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Ocean acidification disorients fish, riles up scientists

clownfish.jpgI may need to start a file for ‘ocean impacts we hadn’t thought of’. First there was the projection that the seas will get noisier as a result of ocean acidification, which whale conservation groups were running with at a UN conference in December. Now researchers report in PNAS that ocean acidification may make fish larvae lose the sense of smell they use to find a home.

Most coastal marine species are swept out to sea during their larval stage and have to find their way to a habitat they can settle down in. Orange clownfish – yes, that’s the famously lost fish from Finding Nemo – must get back to reefs, often ending up in the same ones where they hatched. Philip Munday and Danielle Dixson of James Cook University in Australia have been studying olfactory cues the clownfish may follow.

But they and their colleagues report that clownfish larvae reared in aquariums at pHs of 7.8 and 7.6 don’t respond to smell tests the same way as control fish. These pHs are low compared to the 8.15 the fish live with today, but a business-as-usual rise in carbon dioxide emissions could take the ocean to 7.8 by 2100 and 7.6 in the following century.

Larvae reared at pH 7.8 favour the pungent smell of a swamp tree that clownfish normally avoid. They also fail to discriminate between the smells of their parents and other adults, suggesting that acid-addled Nemos might end up inbreeding more often. The pH-7.6-reared fish don’t swim toward any of the tested scents, suggesting they’ve stopped smelling altogether.

It’s not just clownfish that smell their way home. If acid damages fish’s olfactory capabilities the researchers say, many other coastal marine species could be affected.

One question the paper doesn’t take up is whether an ocean pH of 7.6 would leave any reefs for clownfish to come back to. The danger to reefs, and other better-known impacts of acidification, have meanwhile been highlighted in a declaration by 155 ocean scientists from 26 countries – and they want action.


The document comes out of the October 2008 installment of the UN conference series “The Ocean in a High-CO2 world”, held in Monaco. Delegates to the Monaco meeting had already summarized key issues in a report on research priorities – just like at the last such meeting, in 2004. But this year, with the UN climate change conference in Copenhagen looming, they went one further and issued an alarm signal to policymakers. From the opening of the “Monaco Declaration”:

We are deeply concerned by recent, rapid changes in ocean chemistry and their potential, within decades, to severely affect marine organisms, food webs, biodiversity, and fisheries….We call for policymakers to act quickly to incorporate these concerns into plans to stabilize atmospheric CO2 at a safe level to avoid not only dangerous climate change but also dangerous ocean acidification.

The statement stresses that the 30% rise in surface-ocean acidity since the eighteenth century may already be affecting marine life, decreasing the ability of many organisms to build shells and skeletons – with potential impacts both ecological and economic. It also takes up geoengineering schemes, pointing out that many don’t fix acidification and some, such as ocean fertilization, exacerbate it.

The top priority, according to the statement, is simply to raise awareness (italics are original):

First and foremost, policymakers need to realize that ocean acidification is not a peripheral issue. It is the other CO2 problem that must be grappled with alongside climate change.

Anna Barnett

Photo: Adult orange clownfish / National Academy of Sciences, PNAS

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