License article

Crowned predator reigns on reef

Culling may be the only way to curb a big threat to Queensland's coral, writes Nicky Phillips.

In 1969, the veteran TV journalist Bob Raymond produced a documentary, Life and Death on the Great Barrier Reef. In it, he broadcast spectacular images of the reef at its best, colourful and crowded with fishes.

Part-way through the report Raymond showed viewers a reef littered with the coral-killing crown-of-thorns starfish which were beginning their ''insidious march'' along the length of the Barrier Reef.

The scientist he interviewed, Dr Robert Endean, said the reef was fast becoming a ''natural disaster''.

This week, more than 40 years later, media outlets around the world reported a similar story. A landmark report by Queensland marine scientists declared 50 per cent of the reef's coral had died in the past 27 years.

And the coral cover could halve again in the next 10 years if nothing changed.

''Coral cover is the simplest index of reef health, and the health of the Great Barrier Reef has gone down very dramatically,'' says Hugh Sweatman, a senior scientist at Townsville's Australian Institute of Marine Science (AIMS), which published the report.


The three main culprits of the coral destruction were intense tropical storms, coral bleaching, and the reef's long time foe, crown-of-thorns. The report pointed out that curtailing tropical storms and warming seas needed global support, which was unlikely in the short term. But controlling the starfish was a way forward.

''If we could stop crown-of-thorn outbreaks tomorrow then we could get the coral cover on the reef back to 1985 conditions in less than 20 years,'' says Peter Doherty, a senior researcher with the institute.

Scientists know it won't be that simple. It took them decades to confirm the role human activities play in the outbreaks; nutrient and sediment runoff from north Queensland farms prolongs the survival of starfish larvae.

Then it took federal politicians another decade to approve management funding; a program to improve water quality began in 2008, and a starfish eradication project kicked off in July.

But as reports of a fourth crown-of-thorns outbreak between Cairns and Cooktown emerge - not to mention the accumulating threats from climate change, ocean acidification and port development - scientists say it is unclear whether water quality improvements or targeted killings will return the world's largest reef to its former beauty any time soon.

''The latest outbreak began 18 months ago so even if we could get the nutrients right and find something to kill them we'd still probably have an infestation run down the reef,'' says the institute's chief executive, John Gunn.

The first recorded crown-of-thorns outbreak emerged off Green Island in the reef's north in the early 1960s. The species takes about 15 years to eat its way down the length of the 2000-kilometre reef, and so it was not until the late 1970s that a second plague set off between Cairns and Cooktown, says Sweatman.

The starfish are one of the coral's few natural predators. They kill by inverting their stomachs onto the coral's hard skeleton and envelope the living tissue inside. An adult starfish can eat a dinner-sized plate of coral a night, and consume 10 square metres a year, says Sweatman.

''Female starfish release more than 50 million eggs a year so you don't have to have a very big survival rate to have very large numbers of juveniles and then adults.''

In 1994 a third wave spread down the reef, the remnants of which can still be found off the southern sections of the reef, near Mackay.

Despite extensive research, scientific debate continued to rage over whether this cycle was natural. The species were native, after all, so the government agency set up to manage the reef made little attempt to control the starfish. Only tourist operators were allowed to kill the pests in frequented hot spots.

It was satellite images showing a plume of nutrients flowing from the Burdekin River into the patch of ocean between Cooktown and Cairns where these outbreaks originate that provided some of the first evidence they were fuelled by human activities.

In a laboratory experiment that followed, the AIMS marine biologist Katharina Fabricius found that when chlorophyll, a measure of the amount of plants and nutrients in the water, doubled, the survival rate of the pest's larvae increased by eight times.

''Our current theory is these large scale outbreaks are a response to nutrient run-off from the land, fertiliser mainly, especially after big floods,'' says Jon Brodie, a James Cook University marine biologist.

''The most recent crown-of-thorns outbreaks fits this pattern.''

In 2008, almost 10 years after the government was first presented with this evidence, the $200 million Reef Rescue program, with the task of reducing nutrient runoff by 35 per cent within five years, started.

A spokeswoman for the Environment Department says while estimates on water quality will not be released until the end of the year, close to 3000 land managers were involved in the program, which was on track to meet its targets.

In July, the federal government also gave the Queensland tourism industry $1.4 million to pay divers to manually kill the starfish using spears loaded with weak acid.

Col McKenzie, who heads the peak body for marine park tourism operators and runs the program, says large adult starfish need to be injected in each arm, up to 28 times in total, before they will die.

''I wish they made your sex life better so we could sell them, but I can't think of a use for them,'' he says. In a 10-day trip, his 15 divers can kill 2000 starfish.

Queensland MP Bob Katter has called for a $10 bounty on the pests. But manual killing can only work on a small scale of a few hundred square metres, says scientist Hugh Sweatman. ''You can't protect the entire Barrier Reef with that,'' he says.

Like most pests, the most effective solution may be biological. Scientists at James Cook University have found ways to exploit a naturally occurring disease within the starfish. ''If we could reduce the frequency and intensity of the crown-of-thorns outbreaks that's the one lever we might have to give the reef a break,'' says Sweatman.