New Zealand and Australian researchers have now shown that an acidifying ocean can harm low-frequency hearing in fish, a troubling finding as the oceans are becoming more acidic, faster than at any other time in geological history.

The researchers detailed the results of their experiment in a study published March 3 in Proceedings of the Royal Society B, which they say demonstrate the need to rein in ocean pollution.

Earth's oceans are getting warmer, giving fish plenty of reason to fret. But they are also becoming more acidic, with a 25% increase since preindustrial times, leaving scientists little time to figure out how sea life is responding. Coral populations, for example, are under pressure from ocean acidification. Another recent study found that acidified water interferes with gilthead seabreams' sense of smell. 

Prior research has also indicated that increasing carbon dioxide in water, which in turn increases acidification, seems to warp and enlarge vital organs called otoliths, or ear stones, found in the auditory system of most fish. But scientists had yet to confirm how these changes could affect hearing. 

"We know the form, but we didn't really know the functional role, so the physiological results made it exciting," said lead author Craig Radford, an associate professor at The University of Auckland. "In fish with asymmetrical ears, the low-frequency components drop out in sensitivity."

To perform the study, Radford and his colleagues caught 39 Australasian snapper and harvested their eggs. After they hatched, Radford exposed half to typical ambient conditions, and the rest to "end of the century" levels of projected carbon dioxide. He then waited 21 days, until the fish became juveniles, a crucial age in an Australiasian snapper's lifespan, before taking 10 from each group to measure their hearing and take images of their auditory system with a micro-CT scanner. 

The scans of the acidified group revealed otoliths that were both larger and asymmetrical compared with the control group. With that in mind, Radford measured the "auditory evoked potentials," or brainwave activity, of these snapper when exposing them to different sound frequencies. It is similar to what doctors use to check for deafness in babies, only Radford had to use tiny electrode needles, rather than suction cups, to get through the snappers' scales to read its brainwaves. 

He found that fish with enlarged, asymmetrical otoliths had weaker neurological responses to low-frequency tones, suggesting hearing loss of about 10 decibels — a 10-fold reduction in sound sensitivity. Prior studies had hypothesized that larger otoliths could improve sensitivity to lower-frequency sounds, but Radford found that the opposite was true, owing largely to this asymmetry, he believes. 

"Could you imagine if your eyes were different levels on your face, and how hard that would be to focus?" Radford said in an interview with The Academic Times. "It's the same thing with our ears. They're in the same spot on your head so they can capture pressure waves, and your brain can commute directionality to the sound so you can predict where it's coming from, whether the front or the back." 

Australasian snapper need to be able to hear — their growth and ability to thrive depend on it. These snapper have a bipartite life history strategy, meaning they are born in the ocean, and return to their home reef upon reaching about the same age as Radford's study subjects. To reach their reef, the snapper rely on low-frequency reef sounds — and the findings suggest carbon dioxide exposure may hamper their ability to hear these critical noises.

"If you mess up their ability to detect these low-frequency sounds, then it might have consequences for population replenishment and all sorts of population-type effects, with large numbers of fish not being able to find their way back to their adult habitat," Radford said.

While Radford seems to have figured out that an acidic environment can change the physiology of fish hearing, he wants to find how fish may be evolving to meet increasing exposure to carbon dioxide. 

"We have no idea how they adapt to changing environments, because it's not a sudden click, and the acidity of the ocean drops by 1 pH," Radford said. "It's a gradual change."

Ideally, measuring any adaptive changes would require generations of fish inhabiting an environment with rising carbon dioxide levels — a difficult and expensive undertaking — but Radford says it would still be useful to track even just a few generations of fish, or fish with different auditory systems. He hopes his research will generate more interest in protecting the ocean. 

"Underwater is actually really noisy, and lots of marine animals use sound," Radford said. "These are all major pollutants in the sea that we can control, and we need to be getting the point across that we need to be looking after the ocean, because the ocean basically controls the world."

The study, "Ocean acidification effects on fish hearing," published March 3 in Proceedings of the Royal Society B, was authored by C.A. Radford and S.P. Collins, University of Auckland; P.L. Munday, James Cook University; and D. Parsons, University of Auckland and National Institute of Water and Atmosphere.