As any diver will tell you, the underwater world here in the Cook Islands is a spectacular place to explore.
The crystal clear waters give way to so much marine life, including the numerous plants and animals that call our coral reefs home.
Animals like crabs and snails skitter and slide across the coral bottom, while fish and turtles swim above. If you explore the nooks and crannies of the reef, you’re likely to find species you’ve never seen before.
The sight of a Crown of Thorns Starfish, or taramea as they’re called locally, alarms many divers because they are considered aggressive and destructive coral reef predators.
Turning their stomachs inside out to digest once healthy coral tissue, taramea are extremely effective coral predators, leaving behind only the dead, white skeleton.
As long as the ecosystem is balanced, healthy and water quality is good, very low numbers of taramea on a reef is normal. But in high numbers, large-scale damage to reefs can occur.
Moving up to 20 metres an hour, an individual taramea can eat its body size in coral tissue every night. An outbreak can be disastrous, particularly on coral reef ecosystems already facing stressors like runoff, marine pollution, sedimentation, as well as coral bleaching, cyclones and other climate induced impacts.
Sadly, some reefs may not recover, and even healthy reefs can take 10 to 20 years to fully recover from a significant outbreak.
Crown of Thorns get their name from the venomous thorn-like spines that protrude from their backs, resembling the Biblical crown of thorns. They’re the world’s second largest sea star and have been known to grow up to 80cm in diameter.
Despite their prickly appearance, in normal numbers taramea play an important role in coral reef ecosystems.
By feeding on fast growing corals, taramea help maintain diversity in coral species, creating space for coral larvae to settle and space for slower growing massive coral species to get bigger.
A single adult taramea can eat up to 10 square metres of living coral a year. As a food source, taramea are preyed upon by other marine animals including the matarea (humphead wrasse), kokiri (titan triggerfish), ‘ue (starry pufferfish) and pu (triton trumpet snails). A pu can eat about one taramea per week.
What is considered normal numbers varies greatly across areas and countries, some considering natural densities of 6-20 taramea per square kilometre, and others less than one taramea per hectare.
Defining an outbreak is also different for different countries, ranging from 1,500 to 6,000 taramea per square kilometre, or 15 to 60 taramea per hectare.
Regardless of the exact numbers, they will be most destructive when they consume coral tissue faster than the coral can regrow. The Ministry of Marine Resources is examining what these ranges are for our local coral reefs.
Taramea grow very quickly, start reproducing at a young age, and have some of the highest fertility rates among marine invertebrates.
These characteristics, when combined with their diet (they only eat coral!) contribute to their reputation as coral killers.
The causes of taramea outbreaks are still largely unknown, but may be influenced by several variables.
Typically, scientists link outbreaks to spikes in ocean nutrient levels due to land-based run-off and coastal upwelling.
The extra nutrients create food for taramea larvae. Outbreaks may also result when taramea predators are overfished because they are no longer on the reef to control population sizes or to limit the number of juveniles that survive to adulthood.
Recently, taramea population sizes have become problematic in parts of the Cook Islands, specifically in Rarotonga and Aitutaki where significant removal efforts are underway.
The Ministry of Marine Resources recognises all of the individual community members, local non-government organisations and dive operators who are volunteering in taramea removal efforts.
To address this ongoing issue, the ministry is developing a national response plan that aims to establish a unified and cohesive framework to monitor and control taramea populations in Cook Islands.
With the goal of a coordinated, joint effort, the ministry will be contacting those parties currently involved in taramea removal efforts to contribute to the development of this plan.