Dugongs have a slow reproductive rate. A female will raise only one calf every three to five years. Any threats to dugong can endanger the population. Experts consider that the decline in dugong numbers is due to unsustainable mortality from human-related causes such as habitat loss, commercial mesh nets (fishing nets), shark nets set for bather protection, and hunting. Hunting appears to have been sustainable in earlier years at levels higher than today. The additional recent causes of dugong decline have now made hunting unsustainable. Dugongs have become entangled and drowned in certain types of mesh nets in the Great Barrier Reef World Heritage Area. Dugongs can only hold their breath for a maximum of eight minutes, and will drown quickly once entangled in a net. These nets are those often described as mesh nets (floating or set mesh nets) and are used by some commercial fishers. Shark nets (nets set to catch sharks near bathing areas) have been responsible for the death of 541 dugongs in the Great Barrier Reef since the early 1960s Dugongs depend on access to healthy seagrass meadows to survive, as do commercial and recreational fishers, as seagrass beds are also important habitats for prawns and many commercial fish species. The dumping of dredge wastes and the discharge of silt from coastal rivers reduces the amount of light available to seagrass communities, thereby limiting their growth. The clearing of land and a variety of other agricultural activities in river catchments can increase the amount of silt washing into the sea, particularly after heavy rains. Trawling by commercial fishing vessels and pollution from urban run-off or ships can also damage seagrass meadows. The actual or potential loss of the dugong's seagrass feeding habitat is potentially the most significant issue for the long-term survival prospects of dugongs in the southern Great Barrier Reef. Dugongs are important in maintaining the health of seagrass meadows. Seagrass loss was considered a major cause of death of dugongs in Hervey Bay in 1992 following a flood. Boating and shipping also create hazards for dugongs. Dugongs may be killed or injured when struck by any part of a vessel and may also be scared away from their feeding areas by vessel traffic. In some circumstances, stress is suspected of contributing to illness in dugongs. Dugong deaths occur in unknown but probably significant numbers from illegal fish netting and illegal hunting. Illegal activities which degrade dugong habitat can also have an indirect effect on their survival.
Dugong numbers have declined significantly since 1986-87 in the southern region of the Great Barrier Reef World Heritage Area and the species is facing the threat of disappearing from the area. The decline was discovered through aerial surveys, which covered 39 000 square kilometres of the inshore waters of the southern Marine Park, in 1986-87, 1992 and 1994, and have detected a population decline from an estimated 3480 animals (+/- 460) to 1682 (+/- 240) within eight years. It has been estimated that the dugong population can only cope with loss from human-caused mortality of less than 1-2% per year.
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