Effects of body weight, water temperature and ration size on ammonia excretion by the areolated grouper (Epinephelus areolatus) and mangrove snapper (Lutjanus argentimaculatus)

The effects of body weight, water temperature and ration size on ammonia excretion rates of the areolated grouper Epinephelus areolatus and the mangrove snapper Lutjanus argentimaculatus were investigated. Under given experimental conditions, L. argentimaculatus had a higher weight-specific ammonia excretion rate than E. areolatus. Weight-specific ammonia excretion rates of fasted individuals of both species showed an inverse relationship with body weight (W, g wet wt.), but a positive relationship with water temperature (t, °C). The relationships for total ammonia nitrogen (TAN) were: E. areolatus: TAN (mg N kg⁻¹ d⁻¹)=21.4·exp^0.11t·W^−0.43 (r²=0.919, n=60); L. argentimaculatus: TAN (mg N kg⁻¹ d⁻¹)=121.5·exp^0.12t·W^−0.55 (r²=0.931, n=60). Following feeding, the weight-specific ammonia excretion rate of E. areolatus increased, peaked at 2 to 12 h (depending on temperature), and returned to pre-feeding levels within 24 h. A similar pattern was observed for L. argentimaculatus, with a peak of TAN excretion being found 6 to 12 h after feeding. Stepwise multiple regression analysis indicated that weight-specific TAN excretion rates of both species increased with increasing temperature and ration (R, percent body wt. d⁻¹): E. areolatus: TAN (mg N kg⁻¹ d⁻¹)=22.8·t−28.8·R−378.2 (r²=0.832, n=24); L. argentimaculatus: TAN (mg N kg⁻¹ d⁻¹)=22.9·t−25.4·R−216.4 (r²=0.611, n=24). The effect of body weight on weight-specific postprandial TAN excretion was not significant in either species (p>0.05). This study provides empirical data for estimating ammonia excretion of these two species under varying conditions. This has application for culture management.