Reefscapers Diary March 2020

According to NOAA projections, the Maldives are currently categorised as “Bleaching Watch” (March), “Warning” (April) and “Alert Level 2” (May-June), an alarming prediction when compared with 2019, which reached “Alert Level 1” only for June.

The degree heating weeks (DHW) could potentially last 8-10 weeks, which would be devastating for much of the coral reefs in the Maldives. The warmest sea surface temperatures (SSTs) are seasonally expected during the last 2 weeks of April, and the corals could then continue to be at high risk through to the middle of June. Realistically, we should be bracing ourselves for serious levels of coral bleaching, followed by possible widespread mortality.

During March at Kuda Huraa, a total of 12 new coral frames were sponsored, and a further 38 frames were monitored (cleaned, repaired and photographed). Additionally, we spent 2 days re-mapping our frame sites around the island, and we are currently processing the ‘orthomosaic’ digital map from the data.

At Landaa, 19 new coral frames were transplanted this month, (9 sponsored by guests, 10 sponsored by Four Seasons) and a total of 154 existing frames were monitored around the island (mainly at the Coral Trail, Spa and Water Villas sites).

Monitoring at the East Blue Pit highlighted two important facts:

• Some corals that were heavily bleached are now showing signs of recovery and growth of new branches.
• The frames at the bottom of the pit are slowly being covered in sand, killing the lower coral fragments. All the 31 frames were lifted from the sand, but it is likely that this will keep on happening. We are trying to find a solution to prevent this, to ensure the maximum survival success for all colonies.

At Kuda Huraa, we have secured 7 coloured buoys to the natural reef, to mark the locations of healthy coral colonies selected for monitoring during the upcoming weeks. To provide a baseline for our coral frames, we selected a variety of wild coral species (Acropora digitifera, A. hyacinthus, A. tenuis, Pocillopora verrucosa, P. meandrina, Porites species). We have started to see some paling of corals on our frames at the Channel site. We will move the most vulnerable frames to the cooler/deeper waters at the House Reef, and we plan to experiment with some shading structures over selected frames.

At Landaa, coral bleaching was observed at the Blu and Coral Trail sites, with bleached and pale coral colonies recorded on the natural reef and our frames. Species of Acropora and Pocillopora were impacted, with Acropora colonies seen to be fluorescing (to cope with the UV-stress).

KH01 – 28 out of 35 fragments are now encrusting (a decrease from last month’s 31/37).

• Acropora hyacinthus has started to regrow onto the plate.
• digitifera is growing over calcified areas of A. hyacinthus.
• cytherea continues to fuse, and appears to be dominant over A. hyacinthus.

KH02 – All 37 fragments of Galaxea fascicularis are healthy and encrusting. Colonies of the same colour morph are fusing, but the two colour morphs are not fusing with each other (despite being in contact) and are not negatively affecting the growth of each other.

KH03 – 28 out of 31 fragments are now encrusting to some extent (3 more than last month).

• 1 of the hyacinthus fragments continues to decline in health, with some areas showing complete tissue loss. Due to the slow speed of advance, it is unlikely to be disease.
• Both top-row hyacinthus fragments are growing well (maybe better suited to greater light intensity).
• The valida fragments continue to grow into branching patterns, but still have small areas of calcification.

KH04 –15 out of 26 fragments are now encrusting (a decrease of seven from last month). The fragments have recovered substantially from the tissue loss in January. So far, the variants of A. hyacinthus are growing equally well together.

KH05 – out of the original 28 fragments, 10 have now started to encrust onto the plate, and the remaining fragments have all developed new corallites. Interestingly, where new corallites have grown, the tentacles of the polyps appear to have a green hue compared to the original brown colour morph of the parent polyps. This could be because they are new and have yet to accommodate the specific zooxanthellae, although brown pigment thought to be the symbiont cells have been observed in the encrusting skeletons. At the start of March, 17 new fragments were added (2 have begun to encrust).

Frame Recognition – Through prolonged tuning of the parameters, we have greatly improved the results of the frame recognition algorithm. We can now successfully detect the frame structure’s position in 99.8% of monitoring photos (up from 85-90%). From the 516 test images, only a single photograph was not processed successfully.

Coral Detection – We have improved the performance of our deep-learning algorithm for detecting the coral fragments. By adding data augmentation (random rotations and flipping of the training pictures), we are now able to detect corals more accurately. Further training still needs to be done for some specific cases, such as newly transplanted fragments (which are small and difficult to detect).

Frame analysis – It has taken a lot of work to develop and improve the algorithm that matches the detected coral fragments to the bar. There are many significant algorithmic challenges, including merging data from the four monitoring pictures. The algorithm is already much more accurate, but improvements are essential to be able to track down the growth of our corals at the colony level.

Catamaran – We have installed an additional battery for the propellers, and we are currently investigating an issue with the onboard cameras. We have noticed that nearby boats can sometimes generate radio interference, so we are researching whether a 4G modem would be more reliable than the radio antenna.

Our coral reproduction experiment is still on-going, as we assess the health of each newly settled colony each week. Bleaching was first observed on 17 March (rock 20, colony A) and two weeks later almost every colony had bleached. Filtered seawater is pumped directly from the ocean to our experimental tanks, and the March-May period is the time of seasonally increased ocean temperatures. We recorded elevated temperatures of 29.1°C to 30.2°C that likely caused heat stress in the corals.

We have now turned off the direct seawater inflow, and have started to supply stored water that has equalised to the same temperature as the tank. Cleaning is now done twice a day, siphoning out any detritus or left-over food. To help the corals cope with the absence of zooxanthellae, a daily fresh supply of rotifers is provided (along with fresh algae to feed the rotifers). A few days after implementing these adapted procedures, the first signs of recovery could be observed on some colonies.

The coral plugs and parent-colony drill-holes are photographed and monitored weekly, with health assessments using the CoralWatch colour chart. We now have 6 weeks of data, conducted with three different trial plug sizes. We have observed some predation, probably by parrot fish taking advantage of the increased roughness and accessibility of the coral surfaces.

During the month of March, a new project was started to relocate coral frames to the island of Voavah. We plan to create an artificial reef barrier on the East side of the island, as an experiment to monitor the effects on beach erosion.

Over the course of a full day, our team of 9 biologists and divers uplifted 114 frames from the Water Villas site, and brought them to the supply jetty using the small MDC whaler boat and a raft. The selected frames were originally sponsored by Four Seasons (not by guests) and had not been recycled during last year’s major re-transplantation efforts by our volunteers. Any living coral colonies were fragmented for propagation at the Water Villas site.

We then loaded the frames onto a large dhoni and transported them across the channel to Voavah. The frames were unloaded from the boat and relocated behind the reef crest in two lines (1m-3m deep). Big thanks go out to our dedicated task force of 13 helpers from Voavah and Landaa.

In addition, a further 140 old frames will be brought from the Water Villas and Parrot Reef sites. The next step will be to replant all the frames with new coral fragments, harvested from the coral rescue project at Gulhifalhu. We plan to monitor them periodically over subsequent years.