Remaining pools will shrink further with at least six days in a row forecast to reach 41 degrees or more, including 44-45 degrees from Monday to Wednesday.
Richard Kingsford, director of the Centre for Ecosystem Science at the University of NSW, said he was surprised by the grim sights of mired animals near Menindee Lakes.
Also worrying will be if further wildlife loss succumb from heat exhaustion, he said.
"Unfortunately this is the sort of thing we do see during drought", he said.
"They are rotting, they're large corpses, they're bloated in the water".
Blair said this week he had met with residents in Menindee but bypassed the gathering on the town's main boat ramp on security advice. They are holding critically endangered Murray cod that died in the fish kill.
"It is an ecological catastrophe on an global scale".
The mass fish kill came to national attention on Tuesday when an emotional video by local farmers Dick Arnold and Rob McBride went viral, racking up millions of views.
Mr McBride said: "This is nothing to do with drought, this is a man-made disaster".
Ranchers and other residents say government water policies are the real cause.
The Board extends its sympathy to the drought affected property owners and townspeople along the Darling River that have been affected by the recent fish kills.
Taxpayers have stumped up more than $13 billion to restore the health of the basin.
Fish kills have been caused by a Low Dissolved Oxygen event associated with the rapid die off of an algal bloom along this section of the river.
"The scale of this disaster is extraordinary and unprecedented", Mr Daley said. "We can not be indifferent to the ecological impact and the effect on local residents who live along the river".
"This is the result of draining the Menindee Lakes twice in four years, killing the system", McBride, whose ranch is about 25 miles south of Menindee, says in the video.
Up to a million fish have died in an Australian river in "an ecological catastrophe on an worldwide scale".
Earlier on Thursday, Mr Blair told the Herald that he had asked his departments to conduct an urgent study into the fish deaths and the subsequent clean-up. He added the SDL project at Menindee, which aims to save 105 gigalitres a water a year, must proceed. Some of them were up to 100 years old, the men said. "You would blow up the [Murray-Darling Basin] Plan".
Ex-Greens MP Jeremy Buckingham has made himself an instant meme after almost spewing while holding a dead fish in a video that was meant to highlight the National Party's water policies.
Upriver the basin is about 20% full but in the lower Darling River levels are as low as 5%, a spokeswoman for the Murray Darling Basin Authority said.
The cotton industry says it is not to blame for the mass deaths of hundreds of thousands of fish in the Darling River and is exhausted of being a "whipping boy" for problems associated with the drought.
"Droughts would have contributed to the blue green algae outbreak", he said.