Rising Ocean Acidity Could Disorient Fish

Fish may be literally incapable of finding home in the acidified seas of a carbon-soaked future.

When they’re raised in waters with an acidity comparable to what’s expected by the 21st century’s end, baby clownfish — an aquarium favorite that relies on smell to find home — failed to respond to familiar smells.

“If acidification continues unabated, the impairment of sensory ability will reduce population sustainability of many marine species, with potentially profound consequences for marine diversity,” wrote researchers led by marine biologists Philip Munday and Kjell Døving.

Their study, published Monday in the Proceedings of the National Academy of Sciences, was prompted by the rapidly rising acidity of Earth’s oceans — the most pressing of a battery of climate- and development-driven oceanic threats, from rising temperatures to nitrogen pollution and spreading dead zones.

As seawater absorbs CO2, the proportion of hydrogen ions falls: Average oceanic pH has dropped by 0.1 since pre-industrial times, and will likely fall by another 0.3 to 0.4 units within the next century. These fractional numbers may sound insignificant, but they represent an unprecedented change in both degree and pace in the last 650,000 years, and marine life may be ill-equipped to cope.

Most research on the environmental impacts of acidification has focused on the vulnerability of shellfish, corals and crustaceans, whose shells are weakened and dissolved by acidic waters. But the latest findings show that fish may also be directly and profoundly affected.

In the study, Munday and Døving’s team raised juvenile clownfish in water with a pH of 8.15, generally equivalent to that of contemporary oceans. The fish were then placed in a plastic chute separated at one end into two channels, each fed by a different tube.

The researchers put different scents into the tubes: oil from a tree to which clownfish are usually attracted, oil from a swamp tree they usually avoid, secretions of anemones (a favorite clownfish habitat) and secretions from their own parents (which the fish usually avoid).

Under standard pH conditions, the fish behaved as expected. But when the researchers put them in water with an expected end-of-century pH of 7.8, they became drawn to scents they had previously ignored. At pH 7.6, they stopped responding to scent altogether.

It’s not yet certain whether the findings can be extrapolated to wild clownfish or other species, cautioned Munday, but he noted that many species use olfactory cues to make important behavioral decisions.

“It is possible that other species may be affected,” he said, and noted that a messed-up sense of smell could be just one of “a range of behavioral effects of increasing CO2 in the ocean that we are not yet aware of.”

It’s possible that fish could adapt, but the researchers are not optimistic.

“Ocean pH has changed little over the past 650,000 years,” they wrote. “It is unlikely that genetic adapation by most marine organisms will be able to … keep pace with such a rapid rate of change.”

WIRED SCIENCE, 2 February 2009. Article.

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