Currently there are 17 impounding reservoirs in Hong Kong, in which a variety of living organisms such as algae, zooplankton, and fish can be found. These living organisms grow naturally in a state of ecological balance. However, the water quality of some of the reservoirs have been affected by an excessive growth of algae. Therefore, to ensure an ecological balance and to maintain good water quality, the Water Supplies Department (WSD) maintains a certain quantity of fish, which feed on algae, in such reservoirs by stocking fish fry into them regularly. This time, I have invited colleagues from the WSD to tell us more about the “fish army” and the work of “reservoir fishermen” at the reservoirs.

Fish army to prevent excessive growth of algae

Waterworks Chemist of the WSD, Mr TANG Ho-wai paid a site visit to Plover Cove Reservoir with colleagues recently. According to Mr TANG, the prolonged sunlight exposure and the relatively high level of nutrients in some of the reservoirs are favourable conditions for the growth of algae. For reservoirs that have experienced an excessive algae growth, the department regularly stocks fish fry, which feed on phytoplankton, into them. This method is particularly effective in regulating the growth of algae. The fish army in the reservoirs of Hong Kong is mainly made up of Silver Carp, Big Head, and Mud Carp.

The Silver Carp is a filter feeder that mainly lives right below the water surface. With its fine gill rakers, the Silver Carp can filter and feed on small phytoplankton. The Big Head is an omnivorous fish that stays in the upper and middle layers of water, feeding on both phytoplankton and zooplankton. The Mud Carp mainly stays in the middle and bottom layers of water and feeds on organic detritus. Members of the fish army perform their functions at different water depths. As Silver Carp and Big Head are river fish which require a high flow of water to induce spawning and cannot reproduce in the still water of reservoirs, fish fry are regularly stocked into the reservoirs by the WSD.

“Gill-netting” by reservoir fishermen

Mr TANG Ho-wai says to us that, to monitor the water quality and ecological environments of the reservoirs, the WSD regularly takes water samples from reservoirs for testing. In addition, gill-net surveys are　conducted regularly to monitor the condition of fish to ensure an ecological balance in the reservoirs. According to Mr TANG Ho-wai, past monitoring records show that the water quality of the reservoirs in Hong Kong remains satisfactory.

On the topic of “fishing”, we have to introduce two artisans (fishing), also known as “reservoir fishermen” – Mr KWOK Tai-hei and Mr YIP Chi-on, from the WSD. They leave for a reservoir every day early in the morning and spend almost the whole day working on a boat, with one steering the boat and the other casting a fishing net into the reservoir. Gill-netting is conducted at various monitoring points regularly to monitor the fish in the reservoirs. Basic information of fish, including the species and sizes, and their share of the population, is recorded. At the same time, they take water samples for testing at the laboratory to monitor water quality.

Monitoring water quality in reservoirs in the front line

Mr KWOK Tai-hei, who has been working at the reservoirs for 29 years, will retire in 18 months. With extensive experience in the job, he shares that the water surface of the reservoirs may look calm, but actually it is no different from the sea under unstable weather conditions. In particular in the monsoon season when high winds come, white caps on wave top over reservoir water surface can also be seen. Furthermore, when casting a fishing net into a reservoir, one should be able to keep the net away from the intake points to prevent the net from being sucked in. Mr KWOK Tai-hei wants to pass on his work experience over the past years to his colleagues and remind them to pay attention to work safety at all times.

Mr YIP Chi-on, who has been in the industry for six years, says that he enjoys “fishing” in the reservoirs. The duties of “reservoir fishermen” also include patrolling at the reservoirs and monitoring the situation inside the reservoirs. For example, if an excessive growth of algae is identified in a reservoir, they need to report to the department as soon as possible to enable timely handling and follow-up actions.

The WSD always attaches great importance to the water quality in the reservoirs. Apart from using fish to control and prevent excessive growth of algae, the department will also explore new technology for enhanced monitoring of water quality, for example, exploring the deployment of unmanned surface vessels to conduct water sampling. I appreciate the dedication of our frontline colleagues in monitoring the sources of drinking water for maintaining good water quality in the reservoirs and ensuring drinking water safety in Hong Kong.