Future oceans will be much quieter places, making it harder for young marine animals that navigate using sound to find their way back home, new research has found.
Under acidification levels predicted for the end of the century, fish larvae will cease to respond to the auditory cues that present-day species use to orient themselves, scientists reported in the journal Biology Letters.
While ocean acidification is known to affect a wide range of marine organisms and processes such as smell, until now its effect on marine soundscapes and impact on the larvae of marine animals was unknown.
The ocean is filled with sounds that carry information about location and habitat quality, study co-author Sir Ivan Nagelkerken said.
"Along with chemical and other cues, because of sound's ability to travel long distances underwater, it is used as a navigational beacon by marine animals, particularly larvae," Dr Nagelkerken said.
"More than 95 per cent of marine animals have a dispersive larval stage, where larvae drift with the currents for anywhere from a few days to a year, before returning to settle in their adult habitat near where they were spawned."
To understand how acidification affects these marine animals, the team led by PhD student Tullio Rossi travelled to a naturally occurring carbon dioxide vent near White Island in New Zealand, where ocean acidification levels are similar to those predicted for the end of the century under business-as-usual conditions.
"This natural laboratory gave us a peek into the future," Dr Nagelkerken said.
"We recorded the soundscape around the vent, then compared the loudness and composition of sounds with control sites a few hundred metres away."
The area around the vent was much quieter, the team found.
"There could be a number of explanations for the decrease in sound," Dr Nagelkerken said.
"For example, as acidification increases, kelp forests may be replaced by turf algae. This results in changing abundance of the animals that produce sounds, such as snapping shrimp whose ubiquitous crackle forms the backdrop to present-day ocean soundscapes."
To understand how acidification affects marine animals' auditory preferences, the researchers studied the impact of increased carbon dioxide levels on settlement-stage mulloway (Argyrosomus japonicas), a common temperate fish species.
They found that the 25- to 28-day-old larvae that had been exposed to higher carbon dioxide concentrations deliberately avoided present-day acoustic habitat cues recorded near White Island, while fish reared in present-day carbon dioxide levels responded positively.
Neither group of fish responded to the "future" soundscape recorded around the vent, despite the hearing of the normal fish being unimpaired.
Ocean acidification is known to increase the size of otoliths - fish ear bones - used for hearing, orientation and balance.
It has been hypothesised that bigger ear bones would increase the hearing range of larval fish, but the hearing in fish reared in future carbon dioxide levels was negatively impacted by ocean acidification, even though they had larger ear bones.
Dr Nagelkerken said the findings suggested that in the future, affected species would have to use other, potentially less reliable cues to help them navigate, even though other senses such as vision and smell are also negatively impacted by ocean acidification.
"Finding a home is the key to population sustainability," Dr Nagelkerken said.
"Those that rely on sound as an orientation cue will be heavily impacted, limiting their ability to survive and contribute to the population."
Article first appeared on ABC Science.