The birds were found poisoned in the wetlands of Dongting Lake on Sunday afternoon, Xinhua reported.
Fifteen of 17 swans that were poisoned died and 31 sarcelles were killed.
The two swans that survived the incident and are now listed under China's second-level of state animal protection, said Gao Dali, chief of the security at the East Dongting Lake National Nature Reserve.
Police are investigating the incident.
Meanwhile, a patrol team of experts and volunteers has been set up to prevent similar incidents.