Fish lose common sense as CO2 levels rise

Lizard Island coral reef with the study species, Spiny damselfish,  Acanthochromis polyacanthus . Image by Jodie L. Rummer

Increased levels of carbon dioxide concentrations alters brain chemistry that may lead to fish becoming more at risk from predators.

The first-of-its-kind study was undertaken by researchers from the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science and the ARC Centre of Excellence for Coral Reef Studies at James Cook University 

“Coral reef fish, which play a vital role in coral reef ecosystems, are already under threat from multiple human and natural stressors,” says lead author of the study Rachael Heuer. “By specifically understanding how brain and blood chemistry are linked to behavioural disruptions during CO2 exposure, we can better understand not only ‘what’ may happen during future ocean acidification scenarios, but ‘why’ it happens.”

Researchers measured behavioural impairment and brain chemistry in the Spiny damselfish, Acanthochromis polyacanthus, a species commonly found on coral reefs in the western Pacific Ocean.

During a three-week period, the scientists collected spiny damselfish from reefs off Lizard Island located on Australia’s Great Barrier Reef. The fish were separated into two groups — those exposed to ordinary CO2 “control” conditions and those exposed to elevated CO2 levels that are predicted to occur in the near future, but have already been observed in many coastal and upwelling areas throughout the world. Following the exposure, the fish were subjected to a behavioral test, and brain and blood chemistry were measured.

The fish were given the choice between control seawater or water containing a chemical alarm cue, which they typically avoid since it represents the smell associated with an injured fish of its own species.

The researchers found that the damselfish exposed to elevated carbon dioxide levels were spending significantly more time near the chemical alarm cue than the control fish, a behaviour that would be considered abnormal. The measurements of brain and blood chemistry provided further evidence that elevated CO2 caused the altered behaviour of the fish.

This is the first time, physiological measurements showing altered chemistry in brain and blood have been directly linked to altered behaviour in a coral reef fish, the researchers say.

“If coral reef fish do not acclimate or adapt as oceans continue to acidify, many will likely experience impaired behavior that could ultimately lead to increased predation risk and to negative impacts on population structure and ecosystem function,” says Heuer. “This research supports the growing number of studies indicating that carbon dioxide can drastically alter fish behaviour, with the added benefit of providing accurate measurements to support existing hypotheses on why these impairments are occurring.”

The study is published in Scientific Reports.