THE recent fish kills at Menindee on the lower Darling River in March 2023 had me gazing back over the various columns I’ve written for Fishing World over the last 28 years. An acceleration of reporting of fish kill events has become increasingly apparent, especially so over the last 10 years, and particularly for the inland rivers of the Murray-Darling system. Prior to 2000 little attention was paid in this column to fish kills, except perhaps for passing mention of the world’s largest marine fish kills involving pilchards across southern Australia in 1995 and 1998.
Those ones were a bit different though, as they were due to incursions of an exotic herpesvirus, probably bought into Australia in frozen pilchards imported for tuna feed. The virus infected our local pilchard’s gills and prevented them from accessing oxygen in the water. This was a major biosecurity incident, but was not an environmentally driven kill.
My files on fish kills in the noughties until 2010 revolved mainly around isolated fish kill incidents associated with industrial developments small and large, as well as issues with intermittently closed estuaries in NSW (usually when they were thoughtlessly opened/drained by the public or councils). From 2010 onwards the La-Nina (wet) period bought us some moderate blackwater flush events in the Murray Darling in 2011. This same wet period was also used by industry proponents to try to obfuscate the actual cause of fish kill/disease incidents in Gladstone Harbour in 2011/12, which were related to dredging activities.
The switch to El-Nino (dry) from around 2013-14 provided a change for a few years with marine heatwaves in South Australia and WA causing some issues. The strong El-Nino in 2015/16 drove us again onto the subject of drought associated kills in the Murray-Darling, where environmental flows in many parts of the system had ceased entirely due to drought and overallocation of water. Australia, a land of drought and flooding rains. Fish naturally do die when water is not available. But when most of the available water is used for irrigation, and most of your fish are disappearing into irrigation pumps (thankfully OzFish have shown the way here implementing major advances in pump screening), the “naturalness” of these drought associated kills can be seriously questioned. Especially so for kills associated with blue-green algal blooms which are due to nutrient loading from human activities.
However, in the past five years things have gone into overdrive in the fish kill space, and the difference from previous decades is that few, if any, of the recent incidents can be called “natural”. The “dry problems” climaxed in late 2019 with intense bushfires which caused major fish kill events in affected South-East Australian rivers, especially when they were filled with ash following drought breaking rains. These fire related fish kills appeared unprecedented, certainly in terms of the land area burnt (7.2 million hectares) and damage inflicted on property and bushland ecosystems in south-eastern Australia. Indeed, many scientists were quick to point to anthropogenic climate forcing as a driver for this increased fire intensity (though lack of appropriate fire management/burning in some National Parks didn’t help). Once the rains returned, we then saw massive “blackwater” events in many areas of the Murray-Darling which killed off entire fish populations over many kilometres of river and adjacent billabongs, wetlands and dams. This was due to organic matter and nutrients flushed from modified catchments and floodplains directly into the river, increasing biological oxygen demand and causing algal blooms which dragged oxygen down near zero in many places. The rains had returned and still our precious native fish were dying in droves.
In the Murray-Darling these days fish kills seem to occur due to low oxygen regardless of how much water is in the system. Politicians, agriculture industry representatives and various other so called “experts” when questioned in the media are usually quick to explain away these sorts of low oxygen kill events as “natural”. Of course, the truth is something else entirely. As explained in my column back in June 2020, when waterways like the Murray-Darling are so heavily modified by water abstraction, river regulation and agriculture in adjacent catchments, they are prone to low/no oxygen events. These human induced modifications turn thriving, functioning river systems into mere agricultural drains.
“Last resort” interventions like installing aerators to try to keep some fish alive are a huge neon warning signs of a human caused fish kill. Of course, water without oxygen is fine for watering crops, and can even be drunk by cattle and other livestock if it doesn’t contain toxic algae. You can even drink it if it is appropriately treated beforehand. But a living river system requires much better water quality than that. In order to support an aquatic ecosystem that ends with (hopefully native) fish at the top of the food chain, sufficient oxygen is not negotiable, it just has to be there all the time, together with many other things.
The most recent fish kills at Menindee in March 2023 again have human fingerprints all over them. After a couple of years of water in the river, recruitment of some fish species had gone well, with populations of bony bream and carp quickest to recover. This is normal, because bonies are low in the food chain, carp are tolerant of poor water quality, while it takes more time for populations of their longer lived predators (Murray cod, yellowbelly) to respond to improved environmental conditions. Other natives like silver perch require specific conditions which are hard to find in rivers which resemble drains. But the good times in the Darling at Menindee these days are all too brief. Eyewitnesses described the scale of the 2023 kill at Menindee as “unfathomable” as millions of bony bream went belly up and died, turning the river white for kilometres. The Guardian reported that apparently the incident was worse than the kills experienced in the same place in 2019 “after a rapid drop in temperature led to an algal bloom de-oxygenating the river”. But of course, healthy rivers don’t just develop a “lack of oxygen”.
In this case, it appears that river regulation “turned off the tap” too quickly. Then, as best described by aquatic veterinarian Matt Landos: “Runoff pollutes the river with synthetic fertiliser and adds a range of chemicals like glyphosate that favour blue green algal blooms. These blooms then collapse and fall dead to the river bed. Driving sediments to become more anoxic and creating bacterial proliferation that consumes the oxygen.” As poignantly pointed out by Dr Landos, “Fish need more than to be wet to survive”. A stretch of river that once supported a diverse commercial and recreational fishery annually harvesting hundreds of tonnes of Murray cod, yellowbelly, silver perch could no longer even support the forage fish base that once underpinned that long lost fishery.
This message is slow to get through to the corridors of power, however. It appears the first reflex of “those in charge” remains to write off these fish kills as “natural events”. Around the same time as the incident at Menindee, the excavators were rolling in to clear piles of fish carcasses from the shores of Kangaroo Lake in Victoria after another kill event. The water regulator spokesperson denied “that fish kill events are becoming more common”, but “suspected recent floods were a contributing factor”. The spokesperson then gave the game away regarding what they thought was acceptable water quality by stating that “the water was safe to use for irrigation” and that the site will be cleaned up so that “Easter (holidays) won’t be affected”. These are all giveaway lines of misinformation from authorities responsible for managing and maintaining our waterways. In their world, if the water is OK for irrigating crops and (if you’re lucky) allows contact for swimming, we should all be happy. Local healthy river campaigners were not fooled, stating “Mass fish kills should not be expected or thought of as just part and parcel of what normally happens in the river”. When will decision makers get the message that we need to move beyond pretending fish kill rates in inland Australia are not accelerating, and admit their cause is not “natural”. Fish indeed need much, much more than to be wet to survive.
