The Department Of Marine & Wildlife Resources

The Department Of Marine & Wildlife Resources

Monday, April 27, 2015

Coral Bleaching in American Samoa

What is Coral Bleaching?

When corals get stressed by abnormal conditions, they will expel the zooxanthellae (an microscopic algae that lives within the cells of the coral which helps the coral to survive). Zooxanthellae provide the coral with many essential nutrients that are then used by the coral to its calcium carbonate skeleton. In return, the coral provides protection for the algae as well as the compounds necessary for photosynthesis. It is impossible for the coral to survive too long without its algae.

Corals are very sensitive animals for them to thrive, the water must be warm, but not too warm, relatively clear, saline, nutrients poor, and be in shallow waters to get plenty of sunlight. Any small changes in these requirements and the corals will likely get stressed. They may tolerate change, but not for too long. 

Once they bleach, the corals are basically starving themselves. If conditions return to normal relatively quickly, corals can regain their zooxanthellae and survive. But the stress occurred also may lead to deceased productivity and growth, and increased likelihood to disease. If the stressful factor remains constant for quite some time it decrease the chance of corals to bounce back and recover.

One of the major findings indicated a mild coral bleaching event in American Samoa. Except for Ta'u and Rose Atoll, around 10% of the coral colonies between 3 and 6 m deep territory- wide, presented signs of bleaching, ranging from pale to stark white due to persistent warm waters. The bleaching seems to mostly impact certain species of fast growing corals. Survey results indicate that the coral bleaching event is mostly happening on the reef flats. Widespread but generally light coverage of partial bleaching has been documented for several coral species to a depth of approximately 40ft from Matu'u near the mouth of Pago Pago Harbor and westward to Leone Bay and Seetaga. It is likely that this pattern can be observed island wide. There are sites that have high bleaching because they have branching  corals that are much more susceptible to stress. Over- all, probably less than 10% of the corals in the reef flats are experiencing bleaching. Monitoring of the bleaching event continues, though there is little that can be done to reverse the process immediately. To reduce downtrend impacts of bleaching events it is recommended to carefully manage other stressors such as land- base sources of pollution, water contamination , over- fishing , and other factors that contributes to coral reef degradation.

Will the corals recover?

Only time will tell, temperature have been gradually falling but we have yet to determine if they have been decreasing quickly enough. It will take months to fully survey and determine the extent of coral bleaching in American Samoa as well as to document incidences of disease and mortality due to this heat - stress. Coral bleaching is a tragedy for our territory whom many depend on the reefs for their food and livelihood. Immediate and intensive management is required to try and help our reefs by recovering coral bleaching to a more healthier reef.