[Coral-List] Over fishing contributing to ocean acidification?

[Gene Shinn]

An interesting article about an article published in Science. Gene

Fish an ally against climate change

     * New Scientist 13:02 16 January 2009 by Catherine Brahic

An unlikely ally may have been found in the fight against the effects 
of climate change. Fish excretions seem to play a key role in 
maintaining the ocean's delicate pH balance, says a study that also 
reveals that there are 2 billion tonnes of fish in the world's oceans.

Bony fish excrete lumps of calcium carbonate, known as "gut rocks" 
which are thought to dissolve in the upper layers of the ocean. A 
team led by Rod Wilson of the University of Exeter in the UK has now 
shown that the sheer amount of gut rocks produced plays a key role in 
buffering the carbon dioxide that acidifies seawater.

"This study really is the first glimpse of the huge impact fish have 
on our carbon cycle - and why we need them in the ocean," says 
Wilson's colleague Villy Christensen of the University of British 
Columbia in Canada.

Protective role

While marine biologists have known for some time that fish produce 
gut rocks, until now no-one had estimated just how much calcium 
carbonate is spewed out into the ocean in this way.

It was widely believed that most marine carbonate is provided by the 
external skeletons of marine plankton. These microscopic organisms 
are likely to be hard hit as climate change increases the acidity of 
the oceans and their skeletons literally dissolve away.

The new study reveals that fish play an important role in stopping 
this from happening.
ferent models to estimate the amount of fish biomass that is in the 
global oceans, and its distribution.

By drinking salt water, fish ingest a lot of calcium, and they 
excrete more or less calcium carbonate depending on their size and 
the temperature of the water. "For a given total mass of fish, 
smaller fish produce more than bigger fish, and fish at higher 
temperatures produce more than fish at lower temperatures," explains 
Wilson.

Surprise finding

The team then used data on how much carbonate fish produce on average 
to calculate how much the fish biomass represented in their computer 
models are likely to excrete.

This revealed that between 3% and 15% of all the calcium carbonate 
produced in the oceans comes from fish. Wilson says this is a 
conservative estimate - he and his team think the real figure could 
be three times higher.

"I expect it will be a big surprise to most of the ocean scientists 
who study the ocean carbon cycle," says Wilson. "Apart from a handful 
of fish biologists around the world, the scientific community were 
previously unaware that fish produce of any of this chalky mineral, 
let alone enough to be significant on a global scale."

Eric Achterberg of the National Oceanography Centre in Southampton, 
UK, says the study offers an insight into an underrepresented marine 
process. "Whether the fish carbonate is really an important 
contribution to the mid-water alkalinity is not certain yet and forms 
an excellent topic of research," he says.

'Unrecognised allies'

Wilson agrees that it is not yet certain whether the gut rocks do 
indeed dissolve in the upper layers of the ocean. Their chemical 
structure suggests that they are very soluble in seawater and should 
readily dissolve. But if future studies show this does not happen, 
this will mean the gut rocks sink to the bottom of the ocean without 
dissolving and buffering the oceans.

Because fish carbonate production goes up with temperature, fish are 
likely to produce more carbonate - and be more effective buffers of 
ocean acidity - as temperatures increase through global warming. 
That's the good news. The bad news is that overfishing may have an 
additional downside: in addition to depleting food stocks, it could 
also deplete the precious carbonate buffer.

Because of the complexity of ocean chemistry, "we cannot really say 
much with any confidence about how overfishing might affect ocean 
acidification says Wilson. "But we definitely need to study this more 
to help make better predictions about these future changes."

"We must buck the current trend of clear-cutting of the oceans and 
foster these unrecognised allies against climate change," says 
Christensen.

Journal reference: Science (DOI: 10.1126/science.1157972)

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No Rocks, No Water, No Ecosystem (EAS)
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University of South Florida
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