Fish may be losing their survival instinct and even becoming attracted to the smell of their predators as climate change is turning the world oceans more acidic, a new study has warned.
Fish living on coral reefs where carbon dioxide seeps from the ocean floor were less able to detect predator odour than fish from normal coral reefs, the study found.
The research confirms laboratory experiments showing that the behaviour of reef fishes can be seriously affected by increased carbon dioxide concentrations in the ocean.
The study led by Philip Munday, from James Cook University in Australia, is the first to analyse the sensory impairment of fish from CO2 seeps, where pH is similar to what climate models forecast for surface waters by the turn of the century.
"These results verify our laboratory findings," said Danielle Dixson, an assistant professor in the School of Biology at the Georgia Institute of Technology in Atlanta.
The pH of normal ocean surface water is around 8.14. The study examined fish from so-called bubble reefs at a natural CO2 seep in Papua New Guinea, where the pH is 7.8 on average.
With today's greenhouse gas emissions, climate models forecast pH 7.8 for ocean surface waters by 2100, according to the Intergovernmental Panel on Climate Change (IPCC).
Munday and Dixson were the first to show that fishes' sensory systems are impaired under ocean acidification conditions in the laboratory.
"They can smell but they can't distinguish between chemical cues," said Dixson.
Carbon dioxide released into the atmosphere is absorbed into ocean waters, where it dissolves and lowers the pH of the water.
Acidic waters affect fish behaviour by disrupting a specific receptor in the nervous system, called GABAA, which is present in most marine organisms with a nervous system. When GABAA stops working, neurons stop firing properly.
Coral reef habitat studies have found that CO2-induced behavioural changes, similar to those observed in the new study, increase mortality from predation by more than fivefold in newly settled fish.
Fish can smell a fish that eats another fish and will avoid water containing the scent.
In Dixson's laboratory experiments, control fish given the choice between swimming in normal water or water spiked with the smell of a predator chose the normal water.
But fish raised in water acidified with carbon dioxide chose to spend time in the predator-scented water.
Juvenile fish living at the carbon dioxide seep and brought onto a boat for behaviour testing had nearly the identical predator sensing impairment as juvenile fish reared at similar CO2 levels in the lab, the study found.
The study was published in the journal Nature Climate Change.
