The RSPCA is investigating eight unexplained swan deaths in the harbourside of Bristol.
One of the swans is recovering at a sanctuary in Somerset
The fatalities occurred between 18 September and 1 October between Bristol Bridge and Redcliffe.
Of the eight, four were taken to the RSPCA in the city and another four taken to the swan sanctuary in Swindon. A ninth is recovering at West Hatch.
The charity said it did not know what caused the deaths, but it could be a contaminant in the water.
Damien Pacini, of the RSPCA, said: "This is a bit of a mystery. We've taken blood samples but the results are very non-specific.
"It could have happened for a number of reasons, including lead poisoning from fishing tackle. Other causes could include water contamination.
"We also checked for botulism, which you would normally associate with summer and low water levels when the birds dig deeper into the bottom of the docks."