New research has shown that fish are losing their sense of smell as rising levels of carbon dioxide (CO2) in the atmosphere make the oceans more acidic.
The world’s oceans have absorbed 525 billion tons of CO2 since the beginning of the industrial revolution — equal to some 22 million tons per day. Carbonic acid is formed when the gas is absorbed by seawater, resulting in acidic water with a lower pH. Current projections show oceanic CO2 levels increasing 2.5 times by the end of the century.
The study, led by scientists from the University of Exeter, compared the behaviour of juvenile sea bass at today’s typical oceanic CO2 concentrations with those predicted for the year 2100. The fish in the more acidic waters swam less and were less likely to respond when they encountered the scent of a predator.
Fish use their sense of smell to locate food, safe habitats and suitable spawning grounds, as well as recognize predators and each other. To test the functionality of the sea bass’ nose, researchers recorded activity in the nervous system while the fish was exposed to waters with varying levels of CO2 and acidity.
“Their ability to detect and respond to some odours associated with food and threatening situations was more strongly affected than for other odours,” says University of Exeter researcher Dr Cosima Porteus. “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.”
The scientists also looked at how elevated CO2 and acidity affected the genes being expressed in the nose and brain of sea bass and found evidence for altered expression of many of those involved in sensing smells and processing olfactory information. Though sea bass were exclusively used in the study, the processes involved in sensing smells are common to many aquatic species.
The results showed that CO2 impacts the nose of fish directly, in addition to the impact on their central nervous system function, which reflects impaired processing by the brain itself. It’s not yet known how fish will adapt to these challenges as oceanic CO2 levels continue to rise. With two different issues caused by acidic water exposure, fish may struggle to adjust rapidly enough.