 OCEaN acidification caused by fossil fuel  emissions may be turning fish deaf.  Clownfish reared in seawater acidified by carbon dioxide grow up with impaired hearing, a study has found.  This could have devastating consequences for the colourful star of the 2003 animated movie Finding Nemo, say scientists.  Not only would it leave the coral reef fish vulnerable to predators, but it could impact on their early development and survival.  For the study, researchers reared newly hatched clownfish in water with different levels of acidity.  after 17 to 20 days, the juvenile fish had their hearing tested by being played the sounds of a predator-rich coral reef.  We kept some of the baby clownfish in today s conditions, bubbling in air, and then had three other treatments where we added extra CO2 based on the predictions from the Intergovernmental Panel on Climate Change for 2050 and 2100,  said Dr Steve Simpson, from the University of Bristol.  We designed a totally new kind of experimental choice chamber that allowed us to play reef noise through an underwater speaker to fish in the lab, and watch how they responded.  Fish reared in today s conditions swam away from the predator noise, but those reared in the CO2 conditions of 2050 and 2100 showed no response.  The findings are published in the Royal Society journal Biology Letters.  Since the Industrial Revolution, more than half the CO2 produced by burning fossil fuels has been absorbed by the oceans.  The effect has been to increase the acidity of the oceans at a faster rate than at any time in the last 650,000 years. 