NEW YORK (NYTIMES) - 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 Dr Cosima Porteus, a postdoctoral fellow at the University of Exeter in London and the lead author of the study, which examined elevated carbon dioxide levels and their effects on olfactory sensitivity, gene expression and behaviour in European sea bass.

"Therefore, even a small decrease in their sense of smell can affect their daily activities."

Dr Porteus and her colleagues exposed juvenile sea bass to the amount of carbon dioxide that is predicted by the Intergovernmental Panel on Climate Change to be in seawater by the year 2100, which is more than double today's levels of carbon dioxide.

When exposed to the elevated levels, the fish had to be about 42 per cent closer to an odour source to detect it, the researchers found, making it harder for them to notice food or predators.

The fish began behaving differently. They did not swim as much, and in some cases they did not 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.

Shorter-lived species might have more time to adapt to changes in ocean pH than longer-lived species, Dr Porteus said.

And, she added, because the high carbon dioxide levels were found to affect the nose and at least two parts of the brain, adaptation would have to take place at multiple levels, which would make it hard to predict how the fish would adapt - if they even had time to do so.

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

And while some species will find ways to adapt, "we don't know the limits of those adaptations," she said.

"There will definitely be winners and losers."

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 per cent 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 per cent increase in acidity, the agency says.

"We should take all of this extremely seriously," Dr Hagedorn said.

Dr Porteus said her future research would aim to determine if fish are already being affected by the rise in carbon dioxide compared with pre-industrial levels.

She also hopes to find out the level of carbon dioxide that starts to affect the sense of smell of fish.

"It's likely that this will become a problem at a lower CO2 concentration than that tested in our study," she said.

While the outlook might seem dour, there is still time to take preventive measures and reduce carbon emissions, Dr Porteus said.