• hello@makingroots.co.uk

Crown of Thorns, Threats and Solutions.

Updated: Apr 13



Do you know what a Crown of Thorns is? No, it’s not direct from the pages of Game of Thrones. And how does your banana skins play into this question…?

Crown of Thorns are a type of Sea Star, naturally found in small numbers in healthy reefs. They eat specific types of coral, and create healthy disturbance on the reef. This disturbance in a balanced and healthy ecosystem promotes biodiversity.

However, due to a number of factors, including reduction in predator numbers, increase in organic pollution, populations can increase to outbreak levels. In high numbers, Crown of Thorns can cause the complete death of coral reef ecosystems. This is because with high numbers, the sea stars will continue to eat coral at a much faster rate than the coral can regenerate.


Are Crown of Thorns a problem?

Crown of Thorns have a large responsibility in the die back of The Great Barrier Reef. They account for more than 40% of the total death of the Reef so far. I’m sure many of us will have heard about the impact of climate change on the reef, but not so many about the Crown of Thorns…

What can be done?

Short term fixes include killing Crown of Thorns in an outbreak. A normal healthy community would mean around 6-10 COT per hectare on a reef. But stressers can cause an outbreak- and increases populations to over 500 in one hectare.

Traditional methods of removing Crown of Thorns can actually be more damaging too. Collecting Crown of Thorns into mesh sacks or plastic bags undewater can magnify the population. COT are known to stress spawn, and when mixed in with a bag of lots of other individuals, the "creative fluids" get flowing and you get a LOT more COT. They can also regrow arms, or even reform from cut off arms and so cutting up COT underwater can actually just create lots more problems. Here's the latest method to effectively remove COT!


How to conduct a Crown of Thorns removal:

Pair up, with a big, leakproof bowl on the surface. One person needs to stay at the top, ensuring no juice spills out of the bowl. The other duck dive down, to collect one COT one at a time.

If a COT is particularly wedged into a coral. Leave it. Trying to get it out will cause a stress spawn and damage the coral.

Bring the Crown of Thorns straight to the surface and into the bowl. Keep all liquid inside the bowl, ensuring no spillage. If it's particularly wave-y it's probably best to reschedule your removal.


Periodically take to your boat or station to put into a bigger container.

Take all the collected COT back to the shore, making sure you keep the COT and juices away from the water. Collect data on the COT, including

  • Diamater

  • Weight

  • Number of arms it has

  • Any missing arms (indicates predators)


Compost or bury your collected COT away from the shoreline.

This snorkel method is great if you have lots of healthy, energetic volunteers and a low budget. If you have a bigger budget and smaller team of scuba divers, you might want to think about vinegar guns.

There is also an in water COT eradication method. This involves injection COT with a household vinegar solution into the arms. This will slowly kill the COT, and provide food for fish. It means you do not have to bring the COT to shore, when done correctly means no stress spawning and can be done whilst on a dive. It means you cannot collect biometrics data, but in extreme outbreak situation this might be a good option.

These vinegar injections can be administered via syringe or specialist guns. We prefer the guns, they’re reusable.

If you’re worried if the vinegar as gone in, try adding a few drops of food colouring to your vinegar. You will see a puff of colour in the water if it hasn't worked.

Long term solutions include composting, managing effluent and educating the fishing communities. Predators include Trigger Fish and Tritan Snails (see picture). Ensuring these are protected from fishing and also souvenir collecting, ensures a healthy food web.


Composting might seem like a mundane aspect of “sustainable lifestyles” but its actually so incredibly important. It not only does it have a relationship to greenhouse gas emissions, but if you live on an island communty or near the reef (Australia), you should be composting to safe the coral. And by saving the coral you can help contribute to protecting the entire reef. Ensuring nutrients are maintained on the island as much as possible prevents nutrient increases in the water, which can cause blooms in algae or in this case COT, which can lead to huge die off of the coral. This is called eutrophication and another example is the red tides in Florida. Composting keeps soil healthy and reduces negative impacts.

Making sure effluent is managed properly, and disposed of properly is also key. As is making sure the fishermen understand that a crown of thorns outbreak can damage their fish stock, as the fish use coral reef as nurseries.

These Crown of Thorn outbreaks are complex issues, that require good management to control them. If you are noticing more COT on your reef, contact a specialist who can help guide you, identify if you have an outbreak and how the best way to reduce them.

If you've enjoyed this blog post, don't forget to like, follow and subscribe to our social media and sign up for email newsletters! Every follow is a sign of support for our not for profit work.

#Divingtips #Conservation

Recent Posts

See All

We are optimised for viewing on Desktop, via Safari & Chrome. If viewing on Mobile, or FireFox you might get some funky photo happenings.

Say hi at hello@makingroots.co.uk

© 2023 by Making Roots