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

Abstract

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 NH 4-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.