Rio Miranda fish deaths caused by natural decoada phenomenon

A natural phenomenon known as ‘decoada’ has caused the death of various fish species in the Rio Miranda, located in the Pantanal region of Mato Grosso do Sul. The phenomenon occurs during floods when rising river levels submerge vegetation, such as grasses and water hyacinths. As this organic matter decomposes, it consumes the dissolved oxygen in the water, leading to fish mortality.

On October 16, 2026, dead specimens including pintado, peixe-palmito, cachara, and dourado were observed floating in the river. The Rio Miranda reached a level of 6.83 meters, surpassing the alert threshold of six meters and approaching the emergency level of seven meters. The State Institute of the Environment (Imasul) is monitoring river levels across the state due to high rainfall and the ongoing risk of flooding.