We explore the science behind the recent toxic algae outbreak and the mass fish loss in the Murray Darling, and look at ways to manage it in the future.
A mass amount of dead fish washed up on a river bank.

CSIRO expert Dr Klaus Joehnk says that high temperatures and dry conditions associated with heatwaves will continue to create ideal conditions for blue-green algae. Credit: Facebook/Debbie Newitt/ABC.


Have you seen the distressing videos and images of mass fish deaths in the news over the new year? In the past month, it has been estimated up to a million fish have died along a 40-kilometre stretch of the Darling River in far west New South Wales.

Blue-green algae has been identified as the primary cause and the dead fish have included native species such as bony bream, Murray cod, and golden and silver perch.

In addition to the distressing loss of fish, news articles have advised people not to swim or drink contaminated water in ‘red alert’ algae areas and farmers have been asked to find alternative water sources for livestock.

Hearing from an ecosystem expert

We sat down with our blue-green algae and freshwater ecosystem expert Dr Klaus Joehnk to discuss the science behind the recent toxic algae outbreak and the mass loss of fish.

A middle aged man wearing a red jacket, standing outside surrounded by gum trees.

According to Klaus, the severe blue-green algae outbreak resulted from a combination of factors, including drought and heatwave conditions, and no water flow.

Why do we get blue-green algae outbreaks?

Every year in the summer months Australian waterways regularly experience the occurrence of blue-green algae (cyanobacteria) outbreaks. Warm, slow moving or stagnant water, high solar irradiance (direct sunlight or lack of clouds) plus plenty of nutrients are the perfect conditions for algae to grow. As these algae start dying there is a rapid decline in dissolved oxygen in the water due to decomposition to levels that can lead to fish deaths. Deaths can be extensive when fish cannot move to safe spots or artificially created refuges.

Why was the outbreak so severe in the Murray Darling Basin?

It is likely the fish kill was a combination of factors:

  1. Drought conditions leading to stagnant water in the region which has received less rain than ever before in some parts, (see Bureau of Meteorology rainfall deficiency map here)
  2. Development of an extensive and concentrated blue-green algae bloom due to heatwave conditions, no water flow, and nutrient concentrations in the water due to a range of human-derived and natural inputs.
  3. The sudden passing of a cold front, leading to water mixing bringing up anoxic (deoxygenated) water.
  4. With the water column (from river surface to riverbed) experiencing low oxygen levels, fish had no refuge to swim to, and thus died.

What’s next?

Can the Murray Darling Basin expect more blue-green algae as summer continues?

Unfortunately yes. High temperatures and dry conditions that come with heatwaves also mean an increase in water temperature and continued ideal conditions for blue-green algae. The decaying dead fish are also contributing to lower water quality and less oxygen in the water.

Menindee from above: a satellite image from the day before the disaster, 4th January 2019. Generated with Google Earth Engine using free Sentinel 2 satellite data.

What are the options for management and control of blue-green algae?

Our water scientists have a long-standing active program building up an understanding of the complex chain of events that leads to an algal bloom, and the aftermath of toxins released into the water.


  • We are developing short-term forecasting capability to predict cyanobacteria bloom development on a short, seven-day term. This is based on remote sensing and models.
  • Fast and cost-effective assessments of water quality, such as on-ground and satellite remote sensing approaches, as well as more continuous monitoring systems can be used to assess the conditions of our inland water. This can identify and predict potential changes in water quality in response to changes due to outside influences, such as land use changes, flooding, fires, and climate. We have already been investing in the development of these monitoring systems in some parts of the basin.
  • We are also working on physical, biological and nutrient controls to manage the algae.
  • We are continuing to work with state and federal agencies, including with the Murray-Darling Basin Authority (MDBA), to support the MDBA’s modelling and forecasting work in relation to water management and planning.


  1. This is a devastating incident and I heard that the fish were being scooped out and buried. As a former marine biologist, can you confirm that government agencies are using this great opportunity for NSW Fisheries, CSIRO, Citizen Scientists and other biologists to work together to identify, study samples of all species that have died to gather data. This provides large samples that gives scientists a chance to compile data that they would not normally have access to. Let’s not waste this and ensure that we learn from it.

  2. Hi all. Two key factors we need to remember:
    1- in drought conditions and low flow groundwater and subsurface flows to rivers include more salts. Salts floc out clay particles therefore increasing light penetration. Ideal condition 1 for algal blooms

    2- alas while we pushed hard for sustainability most farmers are no longer soil testing. That is matching nutrients to plant requirements. This adds further to the historical nutrient stores in our river sediments of day the last 50 years of fertiliser excesses. So lots of nutrients enhancing opportunity for blooms.

    Afraid right across Australia’s rivers estuaries and nearshore ocean environments we are building substantial nutrient loads that will ensure Long term problems.

    Think our modern life – still phosphate based detergents cross connections between storm water and waste streams and less than optimum sewage treatment plants. Add that to mismatch between fertilisers applies and plant needs and the problems will continue

    Happy to provide more details as needed
    Colin Creighton AM

  3. Would it be possible to install a wind-driven pump (like the ones used on farms to bring bore water to the surface) at trouble spots along rivers to pump water from the bottom of a river and then drop it from a height so as to oxygenate the water? Would this help?

  4. Much as I admire and appreciate the scientific approach, they are really only telling us the obvious – predicting an algal bloom a short while before the event does not give much time to prevent it if there are no dam gates to open. Reality is that the management of the water flowing through the Darling River and associated catchments has been seriously mismanaged by the MDBA in response to a host of conflicting commercial and political demands and much of the water that could have prevented this occurrence has been used to meet the Green ideals of flushing fresh water out to sea. They have all forgotten to look after the environment in their own back yard.

  5. I hope that the “physical, biological and nutrient controls to manage the algae” include an evaluation of vegetated floating islands. Islands with correct designs hugely increase the surface area of biofilm in contact with the water column, denitrifying nitrates so that gaseous nitrogen is returned to the atmosphere and taking up phosphates to at least temporarily reducing their biological availability. Floating islands also shade the waterbody, reducing water temperatures, and they reduce the amount of sunlight entering the waterbody, reducing algal photosynthesis. See http://www.fiatechnology.com.au (I declare a financial interest in this company) or websites by some of the other companies that make and supply vegetated floating islands.

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