Managing our water-spoiling pests

Image: NCCP/Tom Rayner

TWO million people live in the Murray-Darling Basin, along with hundreds of species of native plants and animals.

However, the Basin is home to some invasive species, too – both above and below the surface.

Willow and carp are among them. Combined they not only alter habitats and displace native species across the Basin, they also reduce water quality and alter its flow.

After years of multidisciplinary research, scientists have developed new tools to help control these two feral pests.

Thirsty willows

The weeping willow (Salix babylonica) and crack willow (Salix fragilis) are invasive European species estimated to occupy and clog more than 30,000 km of river streams across southeast Australia and displacing native vegetation. The loss in many areas of the great river red gum has reduced food sources, such as gum flowers, and taken away habitats for native mammals, reptiles, fish and birds.

The deep canopy of willows also blocks sunlight from reaching the water, making the water colder, and changing the stream ecology.

Ovens River, Myrtleford Victoria – Black willow. Image: Tanya Doody/CSIRO

“Willows have also carved out a new niche for themselves within the actual waterways—in-stream,” says CSIRO Senior Research Scientist Dr Tanya Doody. “All it takes is a broken two centimetre twig to start growing a dense system of roots and begin an infestation. This then disrupts what was once free-flowing water, and erodes the waterway.”

Doody has developed a set of tools to help water managers understand where and how many hectares of willows exist and how much water they use.

Using quantifying water balance calculation tables and an economical remote sensing technique, managers can better understand the total water savings for each infested stream if willows are removed.

The tables were calibrated using more than four years of field research and measurement of willow water use, and narrows the results down to climatic zones. The satellite technique operates on a fine scale of 2 m resolution, so that the canopy area of willows and native vegetation are distinct from each other. It also allows further mapping and monitoring of willow distribution, spatially and temporally.

“Catchment managers can now calculate not only the cost of removing willow trees, but the likely water savings, as well as the value of the water saved,” she says.

The riverine willow infestations drain a substantial amount of water from waterways—from 3.9 to 5.5 ML per year for each hectare of willow canopy in cool temperate and semi-arid climates respectively, she says.

“To put this into perspective, one megalitre is one million litres, which is the volume of water used by three average households in a year. So the returns from removing one hectare of willows is enough for about 17 households each year.”

Below the surface

It took widespread flooding in the early 1970s for carp (cyprinus carpio) to establish in the Murray-Darling. The Millennium Drought-breaking floods in 2012 took them to feral numbers.

In some parts of the Murray-Darling Basin carp comprise more than 80 per cent of fish biomass, exceeding 350 kilograms per hectare.

Ecologist Dr Tom Rayner holds an adult carp collected during fish surveys in the Macquarie Marshes, NSW. Image: Roger Scott.

They are mobile, hungry, breed quickly and can survive in polluted, shallow waters. They feed by sucking through gravel and mud, dirtying the water and blocking sunlight from aquatic vegetation. This impacts plankton, aquatic invertebrates, waterbirds and native fish. This can cause blue-green algal blooms. Carp can also get into irrigation infrastructure and block pumps, causing significant financial losses.

Dr Klaus Joehnk, Team Leader of modelling water ecosystems, is developing an all-in-one hydrological, demographic and epidemiological modelling tool to guide a national release strategy of the carp herpes virus. The strategy should be ready by the end of 2018 via the National Carp Control Plan.

“We are trying to maximise the efficiency of the virus, which is transferred through body contact, but is also dependant on water temperature and other environmental factors,” says Joehnk. “We’ve integrated this knowledge across our team, which includes health & biosecurity epidemiologists and RMIT University mathematicians, to tackle the question of when, where, and under which conditions carp aggregate in the connected river system.”

Looking ahead

Managing the ecosystems through the removal of these pests is critical.

The removal of willows is labour intensive and expensive: in Victoria alone, the cost to the state is $2 million per year. Doody adds the biggest problem is what happens to the site after the trees are gone: including the impact on bank stabilisation and the effect on local biodiversity, ecological function and water quality.

Just outside Canberra, willows along a nine kilometre-stretch of river were recently sprayed, costing 100s of 1,000s of dollars. But the biodiversity remains as low as when the willows were there.

“Native vegetation should be reintroduced, but that doesn’t always happen,” says Doody. “The process of managing willows is long, risky and not always easy.”

Joehnk also cautions that the initial release of a carp herpes virus needs to be followed by other measures to keep carp biomass at low levels—such as introducing daughterless carp to reduce breeding opportunities.

“The knowledge output of this simulation tool allows for a strategic release of the virus. This minimises the hazards associated with massive carp kills, such as organising the removal and processing of dead carp biomass.”

Combined, both sets of tools will help greatly with managing invasive species, and returning water quality and quantity back to the Murray-Darling Basin.

Read our earlier story on CSIRO’s work on the viral control plan for carp.