The head of the national commission for bird flu monitoring, Mate Brstilo, told reporters on Saturday morning that it was not yet known whether this was the lethal H5N1 strain which would be established on Monday.
The swan died on the location which is 10 kilometres away from the island of Ciovo where another dead swan infected with H5N1 virus was found last week.
Brstilo said that the commission would take all necessary measures to contain this contagious disease, and that the culling of domestic poultry in the vicinity of the natural park was also being considered.
Agriculture Minister Petar Cobankovic has made a decree with measures so as to prevent and contain the occurance of bird flu in the country.
Poultry must be kept indoors throughout Croatia.
The hunting of feathered game is also forbidden.
The minister's decree stipulates a ban on the export of live poultry from suspected areas. Vehicles transporting poultry should be cleaned with appropriate disinfectants.
The decree takes effect upon its publication in the Official Gazette.
Fines for failure to abide by the bans are between 2,000 to 20,000 kuna.
(1EUR = 7.4 kuna)