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By Melissa Lyne 19 April 2018 5 min read

Red gums frame ribbon weed in Bora Creek, Macquarie Marshes, NSW, during an environmental flow event. Image: Tom Rayner ©  2010

THE flow of water drives the life cycle of fish. Whether it’s a salmon swimming upstream to spawn, or a cartoon clownfish riding the East Australian Current on the back of a turtle, in any body of water, flow holds the power to make or break even the best of swimmers.

The waterways of southeast Australia are no exception. However, the flow regime throughout Australia’s vast Murray-Darling Basin has suffered through years of river water manipulation to suit human needs. This has disrupted the conditions that once supported booming native fish populations.

Scientists studying the troubled waterways are now combining fish research and computer models to inform how components of the Murray-Darling flow regime can be restored using environmental water allocations. The aim is for water to be delivered at the best time of year to the right places, and in the best possible fashion to encourage native fish populations to again flourish.

The challenge ahead

“Australia is the second most arid continent in the world, bar Antarctica,” says CSIRO freshwater scientist Dr Rick Stoffels. “So fresh water is a hotly contested resource.”

Dr Danial Stratford from the modelling water ecosystems team adds, “There is significant investment in environmental water, so it is important to ensure the best possible outcome for each investment. And fish are one of the key assets in the Basin that people are concerned about.”

Stoffels and Stratford are two of the CSIRO scientists currently developing computer models based on the flow requirements of a variety of native species throughout their life cycle in the Basin, including fish.

Stoffels has the mammoth task coordinating data collection across the entire Basin on how fish respond to flows. The data will help build an understanding of how flow variability affects the ‘processes’ of critical fish populations.

“These are things like growth rate, spawning, movement, and survival, and how these interact to determine whether populations increase or decrease in the long run,” he explains.

Stoffels says agricultural and domestic water extractions for irrigation, weirs and dams are a worldwide biodiversity threat because they disrupt natural river flows. Stratford adds that it’s the small-to-medium flow peaks that are affected, and often lost, with Murray-Darling water regulations.

Why is flow important?

The waterways of the Murray-Darling have naturally variable flows, with both drought and flooding common. Each high and low flow serves important ecological functions, supporting native plants, animals and habitats. For example, a high velocity river flow can trigger fish migration and spawning. The flow carries fish larvae and eggs downstream, picking up nutrients along the way, and emptying into wetlands. It’s here, within the relatively calmer body of water, that the young have shelter and a food-rich habitat.

Flow can also affect water quality and other environmental conditions. Extremely low flows can result in high water temperatures, limit the availability of dissolved oxygen and elevate contaminant or salt concentrations to toxic levels. It’s at this stage that restoring some flow, through managed environmental water, is crucial for native fish.

Combining the research and applying the models

Existing fish ecology research informs a range of models that can now link the hydrology of environmental flows to species responses, analysing what supports fish life history and ecology.

“By linking the hydrological and ecological models this way we can understand the flow events that are important to fish,” explains Stratford.

“We can then also simulate possible flow scenarios for fish communities and understand the likely benefits of different management actions. This allows us to suggest the best use of environmental water to trigger spawning events for native fish, maximise their outcomes for recruitment, and keep the adults alive,” he says.

Chowilla Floodplain during peak drought in 2008, Renmark, South Australia. Image: CSIRO/Tanya Doody

The CSIRO models evaluate fish population outcomes against different management, water planning and climate scenarios, designed to consider multiple short- and long-term objectives within a complex and connected environmental system.

Models are being used to support the implementation of the Basin Plan, and can provide a consistent and unbiased framework for assessment of the relative benefits of different scenarios involving watering actions and volumes.

“We are building scientifically-defensible demonstrations that environmental water is being used effectively and efficiently,” says Stoffels.

Can the models deliver the best outcomes?

Stoffels points out that there isn’t luxury of controlled experiments.

“This isn’t a trivial matter and is actually a great challenge for freshwater scientists,” says Stoffels.

“We don’t have six Murray Rivers, very similar in every respect apart from the flows they experience, three of which receive a specific environmental flow, and three controls that don’t. We have a single Murray River to which water managers deliver a specific flow.

“So, how do we know what environmental water has done to the critical processes we wish to enhance? Without a control in place, our best form of knowledge is data and models.”

Flooding in 2011 in Yanga National Park, Balranald, New South Wales. Image: CSIRO/Tanya Doody

The flow data can be decomposed into two components: ‘environmental water’ and ‘non-environmental water’. The comparison of the two can infer the actual impact of the allocation versus the scenario without an environmental flow delivery.

Stratford adds, “Our models provide an adaptive learning process. When a flow event occurs we can anticipate a specific ecological outcome. This does not always occur as expected, but its does provide us with an opportunity to understand what did happen, and to improve our models.”

“As each year of monitoring passes and more quality data accrues, the models are updated, and uncertainty around how fishes respond to flows is reduced. In turn, as uncertainty is reduced, our capacity to manage riverine flows more effectively and efficiently increases.

“Providing the correct allocation of freshwater to the environment is crucial in order to increase the size of the native fish populations,” adds Stoffels. “We are always improving on how, when and where the most significant benefits can be achieved.”

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