Action should be taken to control the threat of invasive alien species at Mai Po Nature Reserve and other natural habitats, environmentalists have warned.
Invasive plant and animal species are those found outside their natural range and posing a threat to native species or parts of the ecosystem.
One such animal, a snapping turtle from the United States, was found in the Mai Po marshes in 1997. Experts believe it was released there by a local person.
The turtle, which is about eight to 10 years old, is about 50 centimetres long and weighs more than nine kilograms.
Dr Bosco Chan Pui-lok, senior conservation officer at Kadoorie Farm and Botanic Garden, said although no snapping turtles had been found breeding in Hong Kong, they not only competed with, but also preyed on, local species.
Dr Chan said many other alien aquatic species such as freshwater snails and fish had also been found and they had established breeding colonies.
Although the impact of alien animals on the local natural environment is not well-understood, Dr Chan said circumstantial evidence and documented cases overseas suggested they were a major threat to biodiversity.