At least 48 tonnes of fish have turned up dead in a lagoon in western Mexico and authorities are investigating whether a wastewater treatment plant is to blame.

Officials in the state of Jalisco said late Sunday it was the fourth case of mass deaths at the Cajititlan lagoon this year in the town of Tlajomulco, south of Guadalajara.

Magdalena Ruiz Mejia, the state's environment secretary, said it was a "grave phenomenon" and that it would take two more days to remove the fish, which began to appear dead last week.

The Tlajomulco municipality said the deaths were due to a drop in oxygen due to a cyclical change in water temperature.

But Ruiz Mejia said such deaths were "more and more" frequent and intense due to "bad management of the body of water." Authorities are checking local wastewater treatment plants.

In a separate incident in July 2013, thousands of fish died in a Jalisco reservoir after a company that made food for livestock without a permit dumped huge amounts of molasses into the water