A heron catches a fish in Orlando, Florida. Photo: AFP
Escalating carbon dioxide emissions will cause fish to lose their fear of predators, potentially damaging the entire marine food chain, joint Australian and US research has found.
A study by the Australian Institute of Marine Science (Aims), James Cook University and the Georgia Institute of Technology found the behaviour of fish would be "seriously affected" by greater exposure to CO 2.
Researchers studied the behaviour of coral-reef fish at naturally occurring CO 2 vents in Milne Bay, in eastern Papua New Guinea.
They found that fish living near the vents, where bubbles of CO 2 seeped into the water, "were attracted to predator odour, did not distinguish between odours of different habitats, and exhibited bolder behaviour than fish from control reefs".
The gung-ho nature of CO 2-affected fish means that more of them are picked off by predators than is normally the case, raising potentially worrying possibilities in a scenario of rising carbon emissions.
More than 90 per cent of the excess CO 2 in the atmosphere is soaked up by the oceans. When CO 2 is dissolved in water, it causes ocean acidification, which changes its chemistry.
The Aims study found the diversity of fish at the CO 2 vents was not influenced, but that fish nerve-stimulation mechanisms were altered, meaning the smell of predators became alluring.
"What we have now also found in our study of fish behaviour in this environment is that the fish become bolder and they venture further away from safe shelter, making them more vulnerable to predators," said Alistair Cheal, co-author of the research.
While fish at the vents faced fewer predators than usual, the consequences for fish in the wider ocean could be significant as more CO 2 was dissolved in the water.
This article appeared in the South China Morning Post print edition as Why fish are losing fear of predators