Turtles could solve the Murray-Darling's carp problem, research shows

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Oct 28, 2017
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ABC Broken Hill
By Jonathon Poulson and Jessica Schremmer
September 21, 2020.
A study has found turtles play a critical role in cleaning up waterways after fish-kill events.(Supplied: Western Sydney University)

Freshwater turtles could be the solution to keeping the Murray-Darling clean and to helping eradicate carp from the river system, a study has found.

Western Sydney University research has revealed that turtle scavenging can remove fish carcasses from the water five times faster than natural decomposition.

It also has found that if turtles are reintroduced, they will dramatically improve the river's water quality by eating the fish carcasses before they begin to rot.

Western Sydney University ecologist Ricky Spencer said turtles played a particularly important role in cleaning up waterways after fish-kill events.

"They love to eat and are always asking for food like fish and meat so they are really important in terms of cleaning up rivers of any dead fish," Mr Spencer said.

Key points:
  • The study shows turtle scavenging can remove fish carrion five times faster than natural decomposition
  • Scientist Ricky Spencer says freshwater turtles will be crucial to ensuring the success of the National Carp Control Plan
  • Roadkill, invasive foxes and water-quality issues have led to a decline of freshwater turtles in the Murray-Darling Basin
Reintroducing freshwater turtles to the Murray-Darling Basin could be a cost-saving measure in the National Carp Control Plan.(Supplied: Western Sydney University)

Turtles could slash carp clean-up costs
The study was initially conducted to help conserve biodiversity in the river system.

But it has found that replenishing the number of turtles could be a significant cost-saving measure in the National Carp Control Plan — colloquially known as "carpegeddon" — which would see carp herpes used as a biological control agent.

"The initial release of the carp herpes virus may actually cost up to $2 billion to take the fish out of the water," Mr Spencer said.

"We are now seeing that scavenging is performing a role that potentially would cost a lot of money to do if we wanted to clear the carp mechanically or go out with boats and nets."

Mr Spencer said adding more turtles to the river system would help to regulate the river's nutrients instead of taking them out completely.

"What would normally happen is that bacteria would break the carp down and release the nutrients into the water column, which can trigger things like blue-green algae and that's potentially what causes our rivers to turn green,"
Mr Spencer said.
The study found water where turtles were introduced (B) returned to normal faster than water without turtles (A).(Supplied: Western Sydney University)

"Turtles regulate the carp, so instead they'll compete with that bacteria and nutrients and they'll store it and then release it in a more regulated fashion."

Shrinking turtle numbers
The University of Sydney PhD student Claudia Santori, who pioneered the study, said freshwater turtles lived in the Murray-Darling in abundance, but numbers were dwindling.

"In the Murray-Darling Basin there are three species of freshwater turtles — the long-neck turtle, the broadshell turtle and the short-neck turtle — and all of them eat carrion or dead animals to an extent," Ms Santori said.

Turtle populations in the basin have been declining because of predators such as foxes, cats and goannas preying on nests, and because of roadkill, marine disease and poor water quality due to water connectivity issues.

"These species have declined considerably over the past few decades, so my interest was really to find out what the ecosystem is going to lose if these species go extinct," Ms Santori said.
Researcher Claudia Santori inspects a net tube to see how the introduction of turtles affects carp carcass decomposition and water quality.(Supplied: Tom Burd)

The carp, which make up almost 90 per cent of the river's biomass, have had a devastating effect on the river's ecosystem and water quality.

"After the fish kills, there were obviously some major impacts on water quality such as decreases in ammonia and other nutrients," Ms Santori said.

'Anything's worth a try'
Menindee Tourism Association president Rob Gregory said if more turtles were introduced to the river system, there would need to be a plan to protect them.

"When the water's down low and if you're wandering around in that black soil country, you can find a lot of empty shells so there's certainly a lot of predators that like them," Mr Gregory said.

"A lot of things must be done to prevent that from happening to control the foxes and cats particularly, but certainly anything's worth a try."

Mr Spencer said if more turtles were introduced to the river, community involvement would be imperative to the project's success.

"Our '1 Million Turtles Program' is where communities can be actively involved in protecting turtle nests and creating turtle islands," he said.

"We're really trying to enable communities to do it because people love turtles and we want to harness that so we can actually start restoring our native turtle populations."
Sep 22 2020


Building turtle populations key to improving basin river health, new study finds

Rebuilding turtle populations could improve the river health of the Murray-Darling Basin, according to a new study from Western Sydney University.

A Western Sydney University ecologist has completed a study of the aquatic scavengers in the Murray-Darling Basin, and has found that replenishing the numbers of Australian freshwater turtles will be critical to the successful cull of European carp and cleaning up after devastating fish kills in our rivers.
Associate Professor Ricky Spencer said the Federal Government planned to use a biocontrol agent to eliminate the introduced species from south-east Australian waterways.

“The European carp cull — which is referred to as Carpageddon — will use a viral biocontrol to rid rivers of this invasive pest, so that it can return to its former ecological state and offer improved habitability for threatened native species,” Prof Spencer said.

“However, unless there is an effective clean-up plan in place, Carpageddon will lead to hundreds of thousands of tons of fish carcases rotting in our waterways, and will cause further habitat damage.”

Prof Spencer led a research team in a series of experiments in 2018, which simulated the environmental conditions after a major fish kill event.

Carp carcases were introduced in four outdoor field sites near Murray Bridge, South Australia.

The bait was submerged using a range of methods that allowed access to turtles, shrimps, prawns and crayfish.

In addition, researchers set up controlled mesocosm experiments — with and without turtles — using a state-of-the-art experimental wetland facility at Western Sydney University's Hawkesbury campus.

In both the field and mesocosm experiments, researchers monitored the decomposition of the carp carcases and recorded any changes to water quality.

The findings, which are published in Scientific Reports, include observations that:

● Carp that were accessible to turtles perished more rapidly than those not accessible to turtles;

● In mesocosms with turtles, carp carcases were stripped to skeletons within five days, whereas carcases took more than 27 days to decompose without turtles present; and

● Turtle scavenging resulted in water chemistry rapidly returning to normal — including rapid decreases in ammonia levels and consistent dissolved oxygen levels — which allowed water conditions (algae/cyanobacteria and turbidity) to be maintained.

Prof Spencer said the findings clearly demonstrated the importance of the recovery of turtle populations.

“Freshwater turtles once occurred in high numbers in Australian waterways, but are now in sharp decline — in large part due to urbanisation, cars and foxes,” he said.

“There are a number of significant threats to turtles, which could lead them to becoming functionally extinct soon — which means they wouldn't be able to play their significant role in the ecosystem that helps to regulate water quality.”

Prof Spencer said if Carpageddon proceeded, the recovery of the waterways would depend on the presence of a healthy turtle population to assist with the clean-up.

“Scavengers like turtles are part of our natural capital,” he said.

“They perform a job in the ecosystem that should be valued and quantified.

“The cost of introducing turtles as a natural clean-up crew should be quantified — just as it would be worked out how much it would cost to remove carp with boats and nets, or to build a filtration plant to clean our drinking water.

“It would probably be cheaper to boost the numbers of turtles in the Murray-Darling, than try and remove every dead fish from wetlands.”


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