Fish Will Start Losing Sense of Smell as Carbon Dioxide Levels Rise, Study Finds

Just as humans rely on their sense of smell to detect suitable food and habitats, avoid danger, and find potential mates, so do fish — only instead of sniffing scent molecules floating through the air, they use their nostrils to sense chemicals suspended in water.

But fish will start losing their ability to detect different smells by the end of the century if atmospheric carbon dioxide levels keep rising, scientists warned in a recent study published in the journal Nature Climate Change.

For fish, the sense of smell is “particularly important when visibility is not great,” said Cosima Porteus, a postdoctoral fellow at the University of Exeter and the lead author of the study, which examined elevated carbon dioxide levels and their effects on olfactory sensitivity, gene expression and behavior in European sea bass. “Therefore, even a small decrease in their sense of smell can affect their daily activities.”

The fish began behaving differently. They didn’t swim as much, and in some cases, they didn’t move for more than five seconds at a time.

In addition, the data showed that elevated carbon dioxide affected the expression of genes in the nose and brain of the fish.

Although the study focused on sea bass, it is applicable to other kinds of fish, Dr. Porteus said, “because all fish use similar mechanisms to smell their surroundings.”

Animals are known to adapt to their changing surroundings over many generations, but it is unclear how quickly fish would do so in the face of rising carbon dioxide levels, especially if those levels increased quickly.

The changing acidity of the ocean, measured in pH, has implications far beyond the sense of smell, including a possible deficit in reproduction, said Mary Hagedorn, a senior research scientist at the Smithsonian Conservation Biology Institute who studies fish, coral and climate change.

Carbon dioxide levels have been increasing over the last 200 years because of the burning of fossil fuels, car emissions, and deforestation, all of which have resulted in more acidic oceans, according to the National Oceanic and Atmospheric Administration. That’s because the oceans absorb about 30 percent of the carbon dioxide released into the atmosphere, and when carbon dioxide is dissolved in ocean water it turns to carbonic acid.

Since the start of the Industrial Revolution, surface ocean waters have experienced a 30 percent increase in acidity, the agency says.

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