After spending four years trying to find freshwater Murray crayfish in South Australia — and failing — an aquatic ecologist is on a mission to reintroduce the species.

The crayfish were once abundant in the state's section of the river but are now non-existent downstream, according to Nature Glenelg Trust's Nick Whiterod.

"They were around in the early '80s, but there hasn't been a record since then," Mr Whiterod told ABC Radio Adelaide's Afternoons program.

"We are fairly confident that they are not around.

"We looked at upwards of 30 sites over a four-year period during winter and haven't found them."

His team put in more than 7,000 hours patrolling and netting areas where the crayfish would populate.

Researchers have tried for four years to find freshwater crayfish in South Australian waters. ( Nature Glenelg Trust: Nick Whiterod )

Mr Whiterod said it would be possible to restock the South Australian section of the Murray by relocating stock from upstream.

"There is still a stretch from Tocumwal up to Albury that is open to recreational fishing and numbers are quite good there," he said.

"The populations are in the thousands in those spots."

Mr Whiterod said a similar restocking program had been successfully completed recently in Echuca after a blackwater event saw numbers drop.

Freshwater crayfish can still be found in healthy numbers further up river. ( Nature Glenelg Trust: Rick Whiterod )

So what happened to the original crays?

The establishment of weirs along the South Australian section of the Murray had slowed water flow and dropped oxygen levels below what crayfish could survive in, Mr Whiterod said.

But there are a few places where he believes they could survive if they are reintroduced.

"It is something as a state that we should look at doing in the near future," he said.

He said he hoped his not-for-profit would be able to undertake restocking downstream of weirs within the next few years.