[Coral-List] Parrotfish (and Urchin Introductions)

Tue Aug 5 12:39:38 EDT 2014

Thanks for pointing out the that Diadema involves both pros and cons. Of
course, we also must keep in mind that Diadema tends to be depth limited --
being normally abundant in the Caribbean in the top 10-15 meters. 

I want to urge everyone to actually read the whole report. Parrotfish come
across from the summaries and news articles as the new 'pandas'. Actually,
the report covers lots of herbivores, including Diadema, and various
acanthurids (surgeonfish and doctorfish). It also covers sedimentation and
nutrient loading. The evidence presented does tend to warrant some extra
attention for maintaining parrotfish populations, but clearly a holistic
approach is needed.  

If we assume, as some recent evidence indicates, that pristine reefs
generally had inverted biomass pyramids (more biomass in predators than
herbivores), then what we usually see now has passed from inverted pyramids
to upright pyramids to fleshy algal dominance. The latter state tends to be
self-perpetuating, due to high recruitment and growth rates, exclusion of
settling spaces for other organisms, overgrowth of living corals in some
areas, and exclusion of critical habitat features for herbivorous fish. In
cases where nutrient loads have been partly at fault, these will often have
to be reduced as a precondition to recovery. Then fleshy algal dominance
must be overcome by perturbations such as storm waves combined with
unusually high abundances of herbivores.  The reef communities must be reset
and put on new pathways. If there is any hope of returning some reefs closer
to near pristine states, they must first pass through the high herbivory
stage, even amid some negative impacts. 

Of course, bleaching, diseases, and ultimately acidification can be expected
to make fleshy algal dominance more and more likely to occur. However, we
cannot blame these things for the current state of many reefs until the
obvious steps of decreasing nutrients and increasing herbivory have been
taken. We have to stop kicking the patients. 

Cheers!

John     

-----Original Message-----
From: coral-list-bounces at coral.aoml.noaa.gov
[mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of Szmant, Alina
Sent: Tuesday, August 05, 2014 11:11 AM
To: David Fisk; coral-list at coral.aoml.noaa.gov
Subject: Re: [Coral-List] Parrotfish (and Urchin Introductions)

Thanks Dave.  I have photos from the 1970s and pre-1982 Caribbean that show
exactly that...Diadema grazing scars on the corals, over-grazing of the
substrate by the sea urchins.  And I am guessing the low rates of coral
recruitment in the few pre-1983 studies (before bleaching started affecting
the corals, and there was 60+ % coral cover, but before we figured out the
coral spawning cycles) was due to over-grazing by too high densities of
Diadema.. We hated Diadema back then and did not hesitate to bludgeon a few
to clear a path to the substrate, much to the delight of the wrasses!

There is no magic bullet to fix what is wrong with Caribbean reefs.  Healthy
fish grazer communities are just as important as healthy (not too many, not
too few) Diadema populations, but even more critical is environmental
conditions the corals can tolerate (i.e. not too hot in the summer as has
been experienced since the late 1980s).  

"Great minds discuss ideas; average minds discuss events; small minds
discuss people." Eleanor Roosevelt

"The time is always right to do what is right"  Martin Luther King

*************************************************************************
Dr. Alina M. Szmant
Professor of Marine Biology
AAUS Scientific Diving Lifetime Achievement Awardee Center for Marine
Science University of North Carolina Wilmington
5600 Marvin Moss Ln
Wilmington NC 28409 USA
tel:  910-962-2362  fax: 910-962-2410  cell: 910-200-3913
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 David Fisk
Sent: Monday, August 04, 2014 11:53 PM
To: coral-list at coral.aoml.noaa.gov
Subject: Re: [Coral-List] Parrotfish (and Urchin Introductions)

If it only was so simple to "put more effort and resources towards
reestablishing this keystone invertebrate herbivore", ie, in this case,
urchins. In itself, this most likely will not fix the issue for the obvious
reason that there needs to be natural controls on the urchin population.

Without a check on numbers by urchin predators, the reefs will be overgrazed
like many in the Pacific where the carbonate base and fabric of some reefs
are diminishing because of large populations of urchins. Urchin overgrazing
results in algal free substrates but there is no new coral recruitment
happening either, and the remaining live corals are undercut and eventually
carried away by waves and storms. Eventually, increased exposure of adjacent
coastal areas to storm waves are one consequence of this situation. I have
seen reefs in the Pacific where it appears that up to 30-40cm of limestone
pavement has been eroded away by urchins, judging by the age and size of the
remaining few large live corals, which were probably less than 50 years old.

There is plenty of evidence in the literature indicating that too little or
too much grazing pressure will lead to different but equally undesirable
outcomes. Furthermore, a single beneficial grazing level and density of
grazers (fish or invertebrate) that will enhance natural coral recruitment
will not necessarily be the same for all locations.

It might be worth trying a small trial study for urchin introductions, but
such an intervention would clearly have to have a longer term management and
monitoring component to head off further problems, bearing in mind the known
consequences of getting it wrong, as well as allowing for the risk of some
unknown detrimental factor coming into play.

Cheers, Dave Fisk
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