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Ocean Acidification Causes Hearing Loss In Fish - Study

Higher levels of CO2 affect the ability of young snapper to hear, according to a new study published by scientists from the University of Auckland, NIWA and James Cook University in Australia.

Humans are releasing more carbon into the atmosphere, resulting in warming of both the atmosphere and oceans. The sea absorbs around 30 per cent of CO2 produced and increasing the amount of CO2 absorbed causes the world’s oceans to acidify.
During the larvae stage, fish develop remarkable swimming and sensory abilities, which are vital to survival and their ability to locate suitable settlement habitat and replenish populations. Understanding the effect of ocean acidification is therefore crucial to determining its potential effects on fish recruitment and population health.
The ability of fish to detect sound is critical for a variety of behaviours, including as a guide for fish settlement and movement near reefs, the selection of mates and to synchronise mating behaviour.
Fish, such as snapper, ‘hear’ through paired small bone-like structures called otoliths which act like the human middle ear.
In a new study, the research team collected eggs from wild-caught broodstock snapper held at the NIWA National Marine Research Centre in Ruakaka, The resulting larvae were reared in tanks under ambient conditions for 21 days before being split between two treatments: normal seawater and seawater with elevated CO2 levels.
At 42 days after hatching, 10 young snapper from each treatment were fitted with tiny micro-sensors to measure hearing ability, a technique similar to how human babies have their hearing measured.
An underwater speaker was placed at the end of both tanks which delivered tone bursts of 10 micro-seconds duration at frequencies of 100, 200, 400, 600 and 800 Hz. The vast majority of fish tested to date hear best below 1000 Hz.
The study fish were then scanned using micro Computer Tomography (microCt), a 3D imaging technique using x-rays to see inside an object, to examine the anatomical structure of the otoliths – both their size and symmetry.
The results show hearing thresholds were significantly less at low frequencies (80-200Hz) for juveniles reared under elevated CO2 conditions, compared with their control counterparts.
Analysis showed that fish raised in the tank with elevated CO2 had significantly larger and asymmetrical otoliths – different shapes between left and right otoliths - than the control group.
It’s thought that asymmetrical otoliths were the reason the CO2-exposed fish were not as sensitive at lower frequencies.
“We are not entirely sure how fish localise sound but they do,” says University of Auckland Associate Professor Craig Radford. “Sound helps them locate their home reef for example so without accurate hearing, they cannot find their way home.
“This study has found that asymmetric otoliths, where the paired otoliths are different shapes, means fish are less sensitive to sound and so means they might not be able to accurately localise the sound of their ‘home’ reef.”
Dr Darren Parsons, from the University of Auckland and NIWA, says the next step would be to study whether or not fish have the ability to adapt to rising levels of CO2 in the ocean.
“If fish morphology is unable to adapt, then there could be serious consequences for the structure and function of future aquatic communities.”
The study was funded by Fisheries New Zealand and the Ministry of Business, Innovation and Employment.

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