The fate of traditional extensive (gel wai) shrimp farming at the Mai Po Marshes Nature Reserve, Hong Kong

Extensive shrimp farming around Deep Bay, Hong Kong, began in the mid-1940's after the construction of intertidal ponds (gel wail among the coastal mangroves. The ponds are increasingly being seen as an example of how wetlands can be used sustainably since they are naturally stocked with shrimp postlarvae (e.g. Metapenaeus ensis) and young fish (e.g. Mugil cephalus) flushed into the ponds from Deep Bay. Once inside, these shrimps and fish feed on naturally occurring detritus on the pond floor. The only gel wai remaining in the Territory, are those at the WWF Hong Kong Mai Po Marshes Nature Reserve, adjacent to Deep Bay. Analysis of the shrimp production between 1990-1995 showed that there were two seasonal peaks, from April-June (Recruitment-I) and from July-October (Recruitment-II). The second peak was significantly lower than the first (p<0.001), especially from those gel wai in the southern part of the reserve which are much closer to a polluted river. The average harvest from each gel wai had also significantly declined from 40.9 +/- 6.0 kg ha(-1) yr(-1) in 1990 to 15.1 +/- 3.6 kg ha(-1) yr(-1) in 1995 (p<0.01). This decline can be attributed to the abundance of predatory fish in the gel wai, and increasing water pollution in Deep Bay which adversely affects the amount of shrimp larvae for stocking the gel wai, as well as the quality of water for flushing the ponds during the rearing and harvesting seasons. Despite this, those gel wai which are not-commercially viable can still support many non-commercial, more pollution tolerant fish and shrimp species. As a result, the management of these gel wai has been altered such that their objective is to provide feeding habitat for piscivorous waterbirds, which is also in line with the aims of the nature reserve.