Dugongs (Dugong dugon) suffered a significant population decline in China during the twentieth century, and their regional status is unknown. Only 5% of 788 respondents reported previous dugong sightings, with a mean last-sighting date of 23 years ago, and only three reported sightings within the last 5 years. Historical records of dugongs peak around 1960 and then rapidly decline from 1975 onwards; no records have been documented after 2008, and no verified field observations have been made since 2000. This rapid documented population collapse also serves as a sobering reminder that extinctions can occur prior to the development of effective conservation measures.
The dugong is a type of marine mammal. It is one of four living species in the Sirenia order, which also includes three manatee species. It is the sole survivor of the once-diverse Dugongidae family; its closest modern relative, Steller’s sea cow (Hydrodamalis gigas), was hunted to extinction in the 18th century.
The dugong is the only sirenian species in its range, which includes the waters of 40 countries and territories in the Indo-West Pacific. Because the dugong relies heavily on seagrass communities for survival, it is restricted to coastal habitats that support seagrass meadows, with the highest dugong concentrations typically occurring in wide, shallow, protected areas such as bays, mangrove channels, the waters of large inshore islands, and inter-reefal waters.
The dugong’s body is large and cylindrical, tapering at both ends. It has thick, smooth skin that is pale cream at birth but darkens dorsally and laterally to brownish-to-dark grey as it ages. The growth of algae on the skin of a dugong can cause its colour to change. The body is sparsely covered in short hair, which is common among sirenians and may allow for tactile interpretation of their surroundings. These hairs are most prominent around the mouth, where a large horseshoe-shaped upper lip forms a highly mobile muzzle. The dugong’s muscular upper lip aids in foraging.
Despite being legally protected in many countries, the main causes of population decline continue to be anthropogenic, and these include hunting, habitat degradation, and fishing-related fatalities. Many people have died as a result of becoming entangled in fishing nets, though exact statistics are unavailable. The majority of issues with industrial fishing occur in deeper waters with low dugong populations, with local fishing being the main risk in shallower waters. Because dugongs cannot stay underwater for long periods of time, they are particularly vulnerable to entanglement. Shark nets have historically resulted in a large number of deaths, they have been banned in most areas and replaced with baited hooks. Hunting has historically been a problem as well, though they are no longer hunted in most areas.
If dugongs do not get enough food, they may calve later and have fewer offspring. Food scarcity can be caused by a variety of factors, including habitat loss, seagrass death and decline, and human-caused feeding disruption. Sewage, detergents, heavy metals, hypersaline water, herbicides, and other waste all harm seagrass meadows. Mining, trawling, dredging, land reclamation, and boat propeller scarring all contribute to an increase in sedimentation, which suffocates seagrass and prevents light from reaching it. This is the most serious problem affecting seagrass, the food of the dugong.