Why the Crown of Thorns are destroying our reefs

The Crown of Thorns starfish are destroying large sections of coral reefs. Is this a natural phenomenon or are we also playing a part? Marcus Haritos reports.
The Crown of Thorns starfish are destroying large sections of coral reefs. Is this a natural phenomenon or are we also playing a part? Marcus Haritos reports.

Lurking within the shallow reefs of the Pacific and Atlantic oceans is a carnivorous predator that feeds solely on coral tissue. Known as the Crown of Thorns Starfish due to the barb-like thorns that engulf their bodies, these destructive and rampant creatures are capable of consuming coral faster than it grows, turning once thriving habitats for thousands of species into dark desolate graves of dead coral, void of any life.

The Crown of Thorns Starfish or COTS have been observed by scientists to be the single biggest cause of coral reef destruction on the Great Barrier Reef. Dr Mike Hall, the principal research scientist at the Australian Institute of Marine Science (AIMS) who focuses on developing new technologies to manage and mitigate the impacts by the COTS, says their fertility rate is the reason they’re such a problem. A female starfish is capable of producing up to 100 million larvae in a single spawning season. If 90 percent survive then there up 90 million full grown COTS in a given area, and it’s almost “like cattle feeding on pasture land” because the coral can’t move. Outbreaks of COTS have accounted for over 40 percent of Great Barrier Reef coral loss over the last three decades.

Yet on healthy coral reefs, the coral-eating starfish plays an important role. It tends to feed on the fastest growing corals known as staghorn and plate corals, which allows slower growing coral species to form their own colonies, encouraging biodiversity. So, the biggest issue facing scientists at the moment is how to control the outbreaks of COTS to maintain a healthy natural state of equilibrium.

Dr Hall discussed the unique geographical make-up being a cause for the huge population spikes. “The Great Barrier Reef is essentially a huge lagoon. By spawning with 100 million larvae inside a lagoon, they’re trapped within a very small surface area with very little current to allow them to escape the water column. So, the Great Barrier Reef is perfect territory for species like the COTS.”

Why the Crown of Thorns are destroying our reefs

It seems that humans may play a part in increasing the likelihood of outbreaks. The increased use of nutrients, pesticides and other pollutants on the land results in more of these entering waterways into the reef. The elevated nutrient levels cause an increase in phytoplankton, which is the main food source for COTS larvae. This enables a much higher than usual number of larvae to survive and develop into COTS at which stage they start feeding on coral.

Another way that humans might be contributing to the outbreaks is through over-fishing. COTS have a number of natural predators such as the Giant Triton Snail, which releases a substance into the water that has been observed to terrify COTS and send them fleeing. But the giant triton has long been unsustainably harvested from coral reefs, primarily for sale to shell collectors. As a result they remain rare and endangered on the Great Barrier Reef.

Meghan Smith is a researcher at the University of the Sunshine Coast. Ms Smith is in the final stages of her PhD project, investigating the reproductive biology and physiology of COTS. Ms Smith says she believes that this knowledge will provide a fundamental understanding of COTS biology. This would enable researchers to develop better bio-control strategies to aid in preventing the population explosions we are seeing on the coral reef systems of the Great Barrier Reef. “With COTS capable of breaking out on neighbouring reefs, we hope to see more work being done to tackle the problem earlier in the life-cycle stage, to prevent outbreaks opposed to ‘cleaning’ them up.” Ms Smith discussed.

Currently, the most commonly used method to control COTS is a manual injection administered by divers using a drench gun which slowly kills the animal. This method is quick and effective only requiring a single injection. Other methods in development include a submersible AI robot with an automated injecting arm to control COTS, called a COT BOT.

Scientists at the Australian Institute of Marine Science (AIMS) observe the number of COTS by towing a snorkeler behind a small boat around the perimeter of a reef. Between 2010 and 2013 there was a significant spike in outbreaks on the Great Barrier Reef. AIMS observed a 30 per cent increase in the number of reefs experiencing outbreaks. But since then that number has decreased considerably to about eight percent. Another way scientists observe COTS populations is by considering their average density across the Great Barrier Reef. This is done by counting the number of COTS per two-minute tow by boat. In 2017, the surveyed reefs had an average density of .1 COTS per two-minute tow. This compared with 1.17 in 1988. COTS outbreaks occur approximately every 17 years, with the last one occurring in 2010, hence the huge amount observed that year.

“At the moment we are seeing a considerable smaller number compared to the past. While population outbreaks fluctuate and we see rises and falls in COTS numbers on reef ecosystems over time, it is important we do not become complacent during the times when we see lower density populations as future outbreaks could prove detrimental to reefs already under stress,” Ms Smith said.

The Great Barrier Reef is a multi-billion-dollar tourism industry and there is an economic incentive to key stakeholders that areas are preserved and maintained. Tourism is generally considered one of the main causes of environmental degradation, yet in the case of the Great Barrier Reef, it seems to be one of the only financial incentives to maintain it. The tourism industry has for many years played a key role in protecting coral reefs from COTS outbreaks and various other threats such as cyclones and coral bleaching. Without such an established tourism industry, the situation would be far worse.

In comparison, the Raja Ampat Archipelago located in East Indonesia is one of the most biodiverse marine regions on Earth. This region is also one of the most susceptible to COTS outbreaks and the results have been catastrophic. But the difference between here and the Great Barrier Reef is that it doesn’t boast a multi-billion-dollar tourist industry. Currently the only individuals invested in monitoring and conserving the region are an array of individuals either for the purpose of tourism exploits, or to mitigate the issue for the sake of the environment.

It is clear that the economic significance of the Great Barrier Reef has provided it with the appropriate measures to combat the problem but regions like Raja Ampat are helpless. Larz Vant Hoff is an ecologist who has worked on the Great Barrier Reef and now based in Raja Ampat. “The need for an international body involved in protecting these areas is dire because quite clearly as is the case in Raja, the Indonesian government does not yet see the incentive in taking action,” Mr Hoff said.

“If everybody started checking and removing when the numbers are high, we’d probably make a difference. We waited far too long on the Great Barrier Reef. I was there when it started and those of us working on the Reef could see that was something going wrong. But science or the government or whatever took 20 years to decide it was humans causing the problem, by which time enormous areas had already been wiped out. There’s always excuses for doing nothing which, is what I expect we’ll see here in Raja Ampat too.”