Researchers spearhead Murray cod recovery

THERE’S really only one tale worth telling after a day’s fishing in the Murray-Darling Basin.
“If you’ve got a rod in your hands and you’re in the Basin, you’re trying to catch a Murray cod,” says Dr Lee Baumgartner.

Dr Baumgartner, a freshwater ecologist, who works for the NSW Department of Primary Industries, has caught a few cod in his time.

“I’ve never been lucky enough to catch one over a metre though. That’s a once in a lifetime fish.

“Traditionally it was a take fish. Everyone used to catch their big Murray Cod and take it down to the pub to show all their mates.”

There are plenty of cod-catching tales still to be heard in pubs across the Basin but these days they’re much less likely to be accompanied by the trophy fish.

“Very few people keep cod these days. It’s mostly catch and release,” says Dr Baumgartner.

“Murray cod were historically very abundant. They were throughout the entire Murray-Darling Basin, all the tributaries, even up into upland reaches.

“People want to see Murray cod returning to the Basin in those sort of numbers.”

Image: Ian Wooden, Narranderra Fisheries Centre

His day job centres on trying to do just that and it’s not without its challenges. A decline in long-term average river flows due to human water needs, the construction of dams and weirs restricting fish migration, and the introduction of pests, especially carp, have seen native fish populations in the Basin dwindle to less than 10 per cent of pre-European levels.

But on Dr Baumgartner’s side are proactive conservation measures. Captive-bred juvenile Murray cod, gudgeon and perch are being released into Basin rivers boosting populations of native species in the fight against carp.

A juvenile Murray cod (centre) surrounded by gudgeons and perch Pic: Ian Wooden, Narranderra Fisheries Centre.

Fishways are being built in dams and weirs allowing fish to pass and complete migration cycles necessary for breeding. Environmental water, water owned by governments and environmental groups, is supporting Basin rivers in times of low flow.

Dr Baumgartner says environmental water is key to supporting healthy populations of Murray cod into the future. “There’s a lot of historical information that suggests when we had natural floods go down the rivers of the Murray-Darling Basin, Murray cod juveniles would be throughout the wetlands in very big numbers. You’re talking thousands of fish per wetland,” he said.

“So when river regulation slowly decreased the inundation of those wetlands, they lost access to nursery habitats.

“Environmental watering programs have a big role to play in terms of restoring nursery habitat for Murray cod.”

It’s a role environmental water is playing in the Murrumbidgee River, according to research from Charles Sturt University led by Dr Skye Wassens.

“These (environmental) flows become really important for a whole range of species, for frogs and turtles and fish, because they create nursery habitats to actually allow them to breed,” Dr Wassens said.

“Often we need repeat flows over a few years or through the season so that we can let those little fish get back into the river after they’ve grown up a bit and we can allow that movement back and forth.”

Image: Ian Wooden, Narranderra Fisheries Centre

Dr Wassens’ scientific monitoring of ecological responses to environmental watering is supported by Commonwealth Environmental Water. The Australian Government already has 736 gigalitres of water on average each year to use for the environment. This is water the government has purchased from other water users, who have offered their water for sale, or was saved by improving the efficiency of irrigation infrastructure.

The Commonwealth Environmental Water Office uses that water across the Murray-Darling Basin to improve river and wetland health.

“Up-to-date scientific research is integral to our work,” says Ian Robinson, who heads up Commonwealth Environmental Water.

“By funding monitoring of environmental flows, we’re improving our ability to actively manage our water to get the most sustainable outcomes possible for rivers and wetlands and the people that enjoy them.

“For example, we’re currently funding monitoring work in the Edward Wakool river system near Deniliquin in New South Wales to investigate how environmental water can be used to support populations of Murray cod.

“The Murray cod is near the top of the food chain so is an important indicator of ecosystem health, including salinity levels and potential for algal blooming.

“It’s definitely a Basin asset that we’re trying to protect and restore through environmental watering. The fish monitoring in the Edward Wakool, led by the Murray Catchment Management Authority in close partnership with community members, is providing up-to-date knowledge that will help inform our water management.”

Dr John Conallin, a fish expert at the Murray Catchment Management Authority, says the Edward Wakool fish monitoring project was born out of community concern for native fish species.

He said locals were devastated to see the tragic effects of water quality problems that followed extensive flooding after 10 years of drought. A decade’s worth of leaf litter had built up on dry river banks and then was suddenly swept up in flood waters. Shortly after, rivers started to run black and dead Murray cod began to surface.
Dr Conallin said the natural phenomenon came about as a result of the large amount of leaf litter that was flushed into the river by the flood waters.

“Microorganisms in rivers break down that leaf litter and they use oxygen in the water to do that,” he said.
“Basically the little bugs strip all the oxygen out of the water as they’re eating the leaf litter and that leads to fish dying.”

To try and save fish from the blackwater, the Murray Catchment Management Authority worked with Commonwealth Environmental Water to release freshwater from irrigation outlets.
“We think it may have created small freshwater refuges and that’s why we still have populations of Murray Cod around the Deniliquin area that benefitted from the additional water,” Dr Conallin said.

Dr Conallin is investigating how blackwater events can be mitigated in the future and believes environmental watering has the potential to reduce the risk of future events.

A juvenile cod taken during river sampling. Image: Ian Wooden, Narranderra Fisheries Centre

Dr Wassens, who is monitoring in the neighbouring Murrumbidgee River, agrees.
“In the same way as we might think of doing controlled burning or reduction burning, environmental water could be used to reduce the build up of organic matter on the floodplains so that when you do have a big natural flood you’re not getting all of that matter getting washed into the river,” she said.

“People sometimes mistakenly think that environmental flows cause blackwater events, but in reality it is the big increases in the period of time between floods that causes a build up of logs and litter.
“Massive floods are impossible to control and the consequences of the resulting blackwater events can be terrible.”
It’s something the Edward Wakool community is determined to try and protect Murray Cod from in the future.

Dr Conallin said community support for the Edward Wakool fish monitoring project had been overwhelming.
“There’s a lot of community action going on within the whole system,” he said.
“For example, we’ve got recreational fishing groups working with us. They’re part of the restocking program in the blackwater hit areas.
“The community’s work will leave an important legacy. If you’ve already caught a Murray cod, you want your kids to be able to catch one.”

Image: Ian Wooden, Narranderra Fisheries Centre