Sorry Fusc,
I have to say that the pressure of indigenous hunting of dugongs is the largest cause of mortality for the species.
It is estimated that >1000 individuals are killed per year in the Torres Strait alone, and are also hunted in remote coastal areas in Qld, North of Broome and the NT. An ecologically sustainable harvest is known to be around 100-200 individuals per year.
Dugongs are part of the Torres Strait and Aboriginal culture, they are of enormous cultural, spiritual and economic (substinence) importance to indigenous people.
It is a complicated social issue re: managment, but through education and cooperative management agreemens, traditional hunting can be managed sustainably by Australian indigenous communities.