By Marlene Cimons
There have been numerous wake-up calls about the effects of climate change on marine life. As ocean waters heat up, they are bleaching corals. Growing levels of carbon dioxide are acidifying seawater, which is degrading the shells and skeletons of sea organisms. The rising temperatures are prompting fish to migrate to colder waters, even causing them to shrink.
Now climate change is starting to affect their sense of smell, a phenomenon that will worsen in the coming years if global warming continues unabated, according to new research. A sense of smell is indispensable to fish. They use it to find food, detect imminent danger and elude predators, to find safe environments and spawning areas, even to recognize one another.
To lose it could threaten their very survival. If this happens, it also would mean big trouble for the fishing industry, tourism and, most importantly, global nutrition, since many of the world’s people — including its poorest — depend on fish for food.
“Future levels of carbon dioxide can have large negative effects on the sense of smell of fish, which can affect fish population numbers and entire ecosystems,” said Cosima Porteus, a researcher at the University of Exeter and author of the study, which appears in the journal Nature Climate Change.
“This can be prevented, but we must reduce carbon emissions now before it’s too late.”
Carbon dioxide combines with seawater to produce carbonic acid, which makes the water more acidic. Since the Industrial Revolution, oceanic CO2 has risen by 43 percent and is projected to be two and a half times current levels by the end of this century, according to the scientists.
Experts believe that about half of anthropogenic carbon dioxide — that is, emissions produced by human activities, such as the burning of fossils fuels — has over time ended up in the oceans, lowering the pH of seawater, and making it more acidic.
Porteus — collaborating with scientists from the Centre of Marine Sciences in Faro, Portugal and the Centre for Environment, Fisheries and Aquaculture Science in the UK — compared the behavior of juvenile sea bass at carbon dioxide levels typical of today’s ocean conditions with those predicted for the end of the century.
They found that sea bass exposed to the more acidic conditions swam less and were less likely to react when encountering the smell of a predator, offered to them in the form of very dilute monkfish bile. Also, they were more likely to “freeze,” a sign of anxiety, she said.
“I found the longer they were in high CO2, the worse they fared,” she said.
The scientists also measured the ability of the fish to detect certain odors in different levels of acidity by recording their nervous system activity. “I recorded the olfactory — smell — nerve response by measuring the electrical activity of the nerve to these different odorants in the water that flowed over the nose of the fish in both normal and high CO2 seawater,” Porteus said.
“The odorants tested were those that would be involved in finding food — amino acids — and in recognizing fish of the same or other species, including bile acids, bile, intestinal fluid, etc., at different concentrations, and at levels they would encounter in the wild,” she added.
The researchers found that seawater acidified with levels of carbon dioxide that are expected by the end of the century — if global warming continues — reduced the sense of smell of sea bass by half, compared with today’s levels.
“Their ability to detect and respond to some odors associated with food and threatening situations was more strongly affected than for other odors,” Porteus said. “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.”
They did not compare the impact of today’s ocean acidity levels with those of pre-industrial times, although they plan further research to do so. “It is possible that sea bass are already being affected by a rise in oceanic pH,” she said.