Fish find it harder to smell in acidic oceans, study finds

As oceans become more acidic, fish struggle to smell their food and predators, a new study has found.

Article by Shreya Dasgupta.

Much of the carbon dioxide in the atmosphere is absorbed by oceans. But with human activity continuing to drive up the amount of atmospheric CO2, the amount that oceans take up rises too. This creates more carbonic acid, lowering the water’s pH and making it more acidic.

Researchers found that even tiny decreases in seawater pH are enough to weaken the European sea bass’s sense of smell, which it relies on to find food and mates and evade predators. In waters containing high carbon dioxide levels predicted for the end of the century, the sea bass (Dicentrarchus labrax) had to be on average up to 42 percent closer to the source of the smell in order to detect it, compared to when they were exposed to waters containing present-day levels of carbon dioxide.

The fish also showed some behavioral changes, the study published in Nature Climate Change found. Those in waters with elevated carbon dioxide levels spent more time “freezing” before and after being exposed to predator-related odors, for example, which the researchers say indicates anxiety.

“This means that the sea bass might not detect predators until these are closer to them, which will decrease the chance of surviving a predator attack,” lead author Cosima Porteus, a fish physiologist and postdoctoral fellow at the University of Exeter, U.K., told Mongabay. “Sea bass, and other fish, might also have to spend more energy searching for food, mates, suitable habitat, instead of using their energy for growth, investing into offspring, etcetera. This has the potential to affect their survival in the wild.”

Overall, the team exposed the fish to 10 different odors, some signaling different food, some associated with cues from other fish, and others indicating predators. When they measured changes in the signaling of nerves responsible for responding to smell, they found that in fish exposed to more acidic waters, neuron activity reduced in response to six of the 10 odors.

According to Porteus, sensitivity to some odors were affected more than others because of the chemical structure of the smell signals.

“Some of these odorants have positively or negatively charged side chains which means that an decrease in pH can affect these charges and how these odorants bind to the receptors in the nose of the fish,” Porteus said. “This means that not only that the sense of smell will be diminished, but also that the quality of the smell will be changed because animals including fish and humans smell a combination of odorants at any one time.”

Ocean acidification can impair behavior in fish. Image by Uxbona via Wikimedia Commons (CC BY 3.0).

The researchers also found that in fish that were in more acidic waters, genes associated with sensing and processing smells were expressed less.

“We wanted to examine if fish had any ability to compensate for this reduced sense of smell, but found that instead of increasing the expression of genes for smell receptors in their nose they did the opposite, exacerbating the problem,” Porteus said.

The study is a warning for the future, the researchers say.

“But there is hope,” Porteus added. “Our study shows that future levels of CO2 can have these effects on the sense of smell of fish, but we can still do something about it — we can reduce carbon emissions now to prevent this from happening.”

Banner image: European sea bass struggle to smell in acidic waters. Image by Citron via Wikimedia Commons (CC BY-SA 3.0)

Citation:

Philip L. Munday, Alistair J. Cheal, Danielle L. Dixson, Jodie L. Rummer, Katharina E. Fabricius. Behavioural impairment in reef fishes caused by ocean acidification at CO2 seeps. Nature Climate Change, 2014; DOI: 10.1038/NCLIMATE2195

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