Ammonia excretion and total N budget for gilthead seabream (Sparus aurata) and its effect on water quality conditions

Diurnal ammonia excretion rate was measured for 3-, 40- and 90-g gilthead seabream at 24°C at 4-hourly intervals in experimental tanks. The excretion rates reached peaks of 70, 36.4 and 25.2 mg N kg^-1 h^-1 for 3-, 40- and 90-g fish, respectively, during the first sampling period (0-4.5 h). In a similar experiment run at 21°C, a peak excretion rate of 21.6 mg N kg^-1 h^-1 was reached in 0-3 h for 45-g fish. The excretion rate dropped from the peak value to a low level of 10-16 mg NH₄-N kg^-1 h^-1 after 15-20 h, which probably represents the endogenous excretion rate. This rate is similar to that of other marine fish and lower than that of freshwater species. The total daily ammonia production for 3-, 40- and 90-g fish was 1032, 365 and 353 mg N kg^-1 day^-1, respectively. When considered as a percentage of the food consumed, the proportion of N excreted was similar for all sizes of fish; 30% was excreted as ammonia-N, 30% as DON. There was no detectable urea excreted. The 40% unaccounted for was made up of faeces (approximately 10%) and growth (approximately 30%), similar to values calculated from field growth rate studies. Using the results from this study, it was possible to calculate the sources and sinks of ammonia-N to the fishponds in Eilat. The total nutrient load supplied by the fish to the pond or for export downstream was 0.164 moles N kg fish^-1 day^-1 for 3-g fish and 0.060 moles N kg fish^-1 day^-1 for 40- and 90-g fish.