One of the many reefs found on the Great Barrier Reef – Steve Parish

The Great Barrier Reef

The Great Barrier Reef is one of the most spectacular living structures in the world.  Found off the coast of Queensland, north-eastern Australia, it spans a massive 23,000 km in length with an area of 344,400 km2. It is larger than the Great Wall of China and it’s the only thing on earth that is visible from outer space. It is one of the most diverse ecosystems in the ocean, with close to 9,000 different species of marine organisms residing on it, including, 1,625 species of fish, 411 types of hard coral, 133 species of sharks and rays, 6 species of sea turtle  and 630 species of echinoderm (starfish)-one of these being the Crown-of-thorns starfish.

Great Barrier Reef fact sheet – Australian Government

Reef slaying crown-of-thorns starfish (COTS)

Acanthaster planci , is a spine covered venomous starfish from the class Asteroidea. They feed on coral polyps making them corallivores. Native to the Great Barrier Reef, they naturally occur in low numbers; however, densities can increase dramatically in events called “outbreaks”. An outbreak would usually be classed as 30+ COTS per hectare of reef or when the consumption of coral is greater than the rate of growth.

Crown-of-thorns starfish (Acanthaster planci) – Wikimedia
Animation of COTS outbreaks 1985-2017 – Australian Institute of Marine Science (AIMS)

“Outbreaks” – not the zombie kind

According to the Great Barrier Reef Marine Park Authority, there have been four significant crown-of-thorn outbreaks: 1960s, late 1970s, early 1990s and 2010. Each outbreak has a massive effect on the reef. A study conducted in 2012 by Australian Institute of Marine Science (AIMS) concluded that coral cover has declined by 50% over the last 27 years with COTS contributing to half of the decline.  Reefs can generally recover from outbreaks however, it can take anywhere between 10 to 20 years to recover fully. It could take even longer with increased pressure from adverse weather conditions, bleaching and poor water quality.

So what are the causes of crown-of-thorn outbreaks?

There isn’t a mutually agreed conclusion between scientists; however, numerous studies have been carried out. A paper published in 1973 points out that reduced salinity can result in COTS larvae survival to increase. An ever growing problem, global warming, is also thought to have an effect. Increased water temperatures and increased primary production can accelerate COTS larval development. The decline of key COTS predators is another theory that would increase COTS populations and outbreaks.

The most widely accepted theory is that outbreaks are catalysed by the increased availability of plankton. Increased nutrients that are washed into the sea by run off of rain water can induce a plankton boom; this increases the amount of food available for COTS larvae which may result in an outbreak.

What are we trying to do about the problem?

There are a number of approaches to try and rectify this problem. In the short term, manually destroying crown-of-thorns with a lethal injection that is non-toxic to other marine animals is the most efficient technique. This video shows the process of injecting the COTS:

The long term strategy is to learn more about COT outbreaks with a new research strategy to look in to how outbreaks start, predator-prey relationships of COTS, stock recruitment relationships and larval connectivity to reefs downstream. In order for us to prevent outbreaks, we need early detection so that we can quickly dispose of the threat. Programs such as Eye on the reef give us greater monitoring which allows us to detect outbreaks sooner.

In conclusion, the Great Barrier Reef is an astonishing place that anyone would enjoy; it is one of the seven wonders of the world and therefore needs protecting. Crown-of-thorns starfish are coral hungry rodents that need to be controlled in order to reduce reef damage. We have a short term method however; we need to do more research to find a more permanent solution to the problem.

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