HOW do small fish survive predators lurking in the deep ocean?
Nature has its way of cautioning them. Species of fish, like mackerel, clownfish and damselfish, have a heightened sense of smell to detect threats and find shelter. Not for long though.
Scientists from James Cook University in Australia said high levels of carbon dioxide (CO2) dissolved in seawater are muddling the smelling ability of fish so much that instead of swimming away, it gets attracted to the odour of its predator. The team led by marine scientist Philip Munday recently studied the impact of high CO2 levels on clownfish and damselfish. They reared the larvae in aquariums with four different levels of CO2: one with an ideal level of 390 parts per million (ppm) and the others with 550, 700 and 850 ppm of CO2.
After rearing them for a few days, the fish were transferred to an aquarium with two inlets. One inlet brought in seawater and the other poured seawater with odour markers from predators. Fish from seawater with 390 ppm CO2 and that with 550 ppm avoided the predator odour stream. Fish reared in 700 ppm CO2 seawater initially avoided it, but later half of them lingered on in the stream. Those reared in 850 ppm CO2 spent most of their time in the stream with predator odour.
When the researchers released them to their natural habitat, fish reared in 850 ppm CO2 got attracted to predators and were eaten five to nine times more often than the others, the scientists said in the Proceedings of National Academy of Science on July 6. “High CO2 levels make seawater acidic and affect the smelling ability of the fish, which then show a risky behaviour,” said Munday.
Such change in the behaviour of smaller fish can affect their population and disturb the food web, said A A Nambi at M S Swaminathan Research Foundation in Chennai. High CO2 levels in seawater would suffocate marine animals and proliferate aquatic plants and algae, creating dead-zones, he said.
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