday holds out hope for patchy areas of coral cover where alarm cues may still work.

“Then there is a lot more opportunity for natural selection to favour those organisms that are even better at picking them up,” he says.

“You might get selection for those animals that are super sensitive and responsive to alarm cues.”