Reefscapers Diary May 2020

Prospective Coral Bleaching Predictions

The NOAA heat stress projections for Maldives are categorised as “Alert level 1” for May, lowering to “Warning/Watch” during June.

Monthly Progress

At Kuda Huraa during May, we have been remapping and tagging our coral frames at the Water Villas and Channel sites, and monitoring wild colonies for coral bleaching. Recent stormy weather has caused some sand accumulation around our frames and even on the natural reef, resulting in mortality of various colonies.

At Landaa, 164 coral frames were monitored this month, mainly at Parrot Reef and Anchor Point sites. Each frame was cleaned and photographed, and repaired if necessary (transplanted with new coral fragments, and given a fresh ID tag).

As the monsoon has now changed from NE (hot and dry) to SW (more rain and wind) we recruited some resort volunteers to help us reposition our Kuda Huraa Water Villas frames from under the shady boardwalks back into the open lagoon. An estimated 30% of the coral colonies have died due to bleaching (and some sand drifting) and the remaining 70% are starting to regain colour and recover. Predation of some Acropora hyacinthus colonies were recorded on some frames, perhaps by large fish.

CoralWatch – Monitoring for Signs of Coral Bleaching

At Landaa, coral bleaching is being monitored in both wild colonies and our frames, however, the mortality rate appears low at present (we are continuing to monitor).

At Kuda Huraa’s Channel site, approximately 40-50% of the frames are bleached, compared to our experimental shaded frames with only 10-20% bleaching. Sand drifting has caused mortality of some colonies.
At the Blue Hole site, the corals started paling during the middle of May, but most of these are now recovering again. Coral colonies of Acropora muricata species seem to have been badly affected.
At the House Reef, little bleaching was recorded on our coral frames and the wild reef. The average temperature was 30.1°C; the lowest temperature was recorded was 28.1°C (on 29 May).

Coral Plates in Aquarium One (plates KH01, KH02, KH05) and Aquarium Two (plates KH03, KH04)

All our coral plates have been losing their colouration, due to the elevated water temperature.

• KH01 – Bleaching was affecting more corals as the month progressed, and by 26 May all the fragments were dead.
• KH02 – The Galaxea fascicularis are showing signs of bleaching and slow growth, but are healthy overall.
• KH03 – The 13 coral fragments bleached heavily, although we are now seeing signs of recovery (colouration returning).
• KH04 – 3 living coral fragments remain (1 heavily bleached, 2 showing signs of recovery)..
• KH05 – our newest G. fascicularis plate is showing slow growth with a little bleaching, but is in good health overall.

Coral Reproduction Spawning Experiment

During May, we saw a 0% mortality rate in our juvenile colonies. We continued to combat the seasonally elevated ocean temperatures by maintaining a closed-flow system and conducting daily water changes. Whilst this prevented the colonies being exposed to continued elevated temperatures, it did expose them to daily fluctuations (within their thermal tolerance).

Multiple scientific studies have identified a direct link between thermal preconditioning and bleaching susceptibility, meaning that corals acclimatised to thermal stress or variable temperatures are shown to present a higher tolerance. We plan to conduct future trials on lab-reared colonies to establish whether this can be replicated, and possibly apply the theory to our coral frames.

‘Coral Core’ Experiment

Our in situ coral plug propagation trial has been running for over 3 months now, and by month end, 33% of the original transplanted coral plugs are surviving (8/24 comprising: 4 medium, 3 large, 1 small). From the current data, we can conclude that Medium and Large fragment sizes offer the greatest chance of success, and these two sizes will be used in future experiments. It is also clear that the chosen site for out-planting plays a vital role in the success of the transplanted fragments.

Coral bleaching on our frames (left: A.digitifera / right: A.muricata) May 2020