[CDHC] Mysterious, widespread ongoing deep reef mortality

[Tyler Smith]

Hi All,

I want to get the communities suggestions/advice on a widespread and  
ongoing mortality event that we are witnessing on deep reefs in the  
Marine Conservation District (MCD), St. Thomas, US Virgin Islands  
(see posted map 'west mcd-octnov07.jpg', directions below, Area where  
mortality has been found in box [excluding point 52]).  I can't post  
pictures to CDHC, so I have uploaded the files to xdrive, an online  
sharing program.  They can be accessed at:

http://plus.xdrive.com/u/1731367875/837505790yUsUshgXGiJ3vcVUXsW

Folder: 'MyPhotos': 'West MCD Mortality Oct07'

Unfortunately, you will need to create an x-drive account for free.

Background: Our research team is involved in a habitat validation  
survey of multibeam imagery from this MPA.  This area south of St.  
Thomas has high coral cover (30-50%) reefs, nearly unbroken over  
dozens of square kilometers.  All reefs are greater than 30 meters  
deep, and are dominated by plating corals in the Montastraea  
annularis species complex.  The area has been a no-take marine  
reserve since 1999 and there is very little conceivable local  
anthropogenic pressure.  Our surveys consist of NITROX or closed  
circuit rebreather dives onto predicted habitat strata at random,  
predetermined GPS points.  We are deploying 30 x 4 m belt transects  
to assess fish and other motile resources, video transects along the  
same transect (but only 10m in length) to assess benthic cover, and  
coral health assessments (line intercept) for as many corals on the  
transect as possible before we kill ourselves (about 20-40 corals).

Problem: There are signs of high prevalence recent mortality in some  
of the reef areas we have been sampling.  This mortality is mostly  
concentrated in "back reef" areas behind the main outer reef rise  
before the shelf edge.  Coral reefs in this area are typically formed  
of pedestals and small pinnacles that are either flat over the area  
(see photo StT-MCD P119-Oct9-07 - 15.jpg, white areas are mortality)  
or anastomose to form rises and valleys.  White plague or plague-like  
incidence is common on corals in these high density reef areas,  
however, the signs of this mortality are like nothing I have ever  
seen before.  Coral appears to be degrading and sloughing from  
multiple points of the colony at once (StT-MCD P119-Oct9-07 - [11,  
14, 19, 22, and 26].jpg).  The degraded portions are dead, not  
bleached (although there is also strange, granular bleaching  
occurring at low prevalence throughout the MCD, see StT-MCD P119- 
Oct9-07 - 17.jpg).  There is a lot of Schizothrix associated with  
these areas, although it is unclear if this is contributing to the  
mortality or simply feeding of nutrients released from degrading  
tissue.   The mortality is widespread within areas and over at least  
a square kilometer (StT-MCD P119-Oct9-07 - 30.jpg, the yellow thingy  
in the upper left quadrant of the photo is a diver [ignore the  
curvilinear transect].  All the white bits in the picture represent  
recent mortality [sorry a little dark].  We have seen this mortality,  
to varying degrees, at four sites that range over a square kilometer).

Fuel for the mystery:  This area is over 15 km from the shoreline of  
St. Thomas, and so should be fairly buffeted from nearshore impacts.   
It has been a no-take MPA for almost 10 years.  We are experiencing  
+0.6°C anomaly and there is a bleaching watch.  Upwelling, via  
internal waves, occurs onto these reefs and is more intense and the  
variability higher during the warmest sea seawater months (now) when  
the water column is more stratified.  However, the minimum temp  
doesn't get below 26°C, which should be quite comfy for coral.  This  
may be an area of slow current, and we deployed an ADCP, fluorometer,  
and temp. probe today.

While I don't know if anyone can shed light on the process causing  
this mortality, I wonder if anyone has seen anything similar and  
might there be any suggestions as to what could cause this type of  
mortality.

Thanks!
Tyler


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tyler B. Smith, Ph.D.
Center for Marine and Environmental Studies
University of the Virgin Islands
2 John Brewers Bay
St. Thomas, USVI       00802
USA
office: 340-693-1394
fax:     340-693-1385


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