About 100 bags of dead fish were collected from the Shing Mun River at Sha Tin yesterday.
Experts says they might have died in the cold weather last week and have just surfaced.
But a report on a river’s water sample last year showed the pH level was 2.9 times higher than normal.
The dead fish piled up along 800 meters of the river bank from residential site Man Lai Court to the Hong Kong Heritage Museum.
It is the third time dead fish have shown up in the river. Residents are annoyed by the smell and have complained to their district councillor.
The Sha Tin district councillor for Fo Tan, Scarlett Pong Oi-lan, said she has received eight complaints so far.
“We have seen a lot of dead fish outside Sha Tin Jockey Club Swimming Pool, Central Park and the public housing estate Jat Min Chuen,” she said. “Some of the big fish’s heads have already come off.”