The ocean’s rising carbon dioxide levels may cause many coral reef fish to swim toward the smell of predators rather than away from them — and thus toward likely death, marine ecologists said Tuesday.

The greenhouse gas’ ability to alter fish behavior for the worse points to an “unexpected potential impact of elevated carbon dioxide in the oceans,” said Philip Munday, a marine ecologist at James Cook University in Queensland, Australia.

Much study has been done on the effects of ocean acidification on coral and shelled animals, but little on how the effects would manifest in other forms of marine life, said Munday, who led the study published Tuesday in the Proceedings of the National Academy of Sciences. “What we wanted to find out was how it affects those that don’t have a skeleton on their outside.”

The scientists put larval fish in water enriched with various levels of carbon dioxide, whose concentration in the oceans has been rising as a result of rising levels in the Earth’s atmosphere. The lowest was 390 parts per million (the current level in the ocean) and the highest 850 ppm (which the scientists estimated would be the carbon dioxide level in the water by the end of the century if current trends continue). Then the scientists allowed each of the larval fish to pick a water source — one that had been scented with a predator’s chemical signature or one that was clear of dangerous smells.

They did the experiment twice: once with baby clownfishes raised in captivity and once with wild-caught young damselfishes.

Many coral reef fish can smell nearby predators — a key ability, biologists said, given what an appetizing snack larval fish make for rock cod, dottybacks and other larger fish.

“They’re kind of like Hershey’s Kisses … everybody’s after them,” said Mark Hay, a marine ecologist at the Georgia Institute of Technology, who was not involved in the study.

Normally, larval fish would flee from the predator odors. But fish exposed to the highest levels of carbon dioxide in the experiment did not: They even seemed to be attracted to the very odor that should have set off their neuronal alarms.

The scientists then tested these fish in the ocean. They lopped off pieces of coral reef and moved them to empty spots in the sand, making temporary one-fish habitats. Experienced scuba divers discreetly watched each fish as it swam around its new home.

They found that the fish that had spent time in the highest levels of carbon dioxide ventured farther away from their coral and acted much more boldly than their counterparts in normal water — striking aggressively at food and exploring without trying to hide, for example. They were also five to nine times more likely to die.

In other words, merely having been exposed to higher levels of carbon dioxide altered the behavior of the fish for some time after they were again in water with normal levels.

Hay said the study’s findings raised questions — such as how carbon dioxide affects a fish’s ability to smell and whether the findings are applicable to many other types of fish and marine life. But he said the study shows that the effects of greenhouse gases on marine life could be far more complex and far-reaching than thought.

“Most organisms don’t have eyes, don’t have ears — they rely extraordinarily deeply on those [chemical] cues to decide whether to eat the next thing or run from it or mate with it,” he said. “Here’s an example of dramatic alteration in the [biological] machinery … that would be catastrophic for young fish.”

amina.khan@latimes.com