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Crown Of Thorns  Cull

When most people think of conservation it usually inspires thoughts of researchers studying animal behaviour, patrolling some vast wilderness, designating an area as a national park/reserve or freeing a trapped animal from a net. So it may seem odd to some that killing animals for conservation purposes is practised. That is what we’ve been doing here on Mafia Island with the Crown of Thornes (Acanthaster planci).

The C.O.T (Crown of Thorns) is a large species of starfish that has between 7 and 31 arms and is covered with long venomous spines which cause painful wounds. They are major predators of hard stony corals and if their population explodes, over 1 per 100m2 of reef, then they can cause major destruction of the reef. Another problem is they have few natural predators and the few they do have, such as the titan triggerfish (Balistoides viridescens) and Giant Triton Snail (Charonia tritonis), are being over collected or over fished, leading to COT numbers increasing. Basically when their population explodes they’re a real pain in the arse.

Surveys on some of the reefs here on mafia have shown there is a problem with COT population i.e more than 1/100m2. So it was decided a cull was necessary for the benefit of the reefs. A few methods were trialled to find the best for killing the COT. The first was bringing them to the surface in buckets and letting them dry out. This worked because once out the water they dried out, however whilst bringing them to the surface they take on water and become increasingly heavy (also once dead they absolutely stunk!). So another method of injecting them with vinegar was attempted. We went to one of the affected sites and injected the COT’s with 20ml vinegar, we then tied a bit of string to an arm to allow for follow up observation of the effect of this method. This proved a successful method, so they technique was adopted, we even added red dye to the vinegar to ensure it was all going into the individuals.

Armed with our bottles of vinegar and syringes we went to several of the sites with COT outbreaks and proceeded to inject as many as we could find. You could see the results immediately sometimes, especially with the smaller ones where arms would go limp and their bodies would fall from the coral they were munching on. We always gave them a vibrant salute as final send off!

So overall the vinegar injecting method was a success and at the sites we carried out a COT cull their numbers decreased. It may seem cruel and odd to some people, having to kill for the sake of conservation, but it’s all for “the greater good”. The cull will ultimately give the reefs here on Mafia a better chance of prospering and will benefit the entire ecosystem. Also it means future visitors to Mafia (including potential volunteers) will get to see and research the pretty and healthy coral reefs!

By Paddy Woodyard - Tanzania Marine Volunteer

Find out more about Frontier's volunteering projects in Tanzania.

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