[Coral-List] Parrotfish (and Urchin Introductions)

Szmant, Alina szmanta at uncw.edu
Tue Aug 5 11:10:56 EDT 2014

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

-----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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