Oliver et al (2004) Monitoring bleaching

sediment is a sign of mortality. Algae rapidly colonise dead coral surfaces, so if there is a thin film or
haze of green, brown or yellow this is also a sure sign that the coral is dead rather than bleached Figure
5. For some fleshy corals you can detect the presence of live tissue by gently touching the coral surface.
Bear in mind that bleached corals may ultimately die, and so dead colonies may be a final stage in the
bleaching process. However dead colonies can also result from disease, sudden weather events
(especially flooding), pollutants, and predation, so if your first observation is only of large numbers of
recently dead corals, you may have to look for other evidence to determine if this has been caused by
bleaching. Two common causes of non-bleaching related, recently dead corals can are crown-of-thorns
starfish (COTS) and disease. The visually diagnostic features of areas affected by COTS and disease
(Figure 6) are set out below.
Mortality due to COTS
• In low to moderate levels of infestation, COTS and other predators such as the snail Drupella,
leave discrete patches of dead white skeleton (feeding scars) which can look like bleaching.
Major outbreaks can leave quite large contiguous areas of dead white coral.
• Often there is a combination of recent scars (completely white) and progressively older “dirty”
scars with a coating of sediment or algae.
• If you see any COTS in the area (look among and under corals), you should immediately
suspect them as the cause of any apparent coral bleaching, especially if the white or dead coral
is patchily distributed
• COTS scars can also be distinguished from bleaching damage by the strong transition from
dead to live tissue within an affected colony
Mortality due to Disease
• Disease usually starts at one point and slowly spreads across a colony leaving dead and
progressively older “dirty” skeleton behind it. It can form circular patches or curved bands
across a colony
• The disease front is often a distinct band with a specific colour (black, red, white)
• Spotty white patches can be found in diseased corals but this is not a feature of bleached
corals
Appearance of corals in a bleaching event
• Colonies turn pale and bleach evenly over the whole colony or on the upper surfaces. Totally
bleached colonies appear exceptionally clean and almost gleam underwater (Figure 7). When
bleaching is moderate to severe and the water is fairly clear, whole reef surfaces appear white
from above the surface and even from the air (Figure 8)
• Partly bleached colonies appear very pale and there is usually a gradual change between the
paler and the darker parts of the colony. The upper surfaces are often the first, and sometimes
the only parts to bleach (Figure 9).
• On very close inspection you can often see the transparent tentacles of living polyps when the
colony is viewed from the side
• There is never a distinct line between bleach and unbleached parts of a colony
• In the early weeks and months of bleaching, colonies will remain bleached, or get even whiter
over periods of days and weeks. On the other hand, colonies killed by COTS or disease will
become covered in sediment and algae in just a few days
• Healthy, fast growing corals (especially Acropora species) will often have pale or white
growing margins or branch tips. This is a sign of active growth and should not be confused
with bleaching. While bleaching can also affect the upper surfaces of a colony only, the loss of
zooxanthellae will extend further down the branches beyond the growing tips.
How do I measure or quantify coral bleaching?
The most common way to quantify the level bleaching is first to determine the percentage of the reef
surface which is covered by corals and soft corals, and then to record the percentage of this cover
which is bleached. This can be accomplished by visually estimating the two measurements, or by using
one of several techniques to accurately measure bottom cover (line or point measurements, direct area
calculations from images). In areas where bleaching is well developed, and coral surfaces tend to be
either normal (“unbleached”) or completely white (“bleached”), this method can yield results quickly
6