Number 200 has been alive for seven years.

She's not yet reached sexual maturity and is likely to live another 25 or so years.

Nick Whiterod puts 200 down on the muddy bank of the Murray River.

She waves her white claws, threatening the six-feet-tall aquatic ecologist into submission. But then, when she realises she's near the water, she scuttles backwards towards it with surprising speed, claws still threatening.

She sinks in a little cloud of bubbles, and in seconds, her aquamarine shell and white tail spikes are submerged in the murky water of the Murray.

There are parts of the river where Murray crays are no longer found. ( ABC RN: Ann Jones )

This area of the river near Echuca has been devoid of crays for about 10 years.

Before that, populations had been falling for decades all the way along the river, in part due to recreational, and at one stage, commercial fishing. River regulation also played its part — locks and dams created across the catchment altered the river's flow, and the spiny crays are picky about where they like to live.

But the biggest blow to the population of this beast — the second biggest freshwater crayfish in the world — happened in a much more noticeable way. It was the eight-year drought that devastated Australia from 2001 to 2009.

Hardly any water was making it to the river mouth at all. But it wasn't the dry that was the major blow to the crays — it was the rains that followed.

The crayfish release team, heading down the Murray River. ( ABC RN: Ann Jones )

"Once the flooding came back in late 2010 to 2011, a lot of the floodplain areas got inundated. And some of these areas hadn't been inundated for up to 10, 20 years," Dr Whiterod says.

"Leaf litter on the bank had accumulated and then when those big floods occurred, microbes are breaking down the leaf litter and stripping oxygen from the water.

"And that's essentially what blackwater is — it's water that comes off the flood plain [with] low oxygen levels or concentrations and it comes back into the main channel and lowers the oxygen concentration.

"It was pretty well 1,800 kilometres of the Murray River [that] was impacted and it was over summer so temperatures might have been 35 to 40 degrees [Celsius]."

The crayfish have a novel way to deal with events such as hypoxic water or contamination. They crawl up out of the water, wait for a short time on the bank and then re-enter to moisten their gills, repeating the process until the water quality situation has rectified.

But it was summer and many very hot crays perished on the banks, or were picked off by predators, or even fishermen, taking advantage of the situation.

Research conducted by Sylvia Zukowski on either side of the blackwater event found that the Murray cray population in affected parts of the river dropped by a total of 81 per cent.

"I guess it's worth noting that before that blackwater event, populations were already down by 50 per cent, in terms of the range of the species across the Murray River," Dr Whiterod says.

A new home beneath the trees

The sight of 200 Murray cray in one spot may have fallen out of living memory.

But on this cold, wintry morning in July, 200 of the crustaceans, caught upstream by NSW Fisheries workers, have been transported to near Echuca in large eskies with their nippers banded.

This stretch of the Murray River, where dead trees litter the water, is perfect cray habitat. ( ABC RN: Ann Jones )

As a representative of the Glenelg Nature Trust, Dr Whiterod has been involved in cray research with NSW Fisheries for about a decade, undertaking monitoring along the river.

At the translocation site, water whips around the curves, slowly eroding the banks and destabilising trees. They fall inwards towards the flow and the leaves are ripped off by the water, leaving sticks, roots and trunks to become perfect homes for crays.

It's why Dr Whiterod chose this place.

Martin Asmus from NSW Fisheries picks up a cray with a particularly wide tail. It's a female, he says, and that wide tail is diagnostic.

He flips her over and underneath are hundreds, perhaps thousands of tiny red berries — the same texture as the tiny beads of an orange, but the colour of ruby-red lollies.

A female Murray crayfish, with fertilised eggs exposed. ( ABC RN: Ann Jones )

These are fertilised eggs and she flips her tail protectively over them.

For his trouble, Mr Asmus is left with several bleeding points on his hand.

"They live up to their name — spinies," he says.

Fishers want to be part of the solution

Two sections of the Murray are open for a few winter months each year for cray fishing, with an upper and a lower-size limit and a bag limit for each day.

The regulations are based on the animals' reproductive output and the maturity of the crays — so they're there for a reason, Mr Asmus says.

But not everyone follows the rules.

"My understanding is it's probably one of the less compliant fisheries," Mr Asmus says.

"There are people who push the envelope, who keep more than they're allowed and they keep undersized crayfish."

Catching Murray crays was once a popular pastime for many Echuca locals. ( ABC RN: Ann Jones )

There are fishers, however, who want cray fishing on the Murray to be sustainable — and they're not just interested in eating them.

"Murray crays are like a totem species for us," says Roger Knight, vice president of the Barham Angling Club and secretary of the Edward Wakool Angling Association.

Mr Knight, also a local Landcare representative, is keen to get his community involved in citizen science activities around the crays that encourage catch and release, but still get people out on the river.

"That's one of the memories I've got as a kid … the excitement of pulling up a cray net. First of all, you see just the white spikey claws and then the rest of the body comes up," he says.

"They're like mini prehistoric animals, and they leave a lasting memory.

"I've taken other kids out when I've got older and crayed — and you just see the look on kids' faces — it's remarkable. You don't see that when they open Christmas presents or Easter eggs."

'You just assume they're still there'

The crayfish are thought to be locally extinct in South Australia and are protected under the Fisheries Act. The species is listed as vulnerable in the ACT and NSW, and threatened in Victoria.

Murray crayfish are the second-biggest native crayfish in Austalia. ( Supplied: Nick Whiterod )

They have a potential life span of 35 years, so if you're one of the people with a permit to fish in the river, you might be eating something older than you are.

"They're the sort of animal that's sometimes falls under the radar," Mr Asmus says.

"You see them every now and then, and you just assume that they're still there.

"There's a possibility that they just sort of disappear one day, and you don't notice."

But that won't be on Dr Whiterod's watch.

"I can't tell you what they taste like, because I've never eaten one," he says.

"The more you find out about them, the more respect you have for them."