MARK COLVIN: Numbers in the world's only giant cuttlefish colony at Whyalla in South Australia have dropped drastically but no-one is sure why.
The rocky coastline off Whyalla is the only place in the world where hundreds of thousands of the marine creatures gather to breed en masse.
Some are up to a metre long. But this year anecdotal estimates say the cuttlefish are smaller in size and are down to 10 per cent of normal numbers.
The State Fisheries Minister is now looking at extending the protected area of the cuttlefish to prevent the population being completely wiped out.
Nance Haxton reports.
NANCE HAXTON: The cuttlefish congregation at Whyalla is unique, attracting divers and documentary makers from around the world.
Hundreds of thousands of the creatures dance and show their unusual mating displays in the waters off the Whyalla coastline from May to September every year. That is, until now.
Tony Bramley runs a dive shop in Whyalla. He says the season is now over more than a month early because the population is devastated - the lowest numbers he's ever seen.
TONY BRAMLEY: Well I've been looking at them for 30 years, every year for 30 years, and this is just not normal.
Whatever's happened this year, 10 per cent of a normal aggregation, it's not enough to maintain the biomass. Another year like this could be the end of this aggregation. That would be a disgrace.
NANCE HAXTON: Rod Hayles also dives in the region and has taken video documentaries of the cuttlefish for more than two decades. He's shocked at their decline in numbers.
ROD HAYLES: The numbers are down. We don't know at this stage what would have caused that.
NANCE HAXTON: Because this really is the only place in the world where the giant cuttlefish aggregate in this way isn't it? So if the numbers are down that is of concern.
ROD HAYLES: It is unique in the world and it's very important that we look after it as best we can.
NANCE HAXTON: Fishing is now banned in part of the breeding grounds after over-fishing in the late '90s decimated the population. But since the ban the cuttlefish have thrived - until this year.
Greens Upper House MP Mark Parnell says the entire breeding ground of the giant cuttlefish should be protected.
MARK PARNELL: We know that the cuttlefish are a short-lived species. They only live for one or two years. Therefore one breeding season failure could be catastrophic for the entire species.
So we have to give this creature every chance possible of surviving and that means declaring the whole of its breeding grounds a no-take sanctuary zone.
NANCE HAXTON: State Fisheries Minister Michael O'Brien says he is now considering extending the protected area for the cuttlefish.
MICHAEL O'BRIEN: And it could well be that we'll extend the closure area.
NANCE HAXTON: Would you also consider doing a study as to really looking at why their numbers have fallen so drastically?
MICHAEL O'BRIEN: We will be undertaking that and I'm going to be enlisting the cooperation of Whyalla locals to assist us in doing that work.
NANCE HAXTON: Tony Bramley says if something isn't done soon it could put at risk one of the world's great spectacles.
TONY BRAMLEY: We need some positive action to be taken immediately and not to just let it go through 'til next year in the hope that it will be alright because that's putting every egg in one basket so to speak.
There won't be any subsequent options if there's another disaster next year because two years is their life span. And they're semelparous, they die after they've spawned. So if we don't get any recruitment next year that may be the end of this aggregation.
And you know this is such an important thing. It doesn't happen anywhere else. And we really have a responsibility to do everything we can, everything in our powers to make sure we don't lose this special event.
MARK COLVIN: Diver Tony Bramley ending Nance Haxton's report.
The world's only giant cuttlefish colony at Whyalla is under threat. Usually hundreds of thousands of the marine creatures breed en masse off Whyalla's rocky coastline, but this year numbers are down by 90 per cent. And no-one knows why.