HOW MANY COTS ARE THERE ON KOH TAO’S REEFS?
COTs occur in low abundance around Koh Tao, so are unlikely to be observed in the 400 square meter survey that we do each week (4 by 100 sq. m replicates). So first we graphed how often we would see any COTS at all during surveys. It turns out that over 375 surveys over 9 years we record COTS in 16.3% of the surveys. 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, we at 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. We 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 we hope that our efforts at controlling them are helping 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 we 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, we see that there are two peaks in abundance each year, one during March and the other in October. We now have some more work to do 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.