Ocean Acidification Is Frying Fish’s Sense of Smell

By the end of the century, the ocean is predicted to become two-and-a-half times more acidic, which is bad news for sea life.

Sea Bass
Do you smell something, Bob? Wikimedia Commons

It’s hard to imagine since our sense of smell pretty much disappears underwater, but fish rely heavily on their sniffers to detect predators, find food and locate mates. Lina Zelovich at JSTOR Daily reports that for some fish, that critical sense is being dulled as the acidity of the ocean increases. And with climate change making the seas even more acidic each year, the problem is set to get worse.

The ocean is a pretty good carbon sink, capturing about a quarter of all the CO2 released by humans. While that’s a good thing for slowing climate change, it has big consequences for the ocean ecosystem. Over the last two centuries, the extra CO2 humanity has pumped into the atmosphere has increased ocean acidity by 43 percent. It’s estimated that by 2100 the ocean could be 2.5 times more acidic than it is now.

The carbon dioxide that the sea absorbs undergoes a chemical reaction with the water to create carbonic acid in a process known as ocean acidification. Though the acid is weak, it’s plentiful enough to cause problems, like disrupting the formation of shells in many marine species. An international team of researchers explored the impact ocean acidification has on fish in a new study published this week in the journal Nature Climate Change.

The researchers first looked at juvenile European sea bass, Dicentrarchus labrax, in ocean water with normal carbonic acid levels. Then they examined the fish in water mimicking the acid levels expected by the end of the century. The differences in behavior were striking. The fish swam less and were more likely to “freeze” for five seconds or more, a sign of fishy anxiety. Most importantly, to smell something they had to get much, much closer to it—not the best move if the source of the scent is a potential threat.

“The sense of smell of sea bass was reduced by up to half in sea water that was acidified with a level of CO2 predicted for the end of the century. Their ability to detect and respond to some odors associated with food and threatening situations was more strongly affected than for other odors,” says the study's lead author Cosima Porteus, a fish physiologist at the University of Exeter in the United Kingdom. “We think this is explained by acidified water affecting how odorant molecules bind to olfactory receptors in the fish's nose, reducing how well they can distinguish these important stimuli.”

Many animals have genetic resources to help them overcome times of change or stress, like developing more sensory receptors. The researchers wanted to see if the fish might have a toolbox of genes they could use to help them overcome the loss of smell, but nothing of the sort was found.

“One way to smell something better is to have more receptors detecting these smells in order to increase the chance that particular smell will be detected, and therefore increase the expression of these receptors,” Porteus tells Marlene Cimons at Popular Science. “Another way is [for them] to make a slightly different receptor that works better under lower pH. However, we did not find any evidence this was the case.”

In fact, the acid-washed fish produced fewer smell receptors, making it even more difficult for them to detect odors.

It’s likely that acidification won’t just impact sea bass. Porteus says the results should apply to almost all fish including cod, salmon, haddock and other economically important species and may apply to marine invertebrates like lobster as well.

In the next phase, the team will compare the acid levels in today’s ocean to preindustrial times to determine whether fish are already experiencing significant problems with their sniffers. The solution to the problem, like it is for a myriad of problems facing the world, is to tackle carbon emissions head on, whether we can smell them or not.

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