Environmental group Green Power has been conducting ecological surveys at Tung Chung River since 2015, including the monitoring of dragonfly species. The latest survey found that the Mangrove Skimmer (Orthetrum poecilops), which had been recorded for many years, has nearly disappeared this year. And this raises the concern about their situations.
Tung Chung River is one of the few remaining large natural rivers in Hong Kong. Its natural setting can still be retained from its source to its estuary, and is of very high ecological and conservation value. On the other hand, the Tung Chung New Town Extension project is in full swing. Its Tung Chung West development area overlaps with the lower course of Tung Chung River, and will bring major changes to the Tung Chung River valley. Green Power continues to monitor the water quality and ecology of Tung Chung River to advocate for its conservation.
Mr Henry Lui, Senior Conservation Manager of Green Power, said that the Mangrove Skimmer is listed as a “Vulnerable” species on the IUCN Red List of Threatened Species. The larvae of most dragonflies grow in freshwater, but the larvae of Mangrove Skimmer are an extremely rare trait among dragonflies which grow in high-salinity coastal habitats such as mangroves, and can even survive in seawater for a period. The mangrove at the mouth of Tung Chung River is one of the few local habitats for the Mangrove Skimmer in Hong Kong.
Mr Lui said that in recent years, 24–28 odonate species have been recorded at Tung Chung River during the wet season (April to September), including the Mangrove Skimmer. The Mangrove Skimmer is an uncommon species, and the number recorded in annual surveys has remained low, at only a few individuals. It has mainly been concentrated east of the river mouth. This year's survey recorded none for the first time. However, Mr Lui added that in April this year, one Mangrove Skimmer was recorded at another survey location west of the river mouth, but there was no record at the eastern location where the species had been more regularly recorded. The situation was considered unusual.
Suspected Muddy Water Discharge from Construction Sites into Watercourse
The Mangrove Skimmer has extremely specific habitat requirements, with adults and larvae mainly inhabiting mangroves. The small channels running through the mangrove at the mouth of Tung Chung River are typical Mangrove Skimmer habitats. These small channels currently collect rainwater from small hillsides, pass through some village houses and farmlands, and finally flow through the mangrove into Tung Chung Bay. Mr Lui observed signs of muddy water discharge from a pipe connected to the small channel, and muddy water was occasionally spotted. He suspected that the muddy water came from nearby construction sites. He said: "Although it is not clear what substances the muddy water contains, and discharge from construction sites may also meet discharge requirements, for a fragile small channel, even a small amount of pollution is enough to affect the survival of organisms, especially more sensitive or rare species."
Mr Lui added that although the section of the small channel flowing out to the bay has been included within the "Coastal Protection Area" in the Tung Chung Valley Outline Zoning Plan prepared by the Town Planning Board, and is protected under land-use planning from development threats, if protection of the sources of the small channels is neglected, the ecology will inevitably be affected. Green Power has previously written to relevant departments and organisations, urging active protection of the areas through which these small channels flow. Mr Lui reiterated the importance of maintaining water quality in the small channels, and the need to avoid cutting off the stream flow by infrastructure or construction sites, so as to ensure that Tung Chung River is properly protected and that precious species including the Mangrove Skimmer are conserved.
