Crown of Thorns on Malapascua

posted in: Environment | 0

As divers, it is drummed into us from the moment we don scuba gear, that we are to make minimal impact on the underwater environment: don’t touch, don’t take, and above all, don’t kill anything.

But there is one creature for which the opposite is true – the dreaded crown of thorns sea star.

Crown of Thorns

The crown of thorns or Acanthaster planci is the second largest sea star in the world. About 30 cm in diameter with up to 21 arms, it can race across a reef covering up to 20 meters in an hour. It is covered in venomous spikes which can pierce a wetsuit and give a nasty sting to humans.

The crown of thorns presents another big problem – it is a voracious consumer of hard coral. When numbers grow too large, an invasion of these starfish can decimate a reef, as has been widely publicised in several areas including the Great Barrier Reef.

The crown of thorn has only one main predator, the Giant Triton, which can keep the numbers of crown of thorns in check.

Triton shell
Tritons, once plentiful, are now a rare sight in the Philippines except on shop shelves

Sadly, the triton has been hunted to near extinction due to its beautiful shell and because of this, numbers of crown of thorns can sometimes grow out of control.

A small number of these sea stars can exist on a reef without any harm being done and when I first arrived in Malapascua 8 years ago, I would see the occasional crown of thorns. Then, in 2005, diving the sites around Malapascua Island every day, I saw their numbers slowly increasing, until the population boomed and suddenly they were everywhere.  Even when snorkelling in front of the dive shop, I sometimes saw 10 or more creatures per square meter, often so many they were piled up on top of each other.

I felt we had to do something – but what?  It is very difficult to control such outbreaks and there was no sure-fire way to eradicate them. Every removal method that we investigated had its drawbacks.

In the end, Thresher Shark Divers got together with two other businesses on Malapascua and arranged to send out local fishermen to ‘catch’ as many as they could. The dive sites around the island are all very shallow, so it was relatively easy for them. We brought the offending creatures back to shore, rented a local piece of land, and buried them.

Crown of thorns collection on Malapascua
Collecting the corn of thorns

The fishermen were compensated with a fee of PHP1 per sea star (approx. USD $0.02 or GBP £0.01), paid for by the 3 businesses.

Whilst this seems like a paltry amount, none of us could possibly have predicted quite how much money we would have to pay out!  Over the course of 3 weeks, the fishermen made a princely sum of PHP 40,000 (approx. US$1,000 or £600), an amount far and above what they would have made fishing.

So if you have already done the simple maths, you would have worked out that we removed 40,000 crown of thorns from Malapascua’s waters!!!!

Crown of thorns Malapascua
Only one of the mounds of crown of thorns taken from the waters around Malapascua

We were initially worried that it may have done more harm than good and that the starfish, which will often spawn when threatened, would soon return in larger numbers.  However, as the weeks and months slipped by, Malapascua remained free of Crown of Thorns.

But now it is 6 years later, and the problem has returned to Malapascua.  So we have an on-going program to eradicate the crown of thorns using similar methods. They have not yet been cleared as well as they were back in 2005, but we are working on it!  Stay posted for more information.



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