COTS Survey: Assessing a Possible Outbreak

Over recent months our survey teams have been noticing an increase in the number of Crown-of-thorns starfish (COTS) present in Lutoban’s Marine Protected Area (MPA).

Scientifically known as Acanthaster planci (wikipedia), COTS occur throughout the Indo-Pacific region. They can be identified by their many arms, as many as 21, and covering of long venomous spines. They can grow up to 80cm across, making them one of the largest species of sea star.

COTS are corallivores, meaning they feed on coral polyps, typically targeting stressed or diseased corals. They locate a feeding site using eyespots at the tip of each arm, then evert their stomach through their mouth and onto the coral surface, digesting the polyps externally by releasing enzymes and absorbing the resulting nutrients. A single COTS can consume up to 10 m² of coral polyps in a year.

When in sustainable numbers, COTS are valuable members of the reef ecosystem for several reasons. They generally prefer fast-growing branching and table-type corals (fast growth for a coral is around 4cm a year), which creates space for slower-growing, and often more resistant, coral species to establish. As they also prefer weak and diseased coral, COTS reduce the chances of disease spreading through the reef, helping to promote strong, healthy reefs.

COTS are also highly successful reproducers. A mature female (30cm and above) can produce anywhere from 60 to 200 million eggs per season, with figures at the upper end of that range reserved for the largest individuals (Kettle and Adams, 1987). Some studies have also found COTS larvae capable of cloning themselves, though how common this is among starfish more broadly is less clear.

Given the right conditions, however, COTS populations can grow beyond what the reef can sustain, resulting in outbreaks. COTS do have natural predators, including Titan Triggerfish, Triton’s Trumpet snails, and Humphead Wrasse, but once populations reach outbreak levels, these predators can no longer keep pace, and natural regulation breaks down. Coupled with other ecosystem pressures such as coral bleaching, affected reefs can struggle to recover, and artificial intervention is sometimes needed to bring the population back to a level the ecosystem can sustain.

An outbreak of COTS is defined as more than 15 individuals per hectare (Moran and De’ath, 1992), a threshold since adopted by the Great Barrier Reef Marine Park Authority, Green Fins, and the Philippines’ DENR as the standard for field control decisions. Our first protocol was therefore to measure the number present at the study site, Lutoban MPA. After a morning presentation covering basic COTS ecology and biology and the purpose of the survey, the team headed down to the site. All volunteers trained in our monitoring program joined the effort, conducting 5m-wide, 100m belt transects covering as much of the MPA as possible.

The data were extrapolated, and the results showed 60 individuals per hectare in the shallow depth range (3-7m) on Lutoban’s northern reef, and 53 per hectare in the medium depth range (9-13m). Although these numbers are well above outbreak level, more replicate surveys and statistical analysis will be carried out before any action is advised to the LGU.

Control methods for COTS have changed over time. Populations were once managed by cutting the animals into fragments and returning them to the ocean, in the hope that natural predators would consume the remains. This method is no longer used, since many echinoderms, including COTS, can regenerate lost tissue and even regrow limbs, meaning fragments can sometimes survive and re-establish. Instead, the method used by MCP and many other organisations is to inject each COTS with vinegar. This has traditionally been considered to kill the individual within 24 to 48 hours with minimal impact on non-target species, and injected individuals are typically consumed by other reef organisms shortly afterward, leaving little trace. See the note below for more recent research on how this can vary between males and females.

Once further surveys have taken place to confirm an outbreak, our science team will determine whether intervention is necessary. We have previously responded to outbreaks in Siquijor and Siaton.

If you’re concerned about a crown-of-thorns starfish outbreak in your local area, we recommend referring to the Green Fins guidelines on handling outbreaks.