IN JANUARY the Great Barrier Reef Marine Park Authority sent a team of divers to the Swain Reef complex in response to reports of crown of thorns starfish outbreaks.

The team culled 41,000 crown of thorns starfish during the mission but that was just the tip of a very thorny iceberg.

The main purpose of the mission was to gain field intelligence on coral coverage and starfish numbers in the Swain Reefs.

Dr Mary Bonin, assistant director for the control program, said 77 reefs in the Swains were surveyed during the January trip. 

"Ninety per cent of the reefs had outbreak levels of COTS," she said.

Dr Bonin cautioned against panic, saying the Swain Reefs contain about 370 reefs and it could not be inferred the other 300 reefs had outbreaks.

"The fact we saw high outbreaks in the area where we were getting reports on outbreaks (tells us) we were in the right spot," she said.

Dr Bonin said the cause of the outbreaks of starfish in the Swains was not yet well understood.

"Three things drive outbreaks," she said.

"COTS have that kind of lifestyle: boom and bust, they are highly fecund, producing lots of offspring.

"On top of that there are human activities that exacerbate (outbreaks).

"Up north nutrient run-off influences outbreaks and (so does) (over)fishing of predators of the starfish."

But Dr Bonin said the Swain Reefs were much further offshore than the northern reefs which had suffered outbreaks and scientists have hypothesised nutrient up-wellings are partially to blame.

The starfish play an important role in the ecosystem in low numbers, keeping in check fast growing coral species so slower species get a chance.

But when there's too many they decimate reefs.

Dr Bonin said an extensive culling mission to the Swains, lasting several months, would take place at a date to be decided.