Dead fish in Markham river

THE Morobe Mining Joint Venture (MMJV) suggested a depletion of oxygen in the Markham River, so I ask that this be explained in simple terms.
Did MMJV take samples from the river and other tributaries, or collect tissue samples to determine the actual cause of death?
Oxygen depletion may affect smaller-sized aqua­tic life species but based on the pictures in papers, they were bigger tough species.
Oxygen depletion often occurs in warmer months and mostly cause partial fish kills but total kills are relatively rare in recreational ponds unless there is an extremely high fish population.
I am not pointing fingers, but there should be a scientific response with proven analysed data to identify the source of this aquatic genocide.

Aiposa  Agopeha
Goroka