The body of the male dugong, more than 40 years old and weighing as much as 350 kilos before its head was removed, was found floating between Ko Yao Yai and Ko Yao Noi islands.
''There are probably only about 10 of this endangered species still in the Andaman zone,'' said Dr Kongkiat Kittwattana, head of the Rare Marine Species, Marine Division, at the Phuket Marine Biological Centre on Cape Panwa.
A team at the centre on Phuket's east coast was examining the dugong's remains today.
Their conclusion is that marks on the body indicated the animal may have been pulled on board a trawler or some other boat where it was probably beheaded.
''The teeth of this remarkable animal unfortunately are believed to have some protective properties and are a valuable commodity for amulets,'' Dr Kongkiat said.
Sea grass beds that are the dugongs' food have been polluted by coastal resort construction and the animals have been put at risk by an increasing number of tourist boats and their propellors.
The animal found floating yesterday was probably two metres long and aged at least 40, Dr Kongkiat said.
The skeleton of the creature is likely to be kept for future education.
A dead male dolphin pulled from the sea off Rawai and measuring close to two metres in length will have an autopsy performed on Friday at the marine biological centre.
Dr Kongkiat said it was usually the case that bottlenose dolphins died from illness while dugongs usually were killed by fishermen.