Current rates of ocean acidification are silencing oceans, which spells trouble for marine animals during their larval stage.  Fish larvae, for instance, will not be able to find their way back home, because they use sound cues to orient themselves, say researchers from the University of Adelaide.

"More than 95 percent 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," study co-author Sir Ivan Nagelkerken said in a statement. "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."

In the latest study, a team of researchers led by Ph.D. student Tullio Rossi travelled to a naturally occurring carbon dioxide vent near White Island in New Zealand. This study area provided researchers with a "peek into the future," since ocean acidification levels present here are similar to those predicted for the end of the century.

After recording the vent's soundscape and comparing the loudness and composition of sounds with control sites a few hundred meters away, researchers found the area surrounding White Island was much quieter.

"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."

Next, researchers examined how increasing carbon dioxide levels impact a common temperate fish species known as mulloway (Argyrosomus japonicas). They found that 25- to 28-day-old mulloway larvae exposed to higher carbon dioxide concentrations did not use present-day acoustic habitat cues, while fish reared in present-day carbon dioxide levels responded normally.

Previous studies have linked ocean acidification to the growth of otoliths, or fish ear bones, which are used for hearing, orientation and balance. Contrary to common belief – which suggests larger ears would increase the hearing range of larval fish – researchers found rising carbon dioxide levels ultimately impact fish negatively.

"Finding a home is the key to population sustainability," Dr. Nagelkerken concluded. "Those that rely on sound as an orientation cue will be heavily impacted, limiting their ability to survive and contribute to the population."

Researchers suggest future generations of fish may have to rely on different cues to navigate vast oceans but that may be tricky. Some of their other senses – vision and scent detection – seem to be negatively impacted by ocean acidification, too.

The findings were recently published in the journal Biology Letters. 

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