Repelling a reef invasion

A collection of reefs off the coast of Borneo are under threat from proliferating Crown of Thorns starfish. With a small team of volunteer divers, one NGO is setting about saving those reefs - one starfish at a time.

Words and photographs by Elizabeth Fitt

Armed with kitchen tongs and tent pegs, gloved hands clutching a plastic crate, a group of scuba divers descends into the shallow waters off a small island deep in the Celebes Sea, Borneo. They are Crown of Thorns starfish hunters. Their mission is singular: to remove as many of the invasive, coral polyp-eating species as they can. The future of that particular reef could depend on it. 

Most of the group are not coral biologists or marine specialists. They are citizen scientists. They are in Borneo as part of the Tropical Research And Conservation Centre’s (TRACC) self-proclaimed mission to save the ocean “one turtle, one shark, one coral at a time”. And they are doing just that. The Crown of Thorns Starfish (COTS) removal program has removed more than 10,000 starfish, saving an estimated 60,000sqm (more than eight football pitches worth) of coral in the last six months.

Jeeth Vendra, TRACC’s 24-year-old science officer, sits at a table on the sandy floor of “Number 4” – the main socialising, meeting and eating space of the camp – surrounded by a barefoot group of slightly bleary eyed, but attentive volunteers clutching coffee mugs. They listen carefully as he gives an early morning briefing for the day’s COTS-busting exercise. The Crown of Thorns, while beautiful, is not only harmful to coral, it packs a nasty punch for humans too – like a wasp sting, but the pain doesn’t dissipate quickly and the puncture site can ache for weeks. The team need to be careful in the field.  

Starfish play their part in healthy marine ecosystems. It’s when things get out of balance that trouble can start. In this instance, overfishing and shell collection in the area has removed the COTS’ main predators, leaving them to reproduce unchecked. This has resulted in an Crown of Thorns outbreak that if not handled effectively could quickly decimate thousands of square metres of coral, with catastrophic consequences for the local marine ecosystem and for the local communities who depend upon it.

Vendra describes how it all began on a leisure dive on TRACC’s base island of Pom Pom, back in January. He noticed more COTS, closer together than usual, alerting him to the presence of a possible problem. He began organising removal dives.

By February, Vendra had realised they needed to develop a management strategy in order to deal with the issue effectively. He enlisted the help of marine biologist Dr Catherine Jadot, who informed him the targeted removal of COTS is the primary strategy used to manage outbreaks, but to do that effectively they need “an understanding of the demographics and dynamics of local populations”.

Jadot and Vendra set up a monitoring program, surveying multiple sites around Pom Pom Island, initially to assess whether or not outbreak levels had been reached. “The results from those surveys showed that all sites had passed the outbreak threshold of 40 starfish per hectare,” Vendra says. They began leading teams of volunteers to remove COTS from the water, burying them on Pom Pom Island. On each collection Vendra recorded the number of COTS removed, time spent in-water and the number of divers who took part. He did this to establish what he calls the “catch per unique effort” for each site – enabling comparisons to be made between them. Under Jadot’s direction, he also began researching gonad-to-body-weight ratios. Both male and female starfish have gonads, the organs that produce eggs and sperm. Vendra and his team take samples from 30 COTS from each collection, dissecting them to remove and weigh the gonads. They also measure overall size, weight and number of arms.

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This feature appears in ISSUE 04: Fluke science of Oceanographic Magazine

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