They are aggressive, territorial, have a violent sex life and can deliver a nasty bite to the people trying to save them, but the fighting spirit of the endangered Mary River cod is helping bring the species back from the brink of extinction.

The fiery freshwater cod was one of the reasons why, in 2009, former Federal Environment minister Peter Garrett rejected the controversial Traveston Dam proposal in the farming catchment of Queensland's Mary River.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 6 minutes 45 seconds 6 m A Big Country: Mary River cod returned to the wild ( Jennifer Nichols ) Download 3.1 MB

It is estimated that numbers dipped as dangerously low as 600 in the wild, due to habitat clearing and overfishing.

But in 1983, one of the nation's pioneers of aquaculture, Gerry Cook, got together with mates to start a Mary River cod captive breeding program that continues to this day.

The Mary River Catchment Coordinating Committee (MRCCC) now manages the Gerry Cook fish hatchery on the shores of Lake MacDonald in the Noosa hinterland.

Mary River Catchment Coordinating Committee chairman Ian Mackay is seeking funding to survey cod numbers in the wild. ( ABC Rural: Jennifer Nichols )

Over the decades more than 500,000 fingerlings have been released back into the wild between the Gold Coast, west to Toowoomba and north to Maryborough.

"We hope that one day we can say 'once upon a time they were endangered, but now they are back in their home range'," MRCCC chairman Ian Mackay said.

Darren Knowles has been bitten and had his face mask ripped off as he dives to collect the Mary River cod eggs from this pond. ( ABC Rural: Jennifer Nichols )

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 5 minutes 25 seconds 5 m Darren Knowles, Ian Mackay and Debbie Seal discuss the efforts to save the endangered Mary River cod. ( ABC Rural: Jennifer Nichols ) Download 2.5 MB

Fiercely territorial fish

Darren Knowles juggles fire fighting with managing the fish hatchery and laughed when it was suggested he should be paid danger money for both of his careers.

He receives a very unwelcome reception when it comes time to collect the eggs laid on removable fly screens in pipes in a pond, for transfer to tanks in the hatchery.

"To get the eggs out you have to actually dive down with snorkel and put your head up a pipe and have a look and unfortunately the male, he's only doing his job, will come out and try and bite you on the leg," Mr Knowles said.

"People have had face masks ripped off and bites on the hands. So there's a few little screams when one comes out like that under the water.

"When you've got a face mask on, it fills up and you come up coughing and spluttering like no tomorrow but it's one of the exciting parts of the job."

Workers and volunteers from the Mary River Catchment Coordinating Committee are about to release bags of fingerlings into creeks and rivers. ( ABC Rural: Jennifer Nichols )

Dad does the parenting

In the wild, the female Mary River cod lays her eggs in the males' hidden home below a fallen log or a shaded, undercut creek bank.

The male then assumes all responsibility for guarding and raising their offspring.

But relationships between the sexes of the endangered species are far from a caring 'roses and chocolates' romance.

"They are a very aggressive and territorial fish, so unfortunately the female of the species has a bit of a hard time around breeding time," Mr Knowles said.

"She comes out and she's got bite marks on her."

Gerry Cook established the hatchery in the 1980s, fearing the Mary River cod would become extinct. ( ABC Rural: Jennifer Nichols )

The MRCCC is seeking funding to determine the success of the hatchery's restocking program.

"There are reports from the Department of Natural Resources folk that it's easier to find a Mary River cod now, which is anecdotal evidence, but it suggests that there is an increase in the numbers in the river, which is absolutely fantastic," Mr Mackay said.

The Mary River cod is a no-take species in all waters except a very small list of stocked dams.

They're totally off limits during the spawning season — from September to the start of December — because, even if the males are caught and released, they will abandon their young.

Debbie Seal and Ian Mackay from the Mary River Catchment Coordinating Committee are passionate about the restocking program. ( ABC Rural: Jennifer Nichols )

Impacts of climate change

MRCCC administration and event coordinator, Debbie Seal, said it was critical that anglers gave the species a break.

"They stay in the same place. The cod will find a hole or an undercut bank or a log and they'll make that their home and they'll stay in that area 98 per cent of the time and they only leave to feed and breed," Ms Seal said.

"We really need to do a scientific study. This is a species that does get very seriously impacted by climate change.

These Mary River cod fingerlings are ready for release into a suitable shady habitat. ( ABC Rural: Jennifer Nichols )

"We've had situations in recent years where there's been extended droughts and a landholder contacted us to tell us about the one surviving pool in the creek and six large cod were in that pool because it was the only habitat left.

"That landholder took it upon himself to put his pump into the pool to aerate it for the cod and he managed to keep all but one of those fish alive until such time as it finally did rain a month later."

In 17 years of working for the MRCCC, Ms Seal has helped dozens of teams of volunteers release the hatchlings into suitable shady habitat.

"It's really exciting but it's mingled with sadness because you know they're not all going to survive," she said.