Quirks & Quarks

As greenhouse gases gradually acidify oceans, fish may lose their sense of smell

Higher carbon dioxide concentrations in the ocean inhibit a fish's ability to smell prey, predators and each other.

Fish rely on their sense of smell to find food and mates, and avoid predators

Sea bass exposed to high levels of carbon dioxide lost up to half their sense of smell. (Citron, cc-by-sa 3.0)

Fish on acid

If someone asks you about how fish smell, you might think about the distinctive scent of fish in a seafood market.

But a new paper in the research journal Nature is more interested in a fish's sense of smell — and how the burning of fossil fuels may eliminate it altogether.

As carbon dioxide is pumped into the atmosphere, some of it is absorbed by the ocean, which gradually compromise fish's ability to detect odours, the paper found.

"Changes in water chemistry that are happening in the ocean are likely affecting the way small molecules bind to the receptors in the nose of the fish, and therefore decreasing how much fish can smell," Canadian biologist Dr. Cosima Porteus, a postdoctoral researcher at the University of Exeter, told Quirks & Quarks guest host Britt Wray.

That reduction in their ability to smell is going to cost them, as fishes rely on their sense of smell to navigate their daily lives in low-visibility ocean waters. Smell normally helps them find food and mates and also helps them detect safe habitats and avoid predators.

Making fish smell worse: The experiment

Porteus drew her conclusions after performing a set of experiments on the European sea bass. She compared its behaviour under current carbon dioxide levels and levels predicted for the end of century and found increased CO2 changed their behaviour in a number of ways.

"We found that fishes exposed to [future carbon dioxide levels] tended to swim less. They responded less to predator smell and tended to freeze more," said Porteus. "This only make things worse because if their sense of smell is decreased under these conditions, they would probably have to swim around more to find these things that they're looking for."

In another experiment, she recorded nerve signals from the fish's nose. She looked at its response to several different odorants involved in a range of activities from detecting food to detecting other fish that were either related to them or were reproductively active, and found that the animal lost up to half their ability to smell.

"We were surprised because although there's high carbon dioxide, the pH only drops by 0.3 units, which is not very large to see a 50 per cent reduction," said Porteus.

Future ocean ecosystems

At this point, scientists don't know if fish can adapt to these changes, as genetic adaptation would take multiple generations. Porteus believes that changes would have to occur at multiple levels within their olfactory system, and involve the nose and multiple areas in the brain as well.

The problem now is that carbon dioxide levels might rise too quickly to allow time for adaptation and only short-lived species might be able to adjust.

"There are likely to be winners and losers in this game," said Porteus. 

But despite the dire outlook, she believes we can still turn things around.

"There's hope in our study because this is a warning for the future," she said. "It's not happening yet, so if we can reduce carbon emissions, this is preventable in a way. But we have to act soon."