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Reducing Nitrogen Pollution Loading from Fish Farming by Changing Feeding Practices: An Example from Hong Kong
- Hong Kong University Press, HKU
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Perspectives on Marine Environment Change in Hong Kong and Southern China, 1977-2001 (ed . B. Morton). Proceedings o f a n Internationa l Worksho p Reunio n Conference, Hon g Kon g 21-26 Octobe r 2001. Hong Kong: Hong Kong University Press , 2003. REDUCING NITROGE N POLLUTION LOADIN G FRO M FIS H FARMING B Y CHANGING FEEDIN G PRACTICES : AN EXAMPLE FROM HONG KON G Kenneth M . Y . Leun g The Swire Institute of Marine Science and Department o f Ecology an d Biodiversity , The University of Hong Kong, Hong Kong Jim C. W . Ch u Agriculture, Fisheries an d Conservation Department , The Government o f the Hong Kong Special Administrative Region , Aberdeen, Hong Kong and Rudolf S . S . W u Department of Biology an d Chemistry, City University o f Hong Kong , Kowloon Tong, Hong Kon g ABSTRACT Traditionally, marine fish farmers in tropical and sub-tropical regions feed their fish with trash fish. This results in high feed wastag e and adds a significant pollutio n load to the receiving water . The Agriculture, Fisheries an d Conservation Departmen t o f the Hong Kong SA R Governmen t ha s develope d a moist pelle t fee d wit h a view t o improvin g feeding efficienc y an d reducing pollution . Thi s pellet feed ha s been use d successfull y by loca l marin e fis h farmer s sinc e 1994 . We constructe d an d compare d th e nitroge n budgets fo r sea-cag e farming o f the Mangrove snappe r Lutjanus argentimaculatus fe d with trash fish and pellet feed using the mass balance equation: consumption = feed input (I) - fee d wastag e (W ) = productio n (P) + mortalit y (M ) + excretio n (E) + faeca l production (F). Feed wastag e from farm s usin g trash fish (30.1 % o f feed input ; 109. 8 kg-N-tonne fish production-1 -year-1 ) was significantly highe r than those using pellet feed (21.0%; 62. 3 kg-N-tonne^-year 1 ), whil e P , M and F wer e simila r i n both treatments . Furthermore, snapper s fe d wit h tras h fis h excrete d mor e nitroge n a s ammoni a (191. 2 544 K.M.Y. LEUNG, J,C.W. CHU & R.S.S. WU kg-N-tonne^-year-1 ) compare d wit h fish fe d wit h the pellet (166. 9 kg- N-tonne^-year1 ). Consequently, th e annua l nitroge n loadin g (W + E + F) generate d fro m fis h farmin g using trash fish and pellet as feed were 310.1 and 243.0 kg-N-tonne-1 -year1 , respectively . Based on our experimental results, nitrogen loading can be reduced by 21.6% (equivalent to 67.1 kg-N-tonne-1 -year-1 ), if pellet feed is used instead of trash fish. Th e implication and significance of the present results are discussed with special reference to mariculture management i n Hong Kong . INTRODUCTION Global production of farmed marine fish has increased considerably in recent years (fo r example, from 210 thousand tonne-year-1 in 198 7 to 300 thousand tonne-year-1 in 1992 ) and i s expecte d t o reac h 1. 0 millio n tonne-year -1 b y 202 5 (Fisherie s an d Agricultur e Organization 1995 ; New and Csavas 1995) . With this rapid development, there has been a growin g concer n ove r th e impac t o f fis h farmin g activities , o n wate r an d sedimen t quality of the adjacent marin e environment (Gowe n and Bradbury 1987 ; Ackefors an d Enell 1990 ; Holmer an d Kristense n 1992 ; Ackefors an d Enel l 1994 ; Wu et al. 1994 ; Enell 1995 ; Wu 1995 ; Wu et al. 1999 ; Karakassis et al. 2000). Environmental impact s o f marin e fis h farmin g hav e bee n wel l documented , especially thos e of salmonid farmin g i n temperate regions. One of the prime concern s is the high nitrogen (N ) loading from intensive cage culture. It has been estimated that 52 to 95 % of N input int o a culture syste m i s ultimately release d t o the environmen t (Handy an d Poxto n 1993) , leadin g t o th e proble m o f eutrophicatio n an d henc e th e mortality o f fis h...