Mangroves in Kei Ling Ha Lo Wai, Sai Kung, one of the last remaining habitats for them in Hong Kong. Image: Thomas Gomersall

The gradual return of Hong Kong’s mangroves

12 December 2023

Mangrove forests are highly valuable habitats, being important carbon sinks, sheltering coastlines from storms, and supporting over 1,500 species globally. Yet in Hong Kong, as elsewhere in their range, they have declined considerably due to urban development and other activities, with only 510 hectares of local mangroves remaining, mostly in Deep Bay, Sai Kung and Tolo Harbour.

“If we look at historical maps or photos, there used to be a lot of mangroves in Hong Kong, even in Victoria Harbour,”says Dr Felix Leung of the Institute of Environment, Energy and Sustainability at the Chinese University of Hong Kong. “But because of reclamation and water pollution, a lot of the mangroves have been destroyed.”

However, a 2022 study co-authored by Leung and published in Remote Sensing found that at least some mangroves here are making a comeback. Using remote sensing, the authors analysed historical aerial photos and satellite images of Deep Bay to map land use changes between 1924 and 2020. Despite significant losses to urban development and conversion to fishponds, mangroves on the Hong Kong side experienced a net increase in coverage, driven mainly by a seaward expansion from 250 to 400 hectares between 1987 and 2020.

One reason for this is because coastal reclamation in Shenzhen has altered water circulation in Deep Bay, causing excess sedimentation in Hong Kong and increasing the area of substrate for mangrove colonisation. Another important factor is the protection of Mai Po Nature Reserve, where expansion was particularly prominent, highlighting the importance of protected areas for these slow-growing trees.

Unfortunately, the seaward expansion of mangroves has simultaneously decreased the area of intertidal mudflats, an important habitat for migratory birds. According to a 2023 study also co-authored by Leung published in Science of the Total Environment this has contributed to a significant decline in wintering waterbird populations, from 90,986 individuals in 2008 to 48,157 in 2022. Thus, restoration efforts in Deep Bay should concentrate more on landward habitats where mangroves have been lost.

“We need mangrove restoration, but not at the expense of mudflats,” says Leung. “The only areas in Deep Bay that are suitable for mangrove restoration are the abandoned fishponds. The fish farmers have grown too old and their children do not want to be in the industry, so why not utilise those ponds to grow mangroves?”

Leung’s research reinforces the importance of WWF-Hong Kong’s mangrove management work at Mai Po. Each summer, mangrove seedlings are routinely cleared from 45 hectares of mudflat to maintain open habitat for birds. More recently, mangroves were planted in two intertidal gei wai (traditional brackish-water shrimp ponds) as part of WWF and HSBC’s joint Powering our Wetlands project. With Leung’s findings emphasising the need for more landward mangrove restoration, WWF hopes to convert fishponds (which currently aren’t brackish enough for mangroves) into gei wai to achieve this.

“It would be good to plant more mangroves inside of gei wai if fishponds can be changed to gei wai,” says Xianji Wen, Director of Mai Po Nature Reserve and Flyway Programme.

Mangrove restoration elsewhere in Hong Kong is also showing promise and offering valuable insights for future efforts. In Tai Tam Harbour for instance, a planting programme initiated by the Agriculture, Fisheries and Conservation Department in 2005 found that of the six native mangrove species planted, the common species, Kandelia obovata, and the rarer viviparous species, Bruguiera gymnorhiza, had the highest survival rates. This shows that common species are the most promising for use in restoration efforts, but that rare species might be successfully reintroduced if they are viviparous. These results were published in Issue No. 27 of the AFCD Journal of Species Exploration in Hong Kong.

Such insights could prove useful to ongoing eco-engineering schemes that aim to use mangrove restoration to enhance urban coastal biodiversity. If threats from water pollution and invasive species can also be adequately tackled, then these restored mangroves could protect Hong Kong from increasingly extreme storms and flooding.

“When there’s heavy rainstorms, the excess water could be retained by mangroves,” says Leung. “This is quite important as climate change is going to increase the frequency of extreme weather events, so mangroves could act as a resilience habitat.”