Crown of thorn starfish removal in Siquijor

 In Crown of thorns, Fieldwork, Siquijor

With a group of volunteers we went to Siquijor where we removed 1300 Crown-of-Thorns Starfish (from now on COTs, the name is way too long) in 4 dives. COTs eat coral and if they occur in normal numbers you see maybe 3 during your dive. During a COT outbreak that number can be anything between 10 and 500. Scientist do not exactly agree why COT outbreaks occur, but it certainly has something to do with over collection of the biggest predator of COT, the triton, the increasing water temperature and the amount of nutrients in the water. A COT looks like a normal seastar but has many more arms and a spiny body which makes it challenging to remove from the reef. Additionally the thorns are poisonous and if you accidentally get stung the consequences are less than funny, with severe pain and possibly lasting scars if you’re unlucky. We noticed however that if you only lightly touch one of the spines either nothing happens or the pain is only very mild. Good for us, because by the time we were finished five of our diving volunteers had accidentally touched a COT due the sheer number of them.

It needs some planning on which methods to use to remove COTs because they have the unfortunate self-preservation imperitus, that if they feel threatened they will spawn and release million of eggs. Cutting them in half basically also means you now have 2 COTs, because they are able to regenerate missing body parts. So what to do? Well, picking them up and removing them is one method, but you have to hurry before they spawn. Injecting them with a fluid that kills them is another method. Nowadays people use vinegar which is very convenient because you can buy it everywhere, but other fluids that have been used before such as oxbile, formalin and copper sulphate which are either expensive, hard to get and/ or not very environmental friendly. If you inject the vinegar at the base of an arm (at least 20 ml), the COT will die between 24 and 48 hours later and will slowly start dissolving and thereby providing easy food for interested fish and invertebrates.

Because it was the first time for most of us the do a COT removal we had to experiment with the methods a bit. Ideally you use an injection gun with a large canister, but as we got summoned to Siquijor in a hurry to help with the outbreak, we didn’t have time to find it – So we had to improvise, with our own method where each buddy pair went down with half a dozen syringes already filled with vinegar and a string of small bags filled with vinegar with which we could refill the syringes as we went along. The assisting buddy looked very interesting: a necklace with syringes, a string with bags with vinegar, mesh bag, stick and bbq tweezers all on top of the normal dive gear.

The syringes work well if the COT are a bit spread out, but eventually after depleting all the refills we had to resort to the more conventional method of picking up a COT and putting it carefully in a bag. When the bag is full or you run out of time (30 minutes after you put the first COT in the bag to prevent spawning, because they don’t like to be on top of each other), you carefully bring the full bag to the surface and hand it to the boatmen. On our second day when we visited some really badly infested dive sites we we forgotten to bring something to put the COTs in. Fortunately our own dive crates workes perfectly well and we ended up with 7 crates full of COTs. We collected 500 COTs in one forty minute dive! Crazy numbers unfortunately! The outbreak is only near San Juan and some divesites are much more infested than others. Afterwards we had to dig a big hole in the sand to bury the COTs, making sure that the hole was deep enough so the COTs could be buries under a decent layer of sand without the risk that some spines were still sticking out.

Interesting three days, but the outbreak is not over yet…

  • EG Dizon

    Good day!

    I am a marine science graduate student studying in the Philippines, and I greatly appreciate your efforts in preserving Philippine coral reefs. I wish you all well in your future programs!


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