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

Abstract

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) + mortality (M) + excretion (E) + faecal production (F). A nitrogen budge? was constructed for individual groupers cultured for 1 mo under laboratory condition and an annual nitrogen budget was also constructed for a cohort of E. areolatus cultured in an open-sea-cage farm and fed with trash fish. Consumption was determined independently both by feeding experiments as well as by summation of P, M, E and F, to provide an estimate of the accuracy of the budget equation. In the laboratory budget, 27.5% of N consumed was channelled to growth, while 64.4% was excreted in the form of ammonia 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 (37.7%) 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-1 of fish production). Such a value is almost 3 times as high as N loss from temperate salmonid farms.