Thousands of fish turned up dead in a western Mexico reservoir after a company that made food for livestock without a permit dumped huge amounts of molasses into the water, authorities said Monday.

The fish were deprived of oxygen after the company poured hundreds of liters of molasses into the canal that feeds into the Hurtado Reservoir in the state of Jalisco, the mayor of Acatlan de Juarez, Emeterio Corona, told AFP.

"The environment and the sustenance of everybody (in the community) was damaged. What will they live off? I'm outraged. This is a tragedy," he said on the banks of the reservoir as fishermen removed the dead fish.

Arturo Arceo, a member of a fishing cooperative in the community of San Pedro Valencia, said at least 500 tonnes of carp and bream were killed.

The mayor has requested government help for the 180 families that live off fishing in the reservoir.