[Coral-List] bioerosion and Bumphead Parrotfish

Szmant, Alina szmanta at uncw.edu
Fri Oct 27 09:24:35 EDT 2006

Hi Doug:

A very interesting and thoughtful post.  One additional observation:
some of the Caribbean grazing parrotfishes and other reef invertivore
fishes (e.g. angelfishes) are well known to eat sponges, including
encrusting & boring species with external oscula that can be grazed.  I
have seen parrotfishes targeting in their grazing the advancing edges of
Cliona colonies.  The virtual disappearance of these big fishes from
many Caribbean and Pacific reefs may be allowing increased proliferation
of the sponges and other organisms on which these fishes feed,
potentially leading to an increased abundance of bioeroding sponges. A
major reason for reef inverts to use cryptic habitats is to avoid
predation.  When predation pressure decreases, these formerly cryptic
organisms will gradually increase in abundance on less cryptic surfaces.
This kind of altered community dynamics is poorly studied, but in my
opinion, critically important.  The point you made that fishes need to
be protected not just within MPAs but on reefs in general, is one that I
have attempted to make many times at reef meetings such as the Coral
Reef Task Force meetings.  Reefs without a full and normal complement of
fish types and sizes are condemned to function under dysfunctional



Dr. Alina M. Szmant
Coral Reef Research Group
UNCW-Center for Marine Science 
5600 Marvin K. Moss Ln
Wilmington NC 28409
Tel: (910)962-2362 & Fax:  (910)962-2410
Cell:  (910)200-3913
email:  szmanta at uncw.edu
Web Page:  http://people.uncw.edu/szmanta
-----Original Message-----
From: coral-list-bounces at coral.aoml.noaa.gov
[mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of Douglas
Sent: Sunday, October 22, 2006 5:41 PM
To: Coral-List at coral.aoml.noaa.gov
Subject: [Coral-List] bioerosion and Bumphead Parrotfish

     One thing that seems to be missing from the bioerosion discussion
so far is the role of large parrotfish in the Indo-Pacific.  In a 2003
article, Bellwood, Hoey & Choat report that of the 3000 or so coral reef
fish species, only the parrotfish family produce significant bioerosion,
and among the parrotfish in the Indo-Pacific, most of that is due to 2
species, the majority of which on the outer, clear-water Great Barrier
Reef is due to just one species, Bolbometopon muricatum.  These huge
parrots travel in schools of 30-50, and each individual is estimated to
remove 2.33 m3 or 5.69 tonnes of carbonate a year, which with the
population of these fish on the GBR crest amounts to 27.9kg/m2/yr.  They
say that annual calcification on coral reefs is usually estimated to be
on the order of 3-10 kg/m2/yr, but may approach 35kg/m2/yr.  They say
that their figure for B. muricatum is the highest erosion rate ever
recorded on coral reefs, and alone balances local accretion.  Remember
 , this was on outer northern GBR, some of the least human-disturbed
reefs in the world.  They went on to look at a total of 44 sites across
the range of this species, from Indonesia to Tahiti, and found that the
population of this species goes down rapidly with increasing human
impact and quickly approaches zero.  These huge fish sleep in holes too
small to fit in, and are easy prey for night time spear fishing (with
waterproof flashlights, which may be the critical piece of technology).
Howard Choat has shown me a photo of a skiff filled with them from one
night's spearfishing of a small group of spearfishermen in the Solomon
Is.  Dulvy and Polunin report that at one point in Fiji, they comprised
a majority of the fish in the fish markets, but now they are rare in the
markets and locally extinct in parts of Fiji.  Gerry Davis and Chuck
Birkeland tell me that big schools used to be common in Guam, but were
spearfished out in the '70s, and now they are rare there.
    It seems to me that if people are concerned about bioerosion, it
would be far easier to monitor the populations of B. muricatum than
microborers or even macroborers.  They're huge, out in the open, and in
most places the job of recording them is very easy- just write 0, 0, 0,
0 on your slate.  Probably most monitoring reports don't say what their
populations are, because they didn't record any.  Well, none is a real
number, folks, and in this case it's important to report.  It's a
measure of what we've lost.
    Further, this raises an interesting question.  Some people have been
hinting that reef accretion might be equal to reef health.  If so, then
perhaps we should encourage the spearfishing of this species, and not
shed any tears if it goes locally extinct.  That will reduce bioerosion,
and that's a good thing, right?
    I think people are on the right track when they say that bioerosion
as well as accretion are important.  And likely neglected.  I do
seriously advocate monitoring populations of B. muricatum, and think it
would be good to do for macroborers and microborers, if a practical
method can be developed for monitoring programs.  But I also point out
that coral reefs are not just geological structures (and I'm not saying
anyone has said they are just geological!), but they are also
ecosystems.  One of the points that Bellwood et al are making is that
while Indo-Pacific reefs, particularly near the diversity center, are
incredibly species-rich, some guilds have very few species, and in this
case the guild of bioeroding fish falls to just one species, which has
been removed from much of its range, with unknown but perhaps major
consequences for these ecosystems.  B. muricatum is also a coral
predator, with about half it's bites on live coral.  So its removal may
have effects on cora
 l populations as well.  Once again, humans are performing an unplanned
experiment on natural ecosystems, with unknown consequences.  Not smart.
I personally think we need to start thinking seriously about protecting
large reef species in places where they have been depleted, which is
nearly everywhere people are.    And not just inside MPA's, which are
usually too small to protect this species which ranges over a large
area, and other large fish like sharks, which also move over a large
area.  Friedlander and DeMartini have reported that big predatory fish
(sharks and Giant Trevally) are amazingly abundant in the Northwest
Hawaiian Islands, which are some of the most pristine reefs in the
world.  In something like 155 transects around the Main Hawaiian Islands
(where the people are), just 1 shark was recorded.  They point out that
removing a huge biomass of predatory fish certainly has an effect on the
food web.  I personally think the big reef fish (including Humphead Wras
 se and some big Grouper species) need to be protected throughout whole
archipelagos or reef systems, until they recover.
   -Doug Fenner

The opinions expressed here are those of the author alone, and not that
of any institution.

Bellwood, D. R., Hoey, A. S., and Choat, J. H.  2003.  Limited
functional redundancy in high diversity systems: resilience and
ecosystem function on coral reefs.  Ecology Letters 6: 281-285.

Dulvy, N. K. and Polunin, N. V. C.  2004.  Using informal knowledge to
infer human-induced rarity of a conspicuous reef fish.  Animal
Conservation 7: 365-374.

Friedlander, A. M. and E. E. DeMartini.  2002.  Contrasts in density,
size, and biomass of reef fishes between the northwestern and the main
Hawaiian islands: the effects of fishing down apex predators.  Marine
Ecology Progress Series 230: 253-264.

Sadovy, Y., Kulbicki, M., Labrosse, P., Letourneur, Y., Lokani, P.,
Donaldson, T. J.  2003.  The humphead wrasse, Cheilinus undulatus:
synopsis of a threatened and poorly known giant coral reef fish.
Reviews in Fish Biology and Fisheries 13: 327-364.
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