You can run but you can't hide, little damsel. Credit: Getty

Fish that usually camouflage themselves among colourful coral reefs are losing their ability to hide from predators as corals are bleached by Earth's acidifying oceans.

Bleaching often leads to coral death, and is a stress response to two key factors: increasing ocean acidity, caused by uptake of carbon dioxide from the atmosphere, and a rise in ocean temperature. It's all too apparent that ecosystems near bleached corals tend to collapse, but the reasons why are not fully understood.

Some ecologists speculate that fish in bleached reefs simply move to areas where corals are still healthy, whereas others suggest that they succumb to increased predation as the corals provide less cover. To test these ideas, graduate student Darren Coker, of Australia's Centre of Excellence for Coral Reef Studies at James Cook University in Townsville, Queensland, and his colleagues have done laboratory experiments to test the effect of coral bleaching on predator evasion in reef fish.

This study is the first to show a direct link between coral bleaching, predator behaviour and prey-fish susceptibility. David Booth , University of Technology in Sydney, Australia.

The team filled aquaria with Pocillopora damicornis corals and added two species of damselfish (Pomacentrus moluccensis and Dascyllus aruanus) that like to hide around them.

The corals were either healthy, bleached, recently dead from bleaching or long-dead and covered with thick green algae. After the damselfish had spent an hour acclimatizing to their surroundings, a predatory reef fish, the dottyback (Pseudochromis fuscus), was introduced into each aquarium for 75 hours.

The team report in the journal Behavioral Ecology1 that predation rates — the proportion of damselfish taken by the dottybacks — were 33% on bleached corals, 37% on recently dead corals and 42% on dead corals covered in algae. These figures contrast sharply with the 25% predation rate that took place in aquaria with healthy corals.

Coker and his colleagues believe that the increase in predation is linked to both the loss of camouflage and of hiding space for the damselfish. The initial bleaching probably makes the brightly coloured fish easier for the predators to see, explains Coker, so fish populations begin to suffer even before the coral is dead.

"Once coral colonies are dead, algae grow on and between the branches of the coral, occupying the available refuge spaces that the fish might normally shelter within," he says — explaining why predation continues to climb long after the corals perish.

"We always assumed that fish either moved somewhere else or were preyed upon after their coral homes died, but we did not know which was more likely," explains marine biologist Shaun Wilson, part of the wider research team at the Centre of Excellence for Coral Reef Studies, and not involved in the research.

"This study is the first to show a direct link between coral bleaching, predator behaviour and prey-fish susceptibility," adds marine ecologist David Booth at the University of Technology in Sydney, Australia.

The finding could ultimately guide conservation efforts too, says Booth. If predation really does damage fish populations near bleached coral, this could potentially be mitigated by reducing predator numbers through increased fishing activities, he suggests.