[Coral-List] What caused coral mortality?

Sheppard, Charles Charles.Sheppard at warwick.ac.uk
Thu Apr 27 03:59:27 EDT 2017

The post by Bruno was indeed very well done, but I wonder how much anyone’s conclusions are shaped by what reefs they have seen, and over what length of time?  Quite a lot I would say.  In the Caribbean, I saw the massive die-off of elkhorn from white band disease about 30 years ago.   It left vast expanses of shallow forest in a state which was, well, deforested, and then it turned to rubble in just a few years.  (See eg Habitat mapping in the Caribbean for management and conservation:  use and assessment of aerial photography.  Aquatic Conservation 5:277-298).  And much still is in that barren state – where now can we see many contiguous hectares of impenetrable elkhorn with its substantial fish community? 

Back then, most of us had not woken up to the fact that warming spikes were to become the big issue only a few years later.  Warming had already started: looking at graphs post-hoc shows that it started rising in its present trajectory around the 1970s or so.  But the diseases that directly did the killing were at the time attributed to the increase in sewage because the pathogens, or some of them, were the same ones you find in sewage.  Possibly they were made more virulent by slightly warmer water of course, but that is another thing.  

At that time some of us were saying that the near-extinction of the shallow coral forest was very important and that local management was needed to control that sewage.  While any local management in last year’s warming was swamped by the effects of that warming, authorities could have done more about eg sewage and the increasing overfishing decades ago, but we were quite strongly opposed by several, including some sections of the booming yacht tourism industry whose boats had no holding tanks and instead spread the stuff far and wide.

It is 5 years old now, (a time greater than it takes to start and finish a PhD…) but the paper by Aterweberhan et al 2013, (Climate change impacts on coral reefs: Synergies with local effects, possibilities for acclimation, and management implications, Marine Pollution Bulletin 74: 526–539) is relevant to the question of synergy vs antagonism between climate change, nutrients, diseases, sediment and overfishing.  Its tables list large numbers of papers talking about these things and their synergies.

Probably nobody now (well I can think of some perhaps!) would deny the primacy of warming today, especially after the last couple of warm years.  But as well as its direct bleaching effect, warming very often exacerbates effects of previously existing stressors too, like sedimentation or herbivore shortage and pathogens.  And here are two aspects that could usefully be separated more than they are: the initial killing agent and then the ability of reefs to recover post-killing.  The warming last year could and did kill anywhere and everywhere, whether or not there was management or whether there were other existing stressors.  Recovery following it, however, may be different and affected by more local things.  After all, recovery if it occurs, will take place after the warming has abated, will it not?    

A case in point:  I have just returned from northern Chagos atolls where, as in 1998, the scene is now truly tragic to at least 15 m depth or more, with over 90% mortality.  After 1998 though, recovery followed swiftly after a lag of a few years because a lot of recruits got going.  There were no local impacts to inhibit the surviving refuges from spawning.  In other countries I have worked, several other factors inhibited recruitment, so no recovery in those places ever really got going.  In Chagos this month there were, we think, sufficient recruits for recovery (in terms of total cover if not species composition) to occur again... that is, if future warming pulses let it happen.

So, IMHO, there is a danger of conflating 4 things:  what killed corals in the mid 1990s and earlier (local factors very often) vs. what killed them in 1998 and last year (warming), and secondly what killed the corals at any one time vs. what then stops some reefs from recovering when the warming fades.

I am sure it is not as black and white as some seem to think, but a more nuanced, coloured picture.  Although, the most memorable phrase I heard from someone snorkelling over a recently killed reef in Chagos was that it was looking like a reef in black and white rather than in colour!

Best wishes
Charles Sheppard

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