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. Hello,my name name is Nanda Yumnaa from 11 Senior High School of Semarang and i would say a some little things about this thread.In japan,there’s a case similiar to this that were called the Minamata Tragedy.Minamata tragedy is a dangerous consumtion of fish near the village of minamata,the fish was contaminated by mercury but the problem is not the fish dying,the human is.Because when the fish was contaminated by the mercury,it didn’t die.Instead when the humans eat the contaminated fish,they died.So my point is,can the green-blue algae produce some kind of residue just like the minamata tragedy that could contaminated the fish and could possibly killed the humans too?

  2. 4-3
    I am adivia khusnul aisha from 11 senior high school, Semarang, Indonesia. I’ve read this article and it has an excellent subject to discuss. We have to deal with the water problem quickly because it causes much like the dead fish that live in water habitats. Algae problems have also had to be urgently addressed because of the world’s dwindling water supply and the increasing need for water for earth’s inhabitants. Human should also learn more about the dangers of algae to the waters of the world.

  3. 4-15.
    I am Dewi Rahayuning, a 12th grade student of SMAN 11 Semarang, Indonesia.
    It is true that blue-green algae can cause death to ecosystems in the river, due to environmental conditions around the river that are being hit by heat waves that allow algae to grow quickly, this event is called blooming. This condition causes high levels of nitrogen in river water, thus blocking the presence of oxygen that should be utilized by fish. Coupled with this blooming event causes sunlight to cause sunlight to not penetrate into river water. So of course this can cause death in fish. In my opinion, the government should also help scientists to overcome this, maybe in terms of funding, so that if scientists need funds to make an algae growth detection tool can be facilitated, and there is a socialization from the local government about planting trees so that there is ground water the good one. That’s all, my opinion. Thank you for your attention.

  4. Would planting out site specific Australian Native trees, bushes, shrubs, reeds, groundcovers and grasses along all our Australian River banks, billabongs, wetlands and floodplains assist in reducing erosion. salinity, and the amount of nutrients from agriculture entering our freshwater systems; along with providing oxygen to the waterways, habitats for animals and the sequestration of CO2?

  5. I would like to ask if the Menindee Lake had not been drained of it’s water would the green blue algae have been so bad

What do you think?

We love hearing from you, but we have a few guidelines.