(AGI) - Hanoi, Oct 3 - Tonnes of dead fish washed up in Hanoi's largest freshwater lake over the weekend, prompting the city's mayor to call it an unprecedented mass fish death, and raising questions about water pollution in the Vietnamese capital. Local witnesses said they started seeing dead fish last Friday in Hanoi's West Lake - one of the city's popular scenic spots - with the situation worsening over the weekend, according to local newspapers. More than a thousand people, including the military, have been deployed in clean-up operations. About 60 tonnes of rotting fish has been collected so far. "This is the first time an incident of this kind is happening in Hanoi's West Lake" said Nguyen Duc Chung, Chairman of Hanoi People's Committee to local media. According to the lake's management board, an estimated 4,000 cubic metres of untreated wastewater is discharged into the lake every day from homes and businesses. According to VnExpress International newspaper, authorities said that some of the dead fish have been sent for testing to determine the cause of death. Vietnam is still reeling from an industrial incident that wiped out the seafood industry on its central coast earlier this year, linked to Taiwan's Formosa Steel. Thousands protested on Sunday against a steel plant run by one of the company's units, Formosa Ha Tinh Steel, which has admitted that its US$10.6 billion steel plant was responsible for massive fish deaths along a 200-kilometre stretch of coastline in this Southeast Asia country in April. (AGI) .