Coral mortality is a measure of the recent or old death of part or all of a colony or reef. A 100% dead colony is counted as standing dead if it is identifiable to the genus level. For a colony with living and nonliving sections, the dead portions are classified as either old dead or recently dead. Mortality extent is described in terms of the percentages of the colony surface area in each class: for example, 75% living, 5% recently dead,and 20% old dead.
We are especially interested in recent mortality,as it is a “bottom line” expression of reef condition during the past year. Causes of recent mortality include disease (F6), fish grazing (F11), algal overgrowth (F13), and extended coral bleaching (F7,D14).5
If lesions cover a relatively small area of a colony,tissue regeneration may occur. However, high levels of acute disturbance (such as hurricanes) or chronic insult (such as nutrient enrichment, S8) may prevent regrowth of new tissue or give a competitive edge to other reef organisms (for example, algae, F13).
“Hotspots” of recent mortality can alert managers to a need for investigation and perhaps protective action. Data collected during the first several months after a major disturbance are especially useful,as they help researchers to gauge the event’s ecological significance.
We recommend that coral mortality be monitored in the course of regular regional surveys supplemented by special campaigns in the wake of major disturbance events.
Mortality extent is usually estimated visually by divers.Details are given in the AGRRA and MBRS methods manuals.6
A promising sign of Benchmark reef recovery would be a regional average of ≤ 2% partial recent coral mortality.
- The highest partial mortality rates in the last eight
years occurred in 1999 due to the 1998 combination
of Hurricane Mitch and a coral bleaching event.
Honduran and Belizean shallow reefs were especially
hard hit, largely due to hurricane damage.
- The 2005 bleaching event did not result in significant
coral mortality.7 Offshore hurricanes and a late start
to this event (October) may have helped prevent
mortality by cooling the waters bathing these reefs.