Mai Po Wildlife

Fish

Although there have been no recent surveys on the fish diversity in Mai Po, 31 species have been previously recorded. They can be separated into 3 groups related to the habitat they utilize: 1) freshwater; 2) brackish water and 3) marine vagrant.

There are 2 species of freshwater fish in Mai Po: Mosquito Fish Gambusia affinis and Catfish Clarias fuscus. Both are found only in the freshwater ponds and complete their entire lifecycle within the freshwater environment.

Brackish water fish, of which there are 22 at Mai Po, generally live in estuarine areas, but migrate between fresh and sea water during their life cycle. They are found inside gei wai, freshwater ponds, and at the inter-tidal mangroves and mudflat. Some can even walk on the mudflat when the tide is low such as the Mud-skipper Boleophthalmus pectinirostris.

Seven species of fish are classified as marine vagrants. These fish spawn in the inter-tidal mangroves and the fry sometimes get flushed into the gei wai on rising tides. The presence of some of these fish in the gei wai may also be caused by the practice of commercial fish farming in the 1970s. Only 3 of the 7 species have been recorded in the recent years.

© WWF Hong Kong

The presence of fish in freshwater pond and gei wai has both advantages and disadvantages for biodiversity conservation and management. Some fish, like the introduced species Tilapia (Oreochromis spp.), which is abundant and dominant in the gei wai reproduce quickly and consume much of the food and larvae (crustaceans, polychaetes, gastropods, insects etc.) of other wildlife. Therefore, they can lower the biodiversity in the waterbody they inhabit. Also Ladyfish Elops saurus feed on gei wai shrimp and grow too big themselves to be eaten by waterbirds. Other problem species to management are mud-dwelling fish (Catfish C. fusaus, Snakehead Channa spp. etc.) which often drill through the earth bund between gei wai creating leaks which need to be repaired.