In April the world learned that more than 90 percent of Australia's Great Barrier Reef had become bleached. That is, warming waters or other conditions cause the algae living in the coral to exit, leaving the coral weak. It's a bleak statistic, because it's reasonable to assume that as the corals themselves suffer, the entire ecosystem they support suffers as well. 

For example, a study finds that bleaching hinders fish from learning to avoid predators.

Imagine you're a fish, and suddenly one of your friends meets its unfortunate end in the jaws of a predator. 

"We found that these animals actually have this really sophisticated way of learning, which involves the linking of chemical alarm cues, which are damage-released cues from conspecifics, and any other smell or even the sight of anything novel… sort of a Pavlov's dog-type scenario."

James Cook University marine scientist Mark McCormick. He and his team found that this learning process breaks down when the coral becomes bleached. Instead of hosting algae within, the bleached coral becomes blanketed by algae.  

"We've used little patches of live coral and little patches of dead and degrading coral, which have similar topographic complexity, and what we've done is we put those little patches within a bed of either live coral or dead and degraded coral." 

Onto each patch, the researchers deposited a small, naïve reef dweller called a damselfish. They wanted to see how the health of the reef influenced the fish’s ability to learn to avoid the odor of a predator called the dusky dottyback. And they found that the presence of degraded coral—even if surrounded by a healthy reef—entirely disrupted that learning mechanism. [Mark I. McCormick & Oona M. Lönnstedt. Disrupted learning: habitat degradation impairs crucial antipredator responses in naive prey, in Proceedings of the Royal Society B]

"It touches on a really big issue, to some extent a really global issue. So, even though this is actually dealing with a relatively small aquarium fish, it's got life history traits that are very similar to virtually all of the other marine organisms."

So, is there anything that can be done to help the world's reefs and their inhabitants?

"These communities are going to have to try and rebuild, and they have really effective mechanisms whereby they can re-seed themselves. But we're talking about timescales of really probably 10-15 years to actually get a healthy reef back after really a cataclysmic change."

And for that to happen, McCormick says, we need to reduce our CO2 emissions and to stop polluting our waterways. In order for things to improve underwater down under.  

—Jason G. Goldman

[The above text is a transcript of this podcast.]