In January this year, over a million fish were found dead in the Darling River. At the time of the incident, arguments raged over whether the mass mortality was due to mismanagement of the Murray Darling Basin’s water, general lack of water thanks to the drought, or a symptom of wider climate change.  

At the time, ABC Science reported that the those with expertise believed far too much water was being pulled from the river and basin system for use in irrigation, “[b]ut New South Wales Water Minister Niall Blair says drought is to blame.”  

While these factors certainly could have contributed, it was eventually concluded that the fish had drowned due to a lack of oxygen in the water.   

To give an idea of exactly how bad this recent crisis was, Menindee local, Graeme McCrabb, told The Guardian that dead fish were floating to the surface in the tens of thousands and those that had somehow survived were “just gasping for breath”. 

While it may seem absurd for fish to be hunting air, this experience can be more easily understood in the context of a smaller waterbody: If you have a lake, dam, or even a backyard pond, which houses pet fish, you may have seen them up near the surface of a morning. While many owners attribute this to hunger, the fish are most likely seeking oxygen. This is because during the day, any plants living in an aquatic ecosystem will assist with aeration, but overnight, this reverses and aquatic plants actually consume oxygen from the water, meaning that there is far less for fish and other aquatic fauna to consume. 

In the lead up to the event, which is viewed as one of the largest mass deaths in the history of Australian fish kills, what little water there was left in the system had stagnated. As we know, algae loves still water and hot climates, and the resulting outbreak was the beginning of the end for Menindee’s fish population. While the initial stage of the incident has been blamed on an extreme bloom of blue-green algae, the more specific cause was a collection of various types of cyanobacteria that thrived in the warm, stagnant conditions.  

Much research was done into why this occurred and how it led to the Darling becoming a watery grave for so many fish, but it was the CSIRO ECOS that summed it up best: “On occasion, conditions are just right and provide a sure-fire way to help cyanobacteria numbers explode. A drop in water levels brings the sunlit surface and nutrient-rich depths close together, while also making for lazy currents that warm in the summer heat … That’s not what decimated the Darling River’s fish numbers, though. It was the death of these cyanobacteria that triggered devastation for other species.”  

Once the area was almost completely overrun by the bloom, a cold front hit and the algae ceased to thrive. When cyanobacteria lose their ideal conditions, their population will drop substantially in a short period of time. This is great if you’re looking to clean up a private water body through the use of aeration rather than manual labour or chemical treatments; but not so great for the other living organisms in places where cold algae means a severe drop in oxygen levels in the water.  

In private waters, cyanobacteria is addressed with both short and long-term methods. Short term methods include algaecides such as Cupricide or more environmentally conscious water treatments such as Algae Lift and Biostim Accelerator. This will not immediately fix your issue however, as when algae die off, it leaves behind a mass of rotting organic material that removes oxygen from the water. Because of this, no matter what removal method is used, the process should be followed up with aeration and biological augmentation. Stimulating species of bacteria known for their nutrient reduction ability, then utilising an aeration system to give these aerobic species of bacteria the oxygen they need to thrive, will help to boost and maintain oxygen levels, prevent fish kills and dampen any future outbreaks of algae by controlling the availability of nutrients which they need to grow. 

In natural waters, however, the death of an algae bloom means excessive organic waste left behind which strips the water of oxygen. Without any intervention to increase oxygen levels and stimulate beneficial bacteria, the ecosystem then crashes, and a fish kill is inevitable. 

In other words, the short answer to what killed all those fish is an imbalance in bacteria which led to greatly depleted oxygen levels and unliveable conditions.  

Moving forward, far closer scrutiny will be placed on the management and quality control of the Murray-Darling River and Basin system. Aerators have been installed at Lake Keepit and Lake Burrendong to replace lost oxygen and rehabilitate the rivers’ ecosystems but a more complete solution is necessary in order to maintain balance going forward.  

This is why the government’s pledge to provide $5 million to build a hybrid fish hatchery/incident response centre is such a good thing, not only for Menindee, but for the entire ecosystem. 

We’re particularly excited about this as our team is dedicated to providing high level service and effective treatments in an ecologically friendly manner. We were deeply saddened to hear of the mass kill back in January so we’re glad that positive steps are being made to restore health and balance to the river system.  

It should also be noted, however, that this will never be completely attainable, within any aquatic ecosystem, if there is not sufficient water. In fact, maintaining proper water levels could have prevented this disaster, as the fish kill was technically a symptom, not actually the underlying issue. 

Greater importance must be placed on keeping sufficient amounts of water flowing through the system in future, otherwise the Murray-Darling Basin and connecting rivers will never be able to flourish and return to their former glory.