Ocean acidification may make fish foolhardy

Rising carbon dioxide may rob fish larvae of their ability to sense predators and survive

Baby fish become confused and reckless in water with high levels of dissolved carbon dioxide, a new study shows. This leads to higher death rates and may mean that rising atmospheric carbon dioxide, which causes ocean acidification, will reduce the number of fish in the ocean.

“It shows we should be concerned with even minor changes in aquatic ecology, because it’s going to have dramatic effects on the survival of fish,” says Grant Brown, a freshwater behavioral ecologist at Concordia University in Montreal who was not involved in the study. “There are very fine-scale, yet extreme critical effects going on.”

Atmospheric carbon dioxide levels are projected to rise over the next century from fossil fuel burning. As carbon dioxide enters the Earth’s atmosphere, some of it is absorbed into oceans. The CO2 dissolves, as it does in carbonated beverages, and lowers the water’s pH. A lower pH is known to hinder the ability of oysters and other marine life to build calcium carbonate shells, but effects on fish are less well-known.

Marine ecologist Philip Munday of James Cook University in Townsville, Australia, and his colleagues showed previously that high levels of dissolved CO2 interfere with the sense of smell of clown fish. The larvae of clown fish and other coral reef fish rely on their sense of smell to stick close to home and avoid predators.

In the current study, published online July 6 in the Proceedings of the National Academy of Sciences, the team tested clown fish and damselfish larvae’s sense of smell using a range of carbon dioxide concentrations similar to those projected in scenarios for the end of this century by the United Nations Intergovernmental Panel on Climate Change.

Levels in the atmosphere today are about 390 parts per million, and previous studies have found that above levels of 450-500 ppm, corals are no longer able to build their calcium carbonate skeletons.

The team found that fish larvae behaved normally when living in water with up to 550 parts per million of dissolved carbon dioxide, which could be reached by midcentury, according to the IPCC. Larvae hid within protective reefs when swimming in waters below that level, and moved away from a predator’s chemical scent. But when scientists ramped up the seawater’s dissolved carbon dioxide levels, fish got confused.

“It seems that the fish lose their ability to discriminate between odors and are less repelled by odors they would normally avoid,” Munday says. “This suggests some fundamental change in the way that information about the odors is being transmitted to the brain.”

ON ACID New research suggests that ocean acidification causes coral reef fish larvae to be attracted to the smell of predators, decreasing their survival. Courtesy of Dr. Simon Foale, ARC Centre of Excellence for Coral Reef Studies

At 700 ppm CO2, half of the larvae chose to swim in water that smelled like a predator, rather than water that didn’t. These larvae behaved more boldly in their natural habitat, swimming faster and farther away from the protective reef. After 30 hours on the reef, only 40 percent of them had survived, compared with 90 percent of fish in waters of 550 ppm or below.

The effect worsened as CO2 levels went higher. At 850ppm CO2, all the larvae were attracted to the predator’s smell and acted recklessly on the reef. Ninety percent were eaten after 30 hours on the reef.

“It suggests that levels of carbon dioxide predicted to occur at the end of the century could affect replenishment of fish stocks and have added consequences for pressures already put on fisheries,” Munday says.

Munday plans to test whether some species of coral reef fish are more tolerant or sensitive to the effects of carbon dioxide. He also wants to know whether some species can adapt to elevated CO2 levels and if predators will be affected.

Understanding how both prey and predators are affected will be critical for understanding how ecological systems will be affected by climate change, Munday says.

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