Crown of Thorns 

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This morning we went to Koh Nang Yuan and No Name Pinnacle and experienced 30 meters of visibility. This made it easy to spot and remove 25 of these corallivorous sea stars and enjoy the company of amazing wildlife.

The crown-of-thorns starfish (Acanthaster planci) is a resilient and tanacious native of coral reefs in the Indo-Pacific region.

On healthy coral reefs, this coral-eating starfish plays an important role, as it tends to feed on the fastest growing corals such as staghorns and plate corals, allowing slower growing coral species to form colonies. This helps increase coral diversity.

However, outbreaks of this venomous invertebrate pose one of the most significant threats to our already fragile reefs.

These starfish can grow to be over 50 centimeters in diameter, and 1 individual can consume over 13 square meters of coral reef per year. Their populations have been on the rise globally since the 1970’s, with outbreaks in over 30 locations from Red Sea to the Great Barrier Reef. In some reefs, outbreaks of these coralivores have been responsible for collapses in corals and associated fish and invertebrate communities. Although a natural part of the ecosystem, due to anthropogenic factors these starfish are now causing great ecological and economic problems for reefs throughout the Indo-Pacific. The major factors contributing to the global rise in Crown of Thorns populations include:

  • Nutrient enrichment of coastal seas: more nutrients yields more phyto-/zooplankton, which means that more of the pelagic starfish larvae that would otherwise starve to death can survive.
  • Predator removal: In the pelagic stage any filter feeder, including hard corals, will eat the starfish larvae. After reaching juvenile stage, fishes such as wrasse, damsels, and snappers will eat the starfishes. But once they become adults very few predators will eat be able to eat the toxic starfishes (except some triggerfish, pufferfish, and large snails like the triton trumpet.) Unfortunately all of theses species are targeted or removed as by-catch in the fishing industry.

Due to this, in any area where overfishing, development, deforestation, and over-use are threatening the reefs, Crown of Thorns may develop into outbreak level populations. On Koh Tao, they have been monitoring the abundance of Crown of Thorns Starfish (COTs) since 2006 as part of the locally developed Ecological Monitoring Program. There are some interesting trends starting to appear in the data.

COTs occur in low abundance around Koh Tao, so are unlikely to be observed in the 400 square meter surveys that are conducted each week (4 by 100 sq. m replicates). So the monitoring began by simply keeping track of how often COTs were even seen. It turns out that over the course of 375 surveys over the last 9 years only 16.3% of the surveys yielded COTs. But this number is not consistent over each year. In fact, it appears that we had a major spike in COTs abundance in 2010, where they were observed in about 25% of the surveys completed. (see graph below).


In 2010, the NHRCP began collecting COTs to relieve damage to remaining coral populations following the mass coral bleaching event that year, in which up to 68% of the hard corals died in some shallow reef areas. They also began promoting the collection of the starfish at monthly Save Koh Tao Meetings with other dive schools from around the island. Furthermore, the monthly collection of COTs was made into a requirement for the Save Koh Tao Adopt-A-Reef Program in selected areas.

Interestingly, the percent of surveys in which a COT is recorded has fallen every year since 2010, reaching a 9 year low of 7.1% of the 56 surveys already completed in 2014 (note, 2007 is excluded as only 3 surveys were completed that year). This could be due to other factors at play, but it is our hope that the efforts over the years are helping to control the situation.

According to available literature, it is thought that COTs reproduce through spawning events once per year, in April for the Northern Hemisphere. So it was decided to graph the abundance of COTs by month, to see when they are aggregating in preparation for a spawning event. Contrary to the publications, it is evident that there are two peaks in abundance each year, one during March and the other in October. More work is being done to understand what these two peaks in abundance mean, is it two spawning seasons or is there some other reason for the two asymmetrical peaks in the graph below?


According to the literature, an outbreak situation is when populations of COTs reach abundance greater than 140 individuals per hectare. In some areas, such as the Great Barrier Reef, abundances have exceeded 1,000 individuals per hectare. But this criterion for outbreaks does not consider the coral coverage or health of the reef. For a healthy reef with 80-90% hard coral coverage 100 indiv./Ha may be sustainable, but for a depleted reef with only 6% coral coverage this may be considered far too many. We first calculated the total average abundance of COTS from all the data, and found it to be 6.6 indiv./Ha, which is low, but still enough to cause a lot of coral mortality.

The next thought was to calculate the average abundance of COTs at the different sites we survey to see where the COTs are around the island (note: only sites with more than 10 surveys were included in the graph at right). Luckily, none of the sites show outbreak levels of COTs when the data is averaged over all years, although there are 5 individual surveys where the abundance ranged between 100-200 Indiv./Ha. The site with the highest average abundance was Mango Bay with an average of 18.8 Indiv./Ha (16 surveys over 9 years). The site with the lowest abundance was Shark Island, but it should also be noted that the deep survey line at Shark Island does not include a high abundance of hard corals for COTs to feed on. Future investigation will be look at the percentage coral coverage compared to the abundance of COTs for each site so a more weighted criteria for predicting COTs outbreaks on our local reefs can be made.

Photo credit to New Heaven Dive School

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