SYDNEY (AFP) - Fish are losing their survival instinct - even becoming attracted to the smell of their predators - as the world's oceans become more acidic because of climate change, new research said on Monday.
The study of fish in coral reefs off the coast of Papua New Guinea - where the waters are naturally acidic - showed the animals' behaviour became riskier.
"Fish will normally avoid the smell of a predator, that makes perfect sense," lead author Professor Philip Munday from Australia's James Cook University told AFP.
"But they start to become attracted to the smell of the predator. That's incredible.
"They also swim further from shelter and they are more active, they swim around more. That's riskier behaviour for them - they are more likely to be attacked by a predator."
Prof Munday said the research, published in the journal Nature Climate Change, was important given that about 30 per cent of the carbon dioxide released into the atmosphere is ultimately absorbed by the ocean, a process which results in the seas becoming more acidic.
Acidification around the reefs studied is at levels predicted to become ocean-wide by the end of the century as the climate changes.
Prof Munday said the fish appeared to have failed to adapt to the conditions, despite living their whole lives exposed to high levels of carbon dioxide.
"They didn't seem to adjust within their lifetime," he said. "That tells us that they don't adjust when they are permanently exposed to these higher carbon dioxide levels and we would have to think about whether adaptation would be possible over the coming decades."
He said the "seep" to which the fish were exposed - in which carbon dioxide from undersea volcanic activity bubbles to the surface - was the perfect "natural laboratory" for the study.
Close to the seep there is no coral growth, but further away lies a unique coral reef zone with carbon dioxide levels similar to those forecast for future decades.
Co-author Jodie Rummer said while the increased carbon dioxide in the water affected how fish behaved, it did not appear to affect their athletic performance.
"The metabolic rates of fish from the seep area were the same as fish from nearby 'healthy' reefs," she said in a statement.
"So, it seems that future ocean acidification may affect the behaviour of reef fishes more than other aspects of their performance."
The research was conducted by James Cook's Coral Centre of Excellence, the Australian Institute of Marine Science, Georgia Institute of Technology and the National Geographic Society.