The effect of temperature on post-feeding ammonia excretion and oxygen consumption in the southern catfish

The post-prandial rates of ammonia excretion (TAN) and oxygen consumption ((M)over dot(O2) ) in the southern catfish (Silurus meridionalis) were assessed at 2 h intervals post-feeding until the rates returned to those of the fasting rates, at 17.5, 22.5, 27.5, and 32.5 degrees C, respectively. Both fasting TAN and (M)over dot(O2) increased with temperature, and were lower than those previously reported for many fish species. The relationship between fasting TAN (mmol NH3-N kg(-1) h(-1)) and temperature (T, degrees C) was described as: fasting TAN = 0.144e(0.0266T) (r = 0.526, n = 27, P < 0.05). The magnitude of ammonia excretion and its ratio to total N intake (Ex(NH3-N)/In-N) during the specific dynamic action (SDA) tended to increase initially, and then decrease with increasing temperature. The ammonia quotient (AQ), calculated as mol NH3-N/mol O-2, following feeding decreased as temperature increased. The relationship between AQ during SDA and temperature was described as: AQ(during) (SDA) = 0.303e(-0.0143T) (r = 0.739, n = 21, P < 0.05). Our results suggest that ammonia excretion and oxygen consumption post-feeding are operating independently of each other. Furthermore, it appears that the importance of protein as a metabolic substrate in postprandial fish decreases with temperature.