Striped catfish lose the plot in low CO₂ water

As the politicians keep on squabbling over the best ways to alleviate climate change, and CO₂ emissions continue rising, much of the gas ends up in our oceans and rivers. At first glance, it didn't seem as if this gradual acidification was going to pose a problem for the planet's fishy residents. Matthew Regan from the University of British Columbia, Canada, says, ‘They are well prepared physiologically and biochemically to tolerate even the most depressing of future CO₂ projections’. However, more recently it has become apparent that fish that have been exposed to future levels of CO₂ experience behavioural problems: in addition to becoming hyperactive and bolder, they also suffer visual disturbance and anxiety, and are attracted to predators. The associated mild chemical imbalances in the fish's bodies affect their inhibitions. Following on from this discovery, Sjannie Lefevre and Göran Nilsson from the University of Oslo, Norway, wondered how fish that already reside in water with high CO₂ would cope if the situation was reversed and they were transferred into normal (low CO₂) water?

Teaching at a graduate course on air-breathing fish in 2014 organised by Mark Bayley at Can Tho University, Vietnam, Lefevre and Nilsson had the ideal opportunity to address the conundrum. The Mekong Delta is home to the striped catfish (Pangasianodon hypophthalmus), which thrives in high-CO₂ water. So, with a flourishing aquaculture industry on hand to supply the fish and a team of enthusiastic students available to run the experiments, they were in the perfect place to test the fish's reactions to low-CO₂ water.

Collecting fish provided by Do Thi Thanh Huong and Nguyen Thanh Phuong from a nearby farm, Regan and his fellow students Andy Turko, Joe Heras and Mads Kuhlmann Andersen transferred some of the animals into low-CO₂ water, while the rest remained in high-CO₂. Then, with the help of Colin Brauner and Tobias Wang, they began testing the fish's reactions to a range of situations, from the arrival of an unfamiliar object (a brick) in their surroundings to how much time they spent schooling with their own kind, to find out how the water conditions had affected their behaviour.

Not surprisingly, striped catfish that resided in their habitual high-CO₂ conditions showed all of the usual reactions to unfamiliar situations, avoiding the frightening brick and remaining out of reach of a threatening predator. However, the fish that had been held in freshwater began behaving strangely. Not only were they unalarmed by the presence of the predator but also they were unfazed by the arrival of the brick. In addition, they were less attracted to a school of their own species and were much more active. However, when the team gave the low-CO₂ catfish a dose of a drug that counteracts the effects of the GABA neurotransmitter – the neurotransmitter that malfunctions and triggers bold behaviour when fish that normally reside in low CO₂ are exposed to high CO₂ – the emboldened catfish lost their courage and began behaving normally.

So, the GABA neurotransmitter had lost its inhibitory effects in the catfish that had been transferred to low-CO₂ water because of the subtle chemical changes in their brains caused by the alteration in their surrounding water. And Regan suggests that other species may be able to adapt their brains to the brave new world that striped catfish already survive in, provided they can keep pace with change.