[Coral-List] Coral diseases and algae

John Bruno jbruno at unc.edu
Tue Feb 24 10:55:44 EST 2009

> By the way the effect of overfishing in Jamaica was not to eliminate
> herbivorous fish, as top-down dogmatists would have us believe, but
> precisely the opposite. In the early 1950s the reef was dominated by
> predatory and invertebrate eating fish, and there were very few
> herbivorous fish. After overfishing and coastal eutrophication the
> fish population switched to overwhelmingly herbivorous species,
> because that is all the food there is now. So the problem is not lack
> of grazers at all, but that the over-fertilized algae grows so fast
> that no grazers can control them.
> However I am not sure this lack of obvious algae interaction applies
> to other coral diseases than YBD. Jennifer Smith and colleagues, and
> Maggie Nugues and colleagues, have made convincing cases for possible
> interactions of coral disease pathogens and algae based on lab
> experiments and small scale field associations. We found very strong
> associations between many diseases and certain algae quite
> unexpectedly from data analysis of large scale studies of coral reef
> health in the Turks and Caicos Islands. All the coral diseases that
> were abundant enough to be tabulated at all sites (White Plague, Black
> Band Disease, Gorgonian Disease) were significantly associated with at
> least one algae genus. However YBD was too rare there to
> tabulate........

One could take Tom's arguments above a step further (perhaps into the  
realm of absurdity) by pointing out that were there indeed conclusive  
evidence that too much macroalgae and a lack of herbivorous fish was  
the proximate cause of coral diseases (including bleaching as the  
proponents of this idea argue) then the logical management action  
would be to cull carnivores to protect herbivores.  Knowing that  
predators in part control prey populations, it must be a confusing  
message for managers to simultaneously hear that they have to maximize  
predator biomass and diversity AND that of herbivorous fishes,  
including not just their densities but also their function, i.e.,  
grazing which is probably strongly controlled by predator presence via  
a Non-Consumptive Effect or TMI.

And to take this whole argument full circle, I'd like to point out the  
very nice paper by Coelho and Manfrino (2007) that compared coral  
loss, disease and bleaching inside and out of MPAs on Little Cayman  
Island in the Caribbean.  They found fish had no measurable effects:  
"Mean live coral cover decline was similar inside (from 29% to 19%)  
and outside (from 24% to  14%) marine no-take reserves. No significant  
difference in disease prevalence or clear pattern in  bleaching  
frequency was observed between protected and non-protected areas. It  
is concluded that  more comprehensive management strategies are needed  
in order to effectively protect coral  communities from degradation."

Coelho, V.R. and C. Manfrino (2007) Coral community decline at a  
remote Caribbean island:  Marine no-take reserves are not enough  
Aquatic Conserv: Mar. Freshw. Ecosyst. 17:666-685 DOI: 10.1002/aqc.822

John Bruno
Associate Professor
UNC Chapel Hill

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