Australian marine scientists are proposing leaving thousands of decommissioned oil rigs in their deep-sea spots indefinitely to become long-term fish nurseries.

The huge chunks are of steel are meant to be removed when they reach the end of their productive life, but the scientists say their approach could save the oil industry millions of dollars and protect vulnerable fish nurseries.

The Deepwater Horizon disaster a year ago showed deep sea oil exploration at its most destructive.

Eleven workers were killed and almost 5 million barrels of oil leaked into the Gulf of Mexico.

Now marine researchers at University of Technology Sydney are pondering the future of the more than 6,000 rigs still in the sea.

"There's a huge number that are due for decommissioning - they've reached the end of their production life, there's no more oil left and the question is what do we do with these structures?" Dr Peter Macreadie said.

"There's actually not much habitat in the deep sea - not a lot of hard structured habitat, not a lot of reef and so by adding more reef the idea is that maybe you can boost production, you can boost the amount of fish."

But it would take some major legislative changes to put the proposal into practice.

"Current legislation requires rigs to be dismantled and removed and recycled onshore if they can be recycled," Dr Macreadie said.

"But we're actually starting to think maybe there's a much better use for those rigs and in some cases there actually isn't the technology to remove these rigs.

"They've been in operation for 20-odd years and now we're wondering actually, in removing them are we actually causing more problems than if we left them in place or if we moved them into the deep sea to form artificial reefs?"

Protection for species

Dr Macreadie says illegal fishing trawlers going through deep-sea areas are decimating coral reef communities and fish species.

"What's really devastating about that is that these organisms that live in the deep sea, they're slow growing, they reproduce very late in life and they are very vulnerable to exploitation," he said.

"To give you an example, the orange roughy takes about 30 years until it can reach sexual maturity and breed.

"They form huge aggregations around structures in the deep sea and structures are very rare.

"When they form those aggregations fishermen have become aware of this and they can remove entire populations, generations and generations with one swoop of their net.

"So the idea is with rigs - which have a lot of hollow internal space - the fish could find refuge within those rigs and be safe from the trawl fisheries."

Dr Macreadie says researchers do not know how long the defunct oil rigs will take to disintegrate.

"But we can say that many of these rigs have been in production for 30-odd years and they're showing very little signs of decay, but at some point they will eventually disintegrate in the deep sea," he said.

Dr Macreadie and his UTS colleagues have written an article published in the US journal Frontiers in Ecology and the Environment.

They hope to stimulate a rational debate about the future of the world's rigs.

Dr Macreadie says he does not work for, consult, or own shares in any company or organisation that would benefit from the proposal.