Nitrogen budgets for the areolated grouper Epinephelus areolatus cultured under laboratory conditions and in open-sea cages

The nitrogen budget of grouper Epinephelus areolatus (Forskal) in a culture system can be represented by the mass balance equation: consumption (C) = feed input (I) -feed wastage (W) = production (P) + moi-tality (A4) + excretion (E) + faecal production (F). A nitrogen budget was constructed for individual groupers cultured for 1 n ~o under laboratory conditions; and a n annual nitrogen budget was also constructed for a cohort of E. areolatus cultured in a n open-sea-cage farm and fed with trash fish. Consumption was determined independently both by feeding expenments as well as by summation of P, M, E and F, to provide an estirnate of the accuracy of the budget equation. In the laboratory budget, 27.5 % of N consunled was channelled to growth, while 64 4 % was excreted in the form of amrnonia and 8.1 % lost in faeces The N-assimilation efficiency was 91 9%, while the net N-retention efficiency was 29.9%. For the annual budget constructed for the open-sea-cage farm, only 8 6 % of total N input into the farm was harvested in the form of fish production, while loss to mortality was 3.7 %. Ammonia excretion was the most important N loss (46.0%), followed by feed wastage (37 .? %) and faecal production (4.0%). 66% of total N input could be accounted for in the laboratory N budget for individual groupers, but only 48% of N input into the culture system could be accounted for in the annual nitrogen budget constructed for open-sea-cage farming. It was estimated that 87 7 % of the total N input to the farm was lost to the environment (equivalent to 321 kg N t-' of fish production). Such a value is almost 3 times as high as N loss from temperate salmonid farms