[Coral-List] Parrotfish (and Urchin Introductions)

Tue Aug 5 13:38:08 EDT 2014

Hi John:

The concept that Diadema were only abundant in shallow reef waters is incorrect, and due to few people having done night dives at depth on Caribbean reefs before the die-off.  My observations at the 70-90 ft depths at night were that the reef looked like a pin cushion with Diadema coming out from the deeper ledges where they sheltered in the daytime.

In the FL Keys, the corals went downhill starting in 1987 with the onset of almost annual bleaching events, in the presence of many, many grazers (Mark Hay documented some of the highest grazing fish bite rates in Keys).  Diadema was already gone, but Keys have always had plenty of herbivorous fishes.  Nutrients in the keys reefs are not the cause of any algal blooms (see Szmant and Forrester 1996 for a comprehensive study of water column and sediment nutrients).  Later work funded by COP again showed Keys reef waters to be very low in both nutrients and Chla.  Nutrient enrichment experiment have shown that the common/dominant reef algae such as Halimeda and Dictyota actually do poorly with nutrient enrichment.  So at least in the case of the FL Keys, to me it is clear that global warming has been THE cause of coral loss leading to too much open substrate for algal colonization.  As Williams and Polunin (2001) showed, fishes can only eat so much even when there are plenty of them.

Alina

"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 John McManus
Sent: Tuesday, August 05, 2014 12:40 PM
To: coral-list at coral.aoml.noaa.gov
Subject: Re: [Coral-List] Parrotfish (and Urchin Introductions)

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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