The Mangrove Ecosystem of Deep Bay and the Mai Po Marshes, Hong Kong

Metal concentrations in shrimp and mantis shrimps from Deep Bay, Hong Kong and the eastern waters of the Pearl River estuary

The Mangrove Ecosystem ofDeep Bay and the Mai Po Marshes, Hong Kong (ed. S.Y. Lee). Proceedings of the International Workshop on the Mangrove Ecosystem of Deep Bay and the Mai Po Marshes, Hong Kong, 3-20 September 1993. Hong Kong: Hong Kong University Press, 1999. METAL CONCENTRATIONS IN SHRIMP AND MANTIS SHRIMPS FROM DEEP BAY, HONG KONG AND THE EASTERN WATERS OF THE PEARL RIVER ESTUARY Y.B. Ho Department of Botany, The University of Hong Kong, Pokfulam Road, Hong Kong ABSTRACT The white shrimp Exopaiaemon setiferus H. Milne Edwards and three mantis shrimps, namely, Oratosquilla oratoria de Haan, Hapiosquilla japonica Manning and Oratoria sp. were trawled from Deep Bay and the eastern Pearl River estuary in September, 1993. The samples were analysed for six metals, including Mn, Fe, Cu, Zn, Cd and Pb using atomic absorption spectrometry. Both the level of Cu and Zn appeared to be regulated by the four species. Cadmium in some of the mantis shrimp samples exceeded the legislative limit of 2 Ilg Cd·g-1 wet wt and may pose a health hazard to human when consumed. It is recommended that more detailed studies on the level of Cd in seafood collected from Pearl River estuary are desirable. INTRODUCTION In the study of metal concentrations in marine organisms in Hong Kong, while many studies have been made on algae (e.g. Ho 1990), mussels (Phillips 1985), oysters (Phillips 1979) and barnacles (Phillips and Rainbow 1988), the same does not apply to the mantis shrimps and shrimps. The only published account was by Phillips et ai. (1982) who reported on the metal levels in the mantis shrimp Oratosquilla oratoria taken from the eastern waters of the Island of Hong Kong, and numerous other seafood species. The mantis shrimps, in particular Oratosquilla oratoria, occur in abundance throughout the coastal waters of China (Wei 1991) and Japan (Ohtomi and Shimizu 1991) and are consumed fresh (Wei 1991; Hirano et ai. 1992) or prepared as shrimp paste (Wei 1991). Species of Exopalaemon are found thorough the coastal waters of China and are either taken fresh or prepared as dried shrimps for consumption (Wei 1991). Thus these species serve as seafood locally and the nearby regions. During the international Workshop on the Mangrove Ecosystem of the Mai Po Marshes and Deep Bay held in September, 1993, trawling excursions in Deep Bay and the eastern Pearl River Estuary were carried out. The white shrimp Exopalaemon 153 154 Y.B. HO setiferus, and three mantis shrimps, namely, Oratosquilla oratoria, Oratosquilla sp. and Harpiosquilla japonica, collected from various sites were collected and subsequently analysed for their metal levels. It is hoped that data obtained from this study would serve as a reference for future studies. Also since mantis shrimps have become a valuable seafood to Hong Kong people in recent years and they are partly supplied from local waters, it is important to know that whether consumption of local mantis shrimps would pose any potential health hazards or not. This paper reports on the metal levels in the shrimps collected from the Deep Bay and the eastern Pearl River estuary and attempts to evaluate whether they would pose any potential health hazard as seafood. MATERIALS AND METHODS The sampling programme was carried out in the period 12-15 September 1993. Samples were collected from 20 stations in the Deep Bay and the eastern Pearl River Estuary by trawling. Four of the stations (stations 16-19) were in inner Deep Bay, five (12-15, 20) were in Outer Deep Bay, and the rest 0-11) were located in the eastern Pearl river estuary. At each station, paired trawls, each lasting 5 minutes, were carried out in opposite directions with a commercial shrimp trawler. During trawling 4 to 6 nets, each with a beam length of 2 m and mesh size of 4 cm at the cod end, was used. Samples from each station were preliminary washed with seawater, placed on a 500 ~m sieve and stored on ice immediately. The depth at each station was measured and the nature of the substrate noted. Location of all the sampling stations was determined by GPS satellite sensing. Figure 1 gives the location of all the sites. In the laboratory samples from each station were put in a plastic bag and kept frozen at -25° C before sorting. Then samples were thawed, carefully washed with tap water, sorted, and each species placed in separate plastic bags and stored frozen again until further treatment. The white...