Fish living on coral reefs where carbon dioxide seeps from the ocean floor were less able than fish from normal coral reefs to detect the scent of predators, according to a new study that seems to confirm laboratory experiments showing that the behavior of reef fishes can be seriously affected by increased carbon dioxide concentrations in the ocean.

"These results verify our laboratory findings," Danielle Dixson, an assistant professor in the School of Biology at the Georgia Institute of Technology, said in a news release. "There's no difference between the fish treated with CO2 in the lab in tests for chemical senses versus the fish we caught and tested from the CO2 reef."

The new study examined fish from "bubble reefs" at a natural CO2 seep in Papua New Guinea, where the pH is 7.8 on average. While normal ocean surface water is around 8.14 pH, climate models forecast an ocean pH closer to the bubble reefs' by 2100 if greenhouse gas emissions continue unabated, according to the Intergovernmental Panel on Climate Change.

Carbon dioxide released into the atmosphere is absorbed into ocean waters, where it dissolves and lowers the pH of the water. Acidic waters affect fish behavior by disrupting a specific receptor in the nervous system, called GABAA, without which neurons stop firing properly.

"It's a step in the right direction in terms of answering ocean acidification problems," Dixson said. "The alternative is just to wait 100 years. At least now we might prepare for what might be happening."

Fish can smell predators and will avoid water containing the scent, and in the lab, control fish given the choice between swimming in normal water or water spiked with the smell of a predator will choose the normal water. But fish raised in water acidified with carbon dioxide will choose to spend time in the predator-scented water.

Juvenile fish living at the carbon dioxide seep and brought onto a boat for behavior testing had nearly the same problem sensing predators as juvenile fish raised at similar CO2 levels in the lab, the new study found.

However, despite the effects of high CO2 on fish behavior, relatively few differences have been observed between the CO2 seep and the control reef in species richness, composition, and abundance. The researchers did find that the number of large predatory fish was lower at the CO2 seep compared to the control reef, which may balance out the increased risk of mortality caused by their abnormal behavior, the researchers said.

The research originally was published in the advance online publication of the journal Nature Climate Change.