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C  O R A L M  O R TA L I T Y

ORITY ON

F5

RI

RE

TH

EF

S FOR HEAL

What

What Is It?

BENCHMARK

Coral mortality is the estimated percentage of a coral

MAR-wide averages of recent mortality

MAR-wide

colony that is dead.

<2% of a colony, old mortality <24% and

standing dead <7%.

Corals are colonial animals that are unique in that they

can experience partial tissue death and still remain

TA R G E T

alive. As corals grow, they are exposed to continual

Maintain MAR-wide averages of recent

disturbances that may result in mortality of part of

their colony (i.e., tissue loss exposing coral skeleton).

mortality below 2%, old mortality below

20% and standing dead below 5%.

Partial mortality is separated into three categories:

RED FL AG

· Recently dead refers to any non-living parts of the

Recent mortality above 5% exceeds

coral in which the corallite structures are either white

the norm and is a signal of a recent

and still intact, or slightly eroded but identifiable to

disturbance.

disturbance.

species. Recently dead skeletons may be covered by

sediment or a thin layer of turf algae.

· Old dead refers to any non-living parts of the coral

(e.g., algae). Examples of chronic disturbance include

in which the corallite structures are either gone or are

competition and predation.  Examples of acute

covered over by organisms that are not easily removed

disturbance include storms and bleaching events.

(certain algae and invertebrates).

Chronic high levels of mortality can potentially lead

to reduced colony size, reproduction and fitness,

· Standing dead refers to colonies that are 100%

plus increased susceptibility to other disturbances

dead, and identifiable to generic level based on

like disease. Total tissue loss can lead to population

colony morphology (e.g., Acropora palmata) or

reductions and, in extreme cases, phase shifts in

corallite character (e.g., Diploria spp., Montastraea

community structure or loss of habitat structure and

cavernosa).

function.

Small corals tend to have no or very low partial

Being able to identify "hotspots" of recent mortality

mortality, while larger colonies often have greater

can allow managers to initiate proactive management

partial mortality. It is fairly common for corals to

responses. Recent mortality is a good indicator for

regenerate partial dead areas if the lesions are small

distinguishing transient versus lethal effects resulting

enough (< 1 cm) relative to their remaining live tissues.

from significant recent disturbances like bleaching

However, corals may not regenerate new tissue if

and disease events. Old mortality represents more of

lesions are too large or too many. Certain kinds of

an integration of disturbance over time (including both

injury may also preclude regeneration. Dead skeletal

chronic and acute). It is often difficult to distinguish

areas quickly become overgrown or eroded by algae

causes of old mortality. Standing dead (100% old,

or other bioeroding organisms.

standing dead colonies) is an important indicator of

historical abundance and distribution.

Why

Why Do We Measure It?

In general, we expect a "healthy" reef will show little

Corals die from both natural and human causes, and

evidence of recent coral death (e.g., average < 3%

it is often difficult to discern between the two. It is

of colony is recently dead). Reefs with high levels of

very difficult to determine what levels of mortality are

recent mortality (e.g., > 5%) would indicate a major,

abnormal. However, establishing a baseline of partial

recent (months to years) or current disturbance event.

mortality can assist in making this distinction.

Reefs with many standing dead coral colonies, or

those that have high old mortality (e.g., average > 30%

High chronic or acute disturbance resulting in

of the colony is old dead), would indicate significant

dead coral tissue may, depending on the extent of

past disturbance events.

disturbance or tissue loss, prevent regrowth of new

coral tissue or may invite invasion by other species

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