Mulberry-dyke fishponds

Hong Kong’s pond-fish culture originated in the Pearl River Delta, where due to frequent flooding, farmers created ponds to store water and protect their crops from flood damage. During the Ming Dynasty, the mulberry-dyke fishpond system evolved. Each winter, farmers drained the ponds, dug the sludge out of the bottom, and used this material to build up the pond dykes, on top of which mulberry trees were planted. In the spring, they harvested mulberry leaves to feed silkworms, and used silkworm droppings as fish feed. This system enabled farmers to produce silk, sell farmed fish, and use the nutrient-rich pond sludge to fertilise the mulberry trees, creating a sustainable cycle. 

Though the fishponds that developed in Hong Kong over the past century did not focus on silkworm production, their ecological design still resembles that of mulberry-dyke fishponds as farmers planted various secondary crops along their dykes. 

Gei Wai

Apart from freshwater and brackish fishponds, Hong Kong also has ponds operating in the traditional gei wai style. Gei wai are traditional tidal shrimp ponds built along the estuarine coastlines. In fact, gei wai in this region predate ordinary fishponds.

Mr Lai Loi-chau, also known as Uncle Chau, chairs the Hong Kong New Territories Fish Culture Association and oversees around 20-30 fishponds in Nam Sang Wai, Lotei, Kam Tsz, and San Tin. He is the son of a fisherman with over 40 years of experience in fishpond farming. In the 1940s, his father moved ashore and began building gei wai in the Nam Sang Wai area of Yuen Long, gradually forming the gei wai landscape that is now characteristic of Nam Sang Wai and Luen Hing Wai.

Gei wai are enclosures created from stone or earth embankments in the coastal intertidal zone at a height aligned with the tidal cycles. A sluice gate is placed on the seaward side of the embankment. During high tide, seawater flows in through the gates, bringing in plankton, algae, and organic detritus as a natural food source; when the tide recedes, the gates are closed, trapping nutrients inside. Usually, fish farmers open the gates every night during the high tide in the spring and autumn to let water and wild shrimp and fish larvae into the gei wai. Throughout the ensuing months, the shrimp feed on the natural nutrients in the water with minimal human intervention. During the following autumn and spring, water is let out at low tide, allowing fish and shrimp to be harvested using nets placed across the gate.  

This method of aquaculture requires very little capital as it uses the ebb and flow of the tides to bring fish and shrimp larvae into the gei wai, which is why they are built at river estuaries along the coast. However, because these enclosures are filled entirely by river water, they are highly dependent on good water quality. According to Uncle Chau, by the 1960s, Hong Kong’s rapidly growing population had led to a surge in pig and poultry farming in the New Territories, causing the Yuen Long and Kam Tin rivers to become polluted and leading to a sharp decline in shrimp larvae. The water quality in Deep Bay simultaneously deteriorated for various reasons, which had severe consequences for a system which depends on the exchange of water with the external environment. In any case, since the demand for fish was rising at the time, Uncle Chau decided to convert the gei wai into fishponds. 

At first, Uncle Chau experimented with a gei wai in Nam Sang Wai. As the embankment was barely a centimeter or two higher than the river level, he had to build its height up by two or three metres by hand. He drained out the brackish water, and waited for rainwater to refill it. While this sounds implausible, many fish farmers recall that gasoline or diesel-powered pumps were not available until the late 1960s. Prior to that, fishponds could only be filled by diverting water from other sources or by collecting rainwater. According to Uncle Chau, several severe droughts in the 1960s left the pond completely dry. 

The process of fish farming

To raise fish in a fishpond, the pond is first filled with fresh water, then a few fish fry are released into it to observe for any abnormalities. At the same time, the water is disinfected by adding tea seed powder or lime. Then, nutrients such as wheat bran are added to encourage the growth of algae, which becomes a natural food source for the fish. Once the water quality has stabilized and everything looks normal, the main batch of fish fry are released, and are fed daily. If farmers want the fish to grow faster and larger, they will use mixed or eco-friendly feeds. The budget-friendly option is to use bread crumbs and other cheaper feed. In theory, fish can be harvested at any time, but most farmers do so before the Lunar New Year. For instance, in ponds where mullet and tilapia are raised together, fish fry are introduced at the beginning of the year (January or February), and are fed every week until they are harvested in November or December.

Before mechanical equipment became available, fish farmers had to do all of the work by hand, which made raising a batch of fish extremely time-consuming. This was because the number of fish that could be kept was limited by the naturally available oxygen in the water, a limit that was greatly increased by the later adoption of mechanical aerators. Additionally, harmful microorganisms accumulated at the bottom of the pond, so during the dry season, farmers had to drain the ponds to allow sunlight to disinfect them. Fish farmers would then carry out “pond scraping” works to repair embankments and restore the pond to its original depth. Previously, the silt had to be dug out by hand, but these days, mechanical excavators are used. If the pH level of the pond becomes unbalanced, farmers add the appropriate amount of slaked lime to adjust it. This maintenance process, known as “sunning and scraping the pond,” is typically carried out every two to three years. Interestingly, as the water level drops, waterbirds gather to feast on the small fish left behind. 

Most of the fish fry raised by local fish farmers in Hong Kong come from mainland China and Taiwan, though some farmers also collect mullet fry from rivers to raise in their ponds. In the 1960s and 70s, Hong Kong had dedicated fishponds for breeding fish fry. One reason for this was that mainland China temporarily stopped exporting carp fry between 1954 and 55; freshwater aquaculture in China relies heavily on the “Four Major Chinese Carps”: black carp, grass carp, silver carp, and bighead carp, all belonging to the Cyprinidae family. As a result of this disruption, local fish farmers began farming African cichlids (tilapia) as an alternative.

Another well-known local pond-raised fish is the flathead grey mullet, which is a coastal fish that can transition between saltwater and freshwater environments. Starting in the 1970s, fishermen started catching mullet fry at the river mouth and tried to raise them in brackish and freshwater ponds. It turned out that pond-raised mullet not only had a better texture and flavour, but grew faster as well. Over time, mullet became a signature fish product of Yuen Long.

The Conservation of Gei Wai

As mentioned above, many gei wei were converted into fishponds during the 1970s and the 1980s due to the deterioration of water quality in Deep Bay. In addition, many fish farmers, especially those who had been building gei wai since the 1940s, were ageing. Those whose children who were not interested in taking over opted to retire. At the same time, developers were buying up fishponds in the Tai Sang Wai area to be developed into what is now Fairview Park. However, the 1970s also saw the rise of environmentalism, bringing more public attention to the ecological value of gei wai and fishponds. Today, there are 21 traditional gei wei preserved in the Mai Po Nature Reserve.

In light of this concern, WWF Hong Kong started supporting local fish farmers and took over the management of these traditional gei wai in the 1980s, aiming to preserve the migratory bird habitats of Deep Bay. Over the decade, several gei wai in the Mai Po area were handed over to WWF for management. By 1995, the Mai Po wetlands had been designated a “Wetland of International Importance” under the Ramsar Convention, and the Hong Kong government classified the area as a “Site of Special Scientific Interest,” acquiring the remaining gei wai for conservation. Of the 21 gei wai within the Mai Po Nature Reserve, 17 are managed by WWF, while the remaining 4 continue to be operated by local fish farmers. These gei wai are now the last of their kind in Hong Kong. WWF continues to maintain and operate these gei wai, harvesting shrimp and fish. The public can visit the facilities or even participate in traditional shrimp harvesting activities through organised programmes.

The still existing gei wai and fishponds in the tidal flats of the northwestern New Territories are like a time capsule showing the evolution of local aquaculture from the low-intervention, sustainable gei wai system to the more intensively managed fishpond system. The area is still a vital centre for freshwater fish farming and an ecological haven for migratory birds. Although traditional practices have diminished over time, the combined efforts of conservation groups and fish farmers have preserved Mai Po’s gei wai, allowing the dynamics of production to continue with the ebb and flow of the tides. These intersecting waterways have brought us local delicacies like Yuen Long mullet and help to sustain the biodiversity of an internationally important wetland, making it a rare example of dual cultural and ecological heritage in Hong Kong’s modern cityscape. Looking ahead, it will be the challenge of our generation to find a balance between development and conservation that enables the gei wai ecosystem to continue to nourish this land. 

This article was written by contract researcher Ng Cheuk Hang.

Reference:

Awareness, Mainstreaming and Sustainability Working Group,《傳統知識專題小組報告》(“Traditional Knowledge Focus Group –  Final Report”), 15 September 2014, Hong Kong Government, Agriculture, Fisheries and Conservation Department.