Los Angeles 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’s ability to alter fish behavior for the worse points to “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 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. 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 those baby 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 clownfish raised in captivity, and once with wild-caught young damselfish.
Many coral reef fish can smell predators nearby — a key ability, biologists said, given what an appetizing snack the larval fish make for rock cod, dottybacks and other larger, predatory fish inhabiting the reef.
“They’re kind of like Hershey’s Kisses ... everybody’s after them,” said Mark Hay, a marine ecologist at 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 swum 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 homebase 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 carbon dioxide levels altered the behavior of the fish for some time after they were again placed in water with normal levels.
Hay said that the study’s findings raised questions — such as how carbon dioxide affects a fish’s ability to smell, and whether this finding is applicable to many other types of fish and marine life. But he said that it shows that the effects of greenhouse gases on marine life could be far more complex and far-reaching than previously 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.”