Climate change is going to leave some fish feeling very lonely in the coming years, as new research shows that increasing carbon dioxide levels prevent them from recognizing their friends.
Like humans, fish prefer to surround themselves with the familiar. Previous research has shown that fish prefer to group with individuals they recognize, rather than strangers. And this behavior is for good reason: Swimming en masse provides higher growth and survival rates, greater defense against predators, and faster social learning. But, according to a new study from the ARC Centre of Excellence for Coral Reef Studies at James Cook University, climate change may make it difficult for fish to recognize familiar faces.
Researchers studied the effect of increasing carbon dioxide levels on the schooling behavior of the tropical damselfish Chromis viridis, a popular aquarium fish that is native to the Indo-Pacific region. Under normal circumstances, young damselfish can learn to recognize each other in a matter of hours, and older fish within a few weeks. When the researchers exposed the fish to higher-than-average CO2 levels, they were unable to differentiate between friend and stranger.
“Since half of all fish species in the world school at some point during their lives, including economically important species, these effects could be critical for species that rely on group-living to avoid predators,” lead researcher Lauren Nadler explained in a news release.
To study the effect of increasing CO2 on schooling damselfish, researchers used climate change models to simulate conditions predicted for the end of the century by the Intergovernmental Panel on Climate Change (IPCC). They gave individual fish the opportunity to choose between two schools—one of familiar fish and the other of strangers. Under normal conditions, the fish consistently sided with their familiar posse, but showed no preference for familiar or unfamiliar schools under higher CO2 levels.
Carbon dioxide is believed to interfere with the proper function of neuroreceptors in a fish's brain. Increased levels change the concentration of ions in the fish's blood, ultimately impairing their basic senses like sight and smell. It can also affect other aspects of a fish's life, including learning and decision making abilities and risk-taking behavior.
But, loneliness is not the only effect of ocean acidification on fish: Previous research found that higher CO2 levels increase fish anxiety, causing them to hide in dark corners. Another study found that ocean acidification is causing one fish species, the cinnamon anemoneﬁsh, to breed more frequently, according to Grist.