A rare camera trap image of an otter active during the day. Imaget: Sharne McMillan

The distribution, diet and demographics of Hong Kong’s otters

9 August 2023

A team of researchers from the University of Hong Kong embarked on a comprehensive two-year study of the local otter population 

Hong Kong has 6,640 hectares of wetlands including the protected Mai Po Inner Deep Bay Ramsar Site. Hong Kong’s subtropical climate and the geographical position of its wetlands make it a vital site for tens of thousands of migrating birds. See the special feature Croucher News published in 2021 for more information on Hong Kong’s wetlands.

The Mai Po site contains Hong Kong largest remaining stand of mangrove and, in addition to hosting migrating birds, provide an important ecosystem for reptiles, amphibians, insects, and aquatic species. Perhaps less well known, Mai Po hosts a population of Eurasian otters.

Until recently there has been very limited scientific information about the otters of Hong Kong. To improve our understanding, and provide a basis for future conservation efforts, researchers at the University of Hong Kong conducted a two-year study of the distribution, diet, population size and demographics of local otters. Their findings were published in Conservation Science and Practice.

To map otter distribution, Professor Timothy Bonebrake and Sharne McMillan of the Department of Biology at the University of Hong Kong and their colleagues surveyed potential habitats in Hong Kong for droppings and prints. They recorded the geographic locations of their findings and installed camera traps. The team also interviewed local fish farmers.

Some droppings were collected for dietary analysis, with bones, scales and other hard remains of prey being extracted and identified to the lowest possible taxonomic level. Meanwhile, droppings that were fresh enough to preserve DNA were used for genetic analyses to infer population size and demographics. In total, 45 droppings were used for this purpose.

“You can use the DNA to confirm the species that the spraint has been collected from, and it can also tell you about the individual which deposited it”, said McMillan. “We were using that information to try and work out the number of individuals that we could identify.”

The results confirmed that the otters used a wide range of habitats including the protected areas of Mai Po Marshes and Inner Deep Bay and adjacent shrimp ponds and fish farms, with the fish farms providing the most common source of food: tilapia and grey mullet. In the Mai Po Nature Reserve, camera traps recorded mother otters with cubs.

Camera trap image of a mother otter with two cubs in Mai Po Nature Reserve. Image: Sharne McMillan, HKU

“Otters need large, connected habitats”, said Bonebrake. “We want to ensure that there is no disturbance, particularly for such a small population we want to see grow.”