Climate Change Makes it Hard for Fish to Tell Friend from Foe

Climate change is expected to bring with it elevated oceanic carbon dioxide levels, and if these levels continue to increase young fish won't be able to distinguish between their own friendly school and strangers, according to a new report by scientists at James Cook University in Australia.

Like humans, fish prefer to form groups with individuals with whom they are familiar, which provides several benefits, such as higher growth and survival rates, greater defense against predators and faster social learning. However, higher carbon dioxide (CO2) levels can impede this ability.

"Familiarity is an important trait for defense, particularly in a predator-rich environment like a coral reef," study leader Miss Nadler said in a press release. "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."

Scientists at the ARC Centre of Excellence for Coral Reef Studies at James Cook University, Australia, have been studying the effect of CO2 on the schooling behavior of the tropical damselfish Chromis viridis.

Climate change models predict that CO2 levels and ocean acidity will more than double before the end of the century. And the elevated CO2 levels used in this experiment reflect the Intergovernmental Panel on Climate Change (IPCC) predictions for ocean water in 2100, based on their 2014 report.

Schools of fish were placed in such conditions, and given a "choice test" where they were placed between two schools - one of familiar fish and the other made up of strangers. High CO2 levels influenced their social recognition skills, for whilst fish kept under normal conditions consistently chose the familiar school, fish reared under high CO2 conditions showed no preference for either the unfamiliar or familiar school.

The research team believed high amounts of CO2 inhibits the function of neuroreceptors in fish's brains. Ion concentrations in their blood changes, thereby altering the way neuroreceptors work and confusing the fish in terms of their normal social interactions.

This study follows the United Nation's IPCC recent report that estimates that tropical fisheries will decrease by 40 to 60 percent due to increasing seawater temperatures, as fish migrate toward the poles. And CO2 doesn't just mean they can't tell friend from foe, it also means their normal senses are impaired, like sight and smell.