Managing the Crown-of-Thorns Starfish

How Australia Deals With Population Outbreaks

Oct 24, 2009

Australia spends millions of dollars every year to monitor and control the Crown-of-Thorns starfish when its population threatens coral reefs.

For decades, Australia has dealt with outbreaks of Crown-of-Thorns starfish (Acanthaster planci) on the Great Barrier Reef, and scientists are still trying to learn the exact causes of the population explosions. This particular starfish species has an interesting biology and relationship with its environment that makes it complex to manage.

Overpopulations of Crown-of-Thorns have been reported in the Coral Triangle and off the coasts of Tanzania and Australia in recent years, so the problem is not unique to Australia. This article expands upon the topic of starfish species whose voracious feeding threatens delicate ecosystems, as described in the article “The Destructive Feeding Habits of Starfish: When Ecosystems are Devastated by Overabundant Sea Stars.”

Biology and Behavior of Crown-of-Thorns Starfish

Of all invertebrates, this starfish species has the highest fertilization rate ever measured, which means that the potential for the production of elevated numbers of Crown-of-Thorns is high from the start. Adults can grow to be up to a meter in diameter, with up to 21 arms covered with venomous spines, about five centimeters long, that are usually a different color than the reddish body.

The young starfish feed on algae before they move on to coral, mainly tabular coral, and spend half their time feeding. When overpopulation occurs in the Great Barrier Reef, it often starts in the north and moves southward, a consistent pattern that might be due to starfish larvae moving with the currents. The starfish eat coral species they don’t generally prefer, and may wipe out most of a coral colony, leaving less than one percent of it alive.

How Scientists Monitor and Control Starfish Outbreaks

An outbreak of Crown-of-Thorns might have to do with the survival rate of larvae, which could be influenced by natural changes in water temperature, salinity, or available food supply. These starfish have only a few natural predators: the giant triton snail, whose population is already low due to overcollection in earlier decades; the starry pufferfish; the titan triggerfish; and maori wrasse.

Overfishing may play a part in Crown-of-Thorns population growth. When not enough fish are available to feed on juvenile starfish, it increases the reproducing adult population. Even if predator fish are not depleted, officials cannot even estimate the minimum number of predators needed to prevent outbreaks because there isn’t a way to log the survival rate of starfish larvae.

Scientists use broad-scale and fine-scale surveys to estimate juvenile, sub-adult, and adult starfish densities in certain areas, to determine if an outbreak is occurring, or will soon occur. The Crown-of-Thorns population must be more than 30 adult starfish per hectare (10,000 square meters) in order to be considered an active outbreak.

Australia Spends Millions of Dollars to Manage Starfish

Once an outbreak occurs, scientists cannot do much to control it. By looking at patterns of previous outbreaks, they know which direction an outbreak might progress and how many years it might take to spread, as well as how many years it will take for destroyed coral to regenerate.

In areas where an outbreak is likely, monitoring of the situation must take place daily because starfish can quickly move from one area to another. Often divers will inject nontoxic sodium bisulfate into the starfish to kill them, but this can be quite costly if up to 500 starfish per day need to be injected.

According to the CRC Reef Research Centre, now called the Reef and Rainforest Research Centre, the Commonwealth and Queensland Governments spent $2 million for a two-year Crown-of-Thorns starfish control program that began in 2001. Apart from that, some independent tourism operators spend up to $300,000 per year to control the starfish and ensure a high cover of coral for tourists to enjoy.

Future Management of Overabundant Starfish

Coral reefs can recover from destruction by starfish the way they do from cyclones or coral bleaching, because even if the smallest part of a damaged reef survives, it gets the regeneration process started. And if currents bring coral larvae from parent colonies located elsewhere, recovery will be swifter (about 10 years) than if no larvae are deposited in the damaged reef. One concern about this is that some species of coral grow more quickly than others, so the recovered reef may not resemble what it once was, with its overall diversity changed.

It is well known that increased usage of agricultural fertilizers around the world has affected the lakes, rivers and oceans on Earth. For the future, scientists will continue to look at the role that high rainfall and runoff play in ocean water quality. When water with high sediment and low salinity levels is washed into the reef after periods of drought, carrying extra nutrients that spur the growth of algae, Crown-of-Thorns starfish larvae have more food available to them. This seems to be the first level of opportunity to effectively manage starfish overpopulation.

The copyright of the article Managing the Crown-of-Thorns Starfish in Ecosystem Preservation is owned by Cheryl Kraynak. Permission to republish Managing the Crown-of-Thorns Starfish in print or online must be granted by the author in writing.