Clam bleaching poses threat to Manihiki pearls

Saturday May 07, 2016 Written by Published in Outer Islands
Coral bleaching is causing the deaths of hundreds of pa’ua in Omoko Reserve. 16050606 Coral bleaching is causing the deaths of hundreds of pa’ua in Omoko Reserve. 16050606

Widespread pa’ua (clam) bleaching has been reported in Manihiki this month as the global bleaching event continues as a result of the current strong El Niño.


In December and January a mass coral bleaching event was recorded in Penrhyn by the Ministry of Marine Resources (MMR).

Since then, coral bleaching has also been recorded in Manihiki where pearl farmers have halted activities that may further stress pearl oysters including seeding and conditioning. Farmers have been advised to continue to hold off on shell handling.

Delays to pearl production work have caused concern, and some farmers may carry out small work trials to be safe. With pa’ua now affected by bleaching, pearl oysters may also be at risk.

MMR pearl biologist Teuru Tiraa-Passfield says lagoon water temperatures on the island have been around 30-31 degrees Celsius since December.

Photos taken this week at the Amoko Reserve kaoa show hundreds of pa’ua have bleached and some are already dead.

More lagoon sites are being checked to gauge the extent of the pa’ua and coral bleaching.  A survey conducted in March showed pa’ua remained healthy.

Bleaching occurs when corals become stressed and turn a ghostly white colour. The coral expel the symbiotic algae that live inside them which produce much of their food and energy. Long periods of prolonged stress can result in coral damage, coral disease and coral bleaching of single or whole colonies.

Pa’ua can also be affected by bleaching which can result in die-off. Bleaching events directly affect the health of the whole ecosystem and the people who depend upon it for food and income.

Coral bleaching caused by climate change and the current El Niño are now the largest and most pervasive threat to the world’s coral reefs, and present a major threat to marine biodiversity in the Cook Islands.

The El Niño Southern Oscillation (ENSO) began a year ago, but Penrhyn was mostly unaffected until December when the average sea surface temperature (SST) hit 32 degrees Celcius.

MMR continues to monitor the situation which has escalated to a critical alert phase due to the four months that pa’ua, pearl oysters, and other marine life have been stressed due to the higher than usual SSTs.

The El Niño period has now extended beyond April, and well above-average SSTs have continued across the central and eastern equatorial Pacific Ocean.

Pearl oysters are particularly vulnerable drastic or prolonged change.

In 2000 warm waters from El Niño conditions fuelled marine bacteria that caused massive mortality of farmed pearl oysters in Manihiki lagoon almost leading to the collapse of the industry.  

In 2011 the cool waters from La Nina conditions led to a huge algal bloom in the lagoon for a prolonged period of time causing mortality of pearl oysters from hypoxia or lack of oxygen.

                - Release

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