Acidified oceans may affect fish populations — study

[NEW YORK] Native fish hatchlings will find it more difficult to reach secure shelters in the oceans of the future that are acidified by carbon dioxide (CO₂) threatening fish populations, new research suggests.

Ivan Nagelkerken and fellow researchers at the Environment Institute, University of Adelaide, showed in Scientific Reports, published last month (April), that in high CO₂ conditions, barramundi larvae move away from the ocean noises they are normally attracted to. Instead, the larvae are attracted to sounds produced by the wrong sort of habitats or to artificial ‘white noise’.

“Elevated CO₂ interferes with the brain functioning of fish so that they do not respond to natural cues and are attracted to unnatural cues”

Ivan Nagelkerken, University of Adelaide

Barramundi or Asian sea bass, an important commercial fish, are born at sea and migrate as juveniles and adults to fresh, protected areas. Hatchlings listen to the sounds of coastal ecosystems to guide them to sheltered waters.

The researchers compared the activity of barramundi larvae in marine tanks with levels of CO₂ predicted at the turn of the century against responses of larvae in current day CO₂ levels. They found the larvae were attracted to the sounds of tropical estuaries but deterred by the same sounds under simulated future ocean conditions.

"Moreover, under elevated CO₂ conditions, larval barramundi were attracted to the wrong sounds," Nagelkerken tells SciDev.Net.

"Elevated CO₂ interferes with the brain functioning of fish so that they do not respond to natural cues and are attracted to unnatural cues,"  Nagelkerken says.

Article co-author Sean Connell explains that if ocean acidification causes larvae to be attracted to irrelevant sounds, they could end up in the wrong habitat or in inhospitable places. “This could also result in smaller adult fish with significant impact on fisheries.”

“Given that this experiment was conducted under laboratory conditions that manipulated CO₂ levels, while holding all other conditions constant, the results observed can be confidently attributed to CO_2,” says Jeff Clements, visiting postdoctoral fellow at Fisheries and Oceans Canada, New Brunswick, Canada.

Clements says it is difficult to predict if the results would be the same with other fish, although one overarching conclusion of ocean acidification studies is that different species respond differently to elevated CO₂ conditions.

He adds: "The results observed here have been reported in other studies with different coral reef fish species, so it is likely that the larvae of other reef fishes would respond similarly. More studies would be needed to apply these results to fish species from other regions."

Ultimately, he says, this work suggesting that elevated CO₂ affects the ability of fish to recognise familiar habitat sounds is one piece in a bigger ecological puzzle. "Ecological processes do not happen in isolation. It is not realistic, therefore, to try and ‘pinpoint’ a single thing to place blame on for a change."

"In nature, other things will occur alongside increases in CO₂ that could impact sound recognition in fish larvae as well. More research is needed to better understand just how much elevated CO₂ might contribute to noise recognition and population replenishment in reef fishes," Clements notes.

This piece was produced by SciDev.Net’s Asia & Pacific desk.