[Coral-List] Parrotfish, nutrients, and control of algae

Michael Risk riskmj at univmail.cis.mcmaster.ca
Sat Nov 10 22:22:53 EST 2007

Hello Tom, listers.

I am in an airport with (free!) wireless, read your recent post. The
devil making work for idle hands, thought I would respond.

The situation in Maldives you describe is outlined in some detail in
the chapter on Maldives I wrote for the McClanahan/Obura/Sheppard book
on Coral Reefs of the Indian Ocean: Oxford U Press, maybe 2000...

I was intrigued by the cultural habits of Maldivians setting up a
"natural" experiment on grazing and nutrients. As you note, Maldivians
do not eat reef fish. A Maldivian cookbook is 400 recipes for curried
tuna. Most of the grazing in the Indo-Pacific is by fish, so the
cultural framework sets up a situation in which the grazing community
does not change from island to island, just the nutrient input.

The capital, Male, is an island about 2kmx1km, on which live maybe
200,000 people. All the sewage is discharged, untreated, through four
pipe at the four corners...Reefs around Male were killed stone dead at
least a decade ago. Hordes of surgeons, tangs, parrotfish etc swim over
the algae-covered remnants. These dead reefs also had the highest coral
tissue values of 15N ever recorded-until our work off Florida.

So there it is, a natural experiment that shows nutrients kill reefs
even when grazers are intact. Why should this surprise anyone? But
evidently it does.

I was further entranced by the implications of living on an atoll and
killing all the coral. Another amusing whoops interesting aspect of
that chapter: we reported the results of some shallow seismic work we
did in Maldives. Turns out that remote(ish) atolls are clean, intact,
show the usual layercake Pleistocene-Holocene sequence-but on Male,
some recent faults run right up through the Holocene. I suggested that
these had been produced by the combination of recent construction, and
the fact that the island had switched over from a constructive to a
destructive stage. At the time, I predicted that this would not result
in a happy ending.

Reef cracking has recently occurred on Male-luckily, so far restricted
to the backreef atoll sequence. Should the atoll rim let go, the
potential loss of life would be in the thousands. It could very well be
that some luckless Maldivians will be the first global witnesses to the
downside of killing reefs with sewage.


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