Rice Fish - A Not-so-trivial Fish

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Just three to four centimetres long, and with an absolutely plain colour – Oryzias curvinotus, or “Rice Fish” as it is commonly known, was once very common and much overlooked. However, as local paddyfield cultivation almost disappeared completely, coupled with the unfavourable factors of land development and invasion by foreign species, the population of the tiny fish has diminished to an alarming level. Conservation efforts are urgently needed!

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Oryzias curvinotus is a local native freshwater fish with a half transparent, silvery white body tinged with light yellow. It is called “Rice Fish” as it is commonly found in paddyfields. Its habitats include other slow flowing water bodies such as the lower courses of streams and rivers and the wetlands beyond. The fish often swim slowly, in shoals, near the water surface.

The mouth of the fish is on the upper part of the head for easy intake of algae, micro-organisms and their remains, helping to clean the debris in the water bodies, and effectively preventing algal blooms in spring and summer. The fish also feed on micro-organisms attaching on the surface of rocks and plants, hence facilitating the photosynthesis of aquatic plants. The species plays a significant role in maintaining the ecological balance of rivers and streams.

In recent years, scientists discovered a special use of another rice fish species, Medaka (Oryzias latipes). Through transgenic technology, fish embryos have a fluorescent protein implanted; this is very sensitive to toxicity. The embryo expresses fluorescence when toxicity is present in a tested solution. The test is rapid and reliable.

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Restoration work shows promise

The New Territories were once filled with many lowland paddyfields and irrigation waterways. As agriculture dwindled during development of the land, paddyfields and waterways vanished together with the fish. The native population of Oryzias curvinotus becames extremely rare as local rivers are threatened by wastewater pollution and channelisation. Worse still, in around 1940, the Hong Kong government followed the conventional measure around the world of introducing Mosquito Fish (Gambusia affinis), a fish native to southeast America and Mexico, into local rivers in an attempt to solve mosquito problems. Unfortunately, the high adaptability, reproductive power and invasiveness of Mosquito Fish has caused an ecological imbalance and impacted the survival of the weaker Rice Fish.

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To save Oryzias curvinotus, a conservation group launched a restoration scheme in Lai Chi Wo in 2018. The group captured a few dozen Oryzias curvinotus from the So Lo Pun population, about 2 kilometres away from Lai Chi Wo, based on genetic screening. After reproduction in the laboratory, a few hundred of the fish were released in the upper course of Lai Chi Wo in stages. The fish were first put in specially designed plastic boxes so that they could gradually adapt to the new environment, and were only released into the wild after several months. The three-year programme has increased the fish population to several thousand. At present, the fish species can be found in most of the restored farmlands and waterways. There is good progress with the restoration work, and it is expected that the fish population can integrate well into the ecosystem in the long term.