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. could dams designated specifically for environmental management be used to release water at appropriate times to avoid these situations and improve the health of the rivers in general? Problem as always would be funding!

  2. Replying to Peter Pearce, from memory a chap by the name of Yeomans taught contour plowing on properties back in the 1950s.
    The contours collected run off from hills and directed water into earthen dams on the property rather than have it flow into creeks, rivers and the sea

  3. J.A. Bradfield the Queensland engineer who built the Sydney Harbour Bridge suggested back in the 1940s that every east coast river be dammed and excess water from the three or more cyclones we receive each year be piped through the Great Dividing Range to catchments west of the Blue Mountains instead of simply allowing this precious commodity, Fresh Water to flow over the spillway, down stream causing coastal flooding, hardship and deaths before carrying mud, farming pesticides and fertilisers out on to the Great Barrier Reef causing algal bloom, an explosion in Crown of Thorn Starfish and coral bleaching.
    He was howled down by ‘self elected experts’,so nothing was done to Drought Proof Australia, that chicken has now come home to roost.
    Rather than bury them underground, Tunnel Boring machines that have completed road traffic tunnels should be dismantled and transported into the upper reaches of the pent up water behind dam walls so they can be reassembled and bore tunnels through the sandstone strata to transport excess water west of the mountains rather than have it flow over the spillway.
    Our farmers cannot feed the world without a reliable supply of cheap fresh water.
    Lets Drought Proof Australia can you imagine how many jobs that would create ?
    That $84 Billion that Malcolm Turnbull suggested gifting to companies last year would go a long way to funding this Snowy River like scheme.

  4. It would be nice to see a satellite of the water catchment held by the MDBA if the drought has effected their levels as a comparison of before and now, as Dr Klaus said water flow. Might be time to stop taking water and let it flow as before the MDBA took control.
    We should be working for the health of all waterways first before commercial interest. I just did a trip to the Victorian high country and it was quite obvious that the heavy rain filled all the dams on properties but the pastures were still dry as no follow up rain has come. When you look at all these dams you notice the farmers use the contours of the land very effectively and not attached to creeks or waterways. To my knowledge along time ago scientist and scholars worked out where to put weirs along the Darling river for the health and benefit of the river system and the local communities along it’s length, we might need to revisit the history of the system before we knew best as to limit the flow of water as this country has always had extreme conditions plenty of droughts and floods, did we get these algae blooms than or was there enough water held by the weir systems. At the end of the day we are responsible to come together and work out how to fix this or lose it. As the system at the moment works for one sector, I’m sorry everyone saying drought and no rain is the problem is not the only answer is bit like can’t see the tree because of the forest.

  5. I’d like to ask a question: to what extent can future plans to address blue-green algae be effective without improving the water flow of the river? I’m assuming that flow would improve if irrigators licences were revoked in part or totally.

    1. Hi Megan,

      Thank you for your question.
      There are a number of factors influencing cyanobacteria growth including water flow, hot weather (temperature), cloud free conditions (‘irradiance’) and nutrient concentrations. Some of these can be controlled or influenced by humans and some cannot. We know that blue-green algal growth can be suppressed when increasing water flow above a certain threshold; this knowledge is included in current management plans. It works well under “normal” conditions but under drought conditions this might be hard to achieve, especially when the river separates into stagnant pools. Management decisions, including those about licensing, are made by regulators such as NSW Department of Primary Industries, or the Murray Darling Basin Authority.

      Thanks again for your interest,
      CSIRO Social Media Team.

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