Thousands of lifeless carp were seen floating or washed up on the banks of the Euphrates river in Musayyib, Babil Province, Iraq.
The region's Agricultural suggested that overcrowding in fish farms and a rapid spread of bacterial disease caused the mass death.
On Friday, the Iraqi Health Ministry said that samples from the water and dead fish had been taken in Babylon province and that tests are being carried out to determine the exact cause of death.
Following the incident, health officials advised Iraqis to stop eating fish.
