1 September 2014 Last updated at 10:40
Hundreds of thousands of fish have been washed up on the shores of Lake Cajititlan in the Mexican state of Jalisco over the past week.
Almost 50 tonnes of dead popoche chub freshwater fish have been removed from the lake.
The local authorities said it was part of a "natural cycle" but state officials said it was due to the lake's "poor management".
Jalisco's secretary for the environment, Maria Magdalena Ruiz Mejia, denied "categorically that this is a natural and cyclical phenomenon".
"We have no evidence to support that it is natural and cyclical, to the contrary, we have a series of variables which lead us to believe this phenomenon is not only recurrent and becoming more frequent and severe, but also that it is caused by the poor management of the body of water," she said.
Ms Ruiz Mejia said mud from local wastewater treatment plants could be to blame for the mortality.