Environmental group Green Power released the latest findings of Tung Chung River Ecological and Water Quality Survey today. The survey found 27 species of odonate (which includes dragonflies and damselflies) and 11 native fish species downstream of Tung Chung River, and 22 native fish species in the estuary. The numbers showed a decline in odonate and fish diversity since new town extension works commenced. River water quality downstream of East Stream has deteriorated with the average conductivity at 119.3 microsiemens per centimetre (μS/cm), the average Ammonia-N value at 0.061 milligrams per litre (mg/L) and the average Phosphate-P value at 0.029 mg/L. Lo Wing-fung, Environmental Affairs Manager of Green Power, calls on the government to adopt more measures to conserve Hong Kong's large, rare and ecologically valuable natural river─Tung Chung River.
Survey Reflects Impact of the TCNTE Project
Tung Chung River has managed to stay natural from its source, estuary to the bay area, something of a miracle. The main stream of the river─West Stream─originates from Lantau Peak and is joined by its eastern tributary─East Stream─from Sunset Peak. The river owes its rich biodiversity to the unique ecology of two streams. Two sections of the river are designated as Ecologically Important Streams for having the second most freshwater fish diversity in Hong Kong.
Lo said the Tung Chung New Town Extension (TCNTE) project comprised large-scale building programmes for residential flats and infrastructure works (including roads, drainage facilities, etc.) sitting on downstream Tung Chung River basin. Relevant works have started in late 2021 for Tung Chung West, and in mid-2022 for large-scale public housing estates and roads. Green Power has been conducting ecological and water quality survey of Tung Chung River since 2018 before TCNTE works began. Our data are useful references for understanding the TCNTE project’s impact on Tung Chung River ecology.
Odonate and Fish Biodiversity Diminished
Odonate survey is conducted in wet season (April to September) every year. In the pre-construction period between 2018 and 2021, the annual average of odonate species was 32, with a record high of 34. The number stood at 33 in 2022 after the works kicked off. It dropped to 24 in 2023 as the works progressed and rose to 27 this year.
Lo said the number of odonate species plunged from the annual average of thirtysomething to twentysomething in mere couple of years. He saw the numbers fell while odonate habitats such as low-lying fields, ponds and creeks went one by one since the site formation works of the TCNTE project began. Gone are the Common Bluetail (Ischnura senegalensis), Asian Pintail (Acisoma panorpoides) and Lesser Emperor (Anax julius) in the surveyed areas. Populations of Orange-tailed Sprite (Ceriagrion auranticum ryukyuanum), Green Skimmer (Orthetrum sabina sabina) and Russet Percher (Neurothemis fulvia) shrank significantly.
Fish diversity plummeted too. Native fish survey is also conducted in wet season (April to September) every year. In the pre-construction period at the river mouth, the four-year average of fish species was 26, with a record high of 33. It fell to a record low of 13 in 2023 after the onset of the works. In the pre-construction period, a record high of 12 fish species, averaged at 11 a year, were recorded downstream. The figure sank to a record low of six in 2022 after the construction began. Auspiciously, numbers bounced back to 22 and 11 at the river mouth and downstream respectively this year.
Lo attributed diminishing native fishes to removal of vegetation cover by the site formation works which resulted in soil erosion and stormwater runoffs that deposited downstream altering riverbed topography and benthos. Though the surveys were conducted outside the channelized portion of Tung Chung River, the sediment built up with the current and changed the aquatic ecosystem far and wide.
Water Pollution by Illegal Sewage Discharge
Green Power carries out water quality survey every month to collect conductivity, ammonia and phosphate data – indicators of water pollution. The last two chemicals are pollutants commonly found in domestic and industrial sewage. Their concentration levels are high in Tung Chung River downstream of East Stream. Lo blamed untreated wastewater disposal from the construction sites and villages for the river pollution and called upon the government to increase site visits and strengthen law enforcement.
Notwithstanding existing mitigation measures, Lo asked the government to step up Tung Chung River conservation efforts to protect it against ecological perils posed by the TCNTE project. Installing sedimentation tanks and treatment facilities at the exit of village stormwater channels and adjacent to construction sites to prevent pollutants and mudflows from entering stream courses will go a long way towards improving water quality and protecting fishes. Odonate habitats such as ponds and creeks ruined by new town extension works should be restored. Lo stressed the importance of setting water quality parameters for health of local aquatic life and recommended "Technical guideline for deriving water quality criteria for freshwater organisms", enacted in 2017 in Chinese Mainland, for reference by the Hong Kong Government. Determining pollution thresholds tolerable for the wellbeing of Hong Kong's aquatic life is a prerequisite for the formulation of conservation policy and evaluation.
