The Hot Zone

Not finding Nemo

Posted by Alison Hawkes on July 27, 2010
Category : The Oceans

Much has been made about how changing the pH of ocean water prevents corals and other critters from calcifying  shells. Ocean acidification is also impacting fish in strange ways, too, says a study in a recent edition of the Proceedings of the National Academy of Sciences.

Researchers led by Philip Munday at James Cook University in Australia explored the way clownfish and damselfish larvae responded to water with dissolved CO2 concentrations of 700 ppm and 850 ppm. Surprisingly, they were attracted to their worst enemies: rock cod and dotty back fish.

Clownfish in the East China Sea, near Okinawa island.

A colorful damselfish

When transplanted back into a natural coral reef habitat their risky behavior didn’t cease. The daredevils’ mortality rate was 5 to 9 times higher than what it would have otherwise been.

How is this happening? When the fish are small they spend time floating amongst plankton and doing the best they can to avoid predators and find suitable habitat by sensing chemical cues. But their sense of smell is impaired under lower pH conditions.  Some of the individual clownfish studied spent upwards of 88 percent of their time in a water stream that contained the predator cue.

It only gets worse the higher the CO2 concentrations. They didn’t hide out long after being scared and ventured away from the reef and explored without concern for their fate.

The researchers warn that clownfish and damselfish are hardly the only species that might be impacted. They write:

Impairment of the olfactory system by elevated CO2 would have far-reaching implications for marine diversity if other species are similarly affected and if it increases mortality of larvae during recruitment to adult populations.

At the current rate, CO2 concentrations could easily reach those tested with the clownfish and damselfish by the end of the century. The fish would be hard pressed to adapt, since the resultant decline in the ocean’s pH (by 0.3 to 0.4 units) would be a rate faster than any time in the last 650,000 years. Where will we find Nemo?

-Alison Hawkes

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