Jalisco state authorities said that the death of 3.2 million fish in western Mexico last week was caused by poorly functioning wastewater treatment plants that failed to filter out untreated material.
An analysis of water samples confirmed that at least 82 tonnes of fish turned up dead because they lacked oxygen due to excessive organic waste in the water.
Fresh water fish locally known as "popocha" began to float up in the Cajititlan lagoon last week. This was the fourth such case at the same lagoon this year.
Jalisco state environment secretary Magdalena Ruiz Mejia said that such deaths were more and more frequent due to “bad management of the body of water.”
Some 500 tonnes of fish died in a Jalisco lagoon during July 2013 after a company that made food for livestock dumped huge amounts of molasses into the water.
See more images here.