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

On Monday, fishermen used shovels, wheel-carts and trucks to pull tonnes of dead fish out of the Cajititlan lagoon that has been the scene of four fish kills this year.

Authorities were investigating whether negligence at wastewater treatment plants was to blame after millions of fresh water fish locally known as "popocha" began to float up last week.

"We don't want this problem to worsen because we would end up in the street," said Rigoberto Diaz, a local fisherman who fears that other species such as tilapia, which unlike popocha is edible, will die too.

Fellow fisherman Mauro Hernan echoed concerns that authorities have yet to confirm the cause of the die-off.

"We were told that the state government will support us. We don't know when we will be able to fish again," Hernan told the AFP news agency

Jalisco state environment secretary Magdalena Ruiz said it was the fourth unexplained fish kill at the same lagoon this year.

"You can't deny that there's a contamination" due to suspected negligence at wastewater treatment plants, she was quoted by AFP as saying.

Authorities are conducting tests on the dead fish while state environmental prosecutors are investigating local wastewater treatment plants.

In a separate incident in July 2013, some 500 tonnes 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.