Dogmas and controversies in the handling of nitrogenous wastes: Is exogenous ammonia a growth stimulant in fish?

Traditionally, waterborne ammonia is considered a toxicant that decreases productivity in aquaculture. However, several recent studies have suggested,but not proven, that growth of salmonids might actually be stimulated by chronic exposure to very low levels of ammonia. In the present study, two 70–71 day growth experiments were conducted under rigorously controlled experimental conditions with juvenile rainbow trout at total ammonia concentrations ([T_Amm])=0, 70 and 225 μmol l^–1, pH 7.6. In the first series, a small-scale laboratory proof-of-principle study at 15°C, there was a significant stimulation of mass gain, gross food conversion efficiency, condition factor and protein production per fish at [T_Amm]=70 μmol l^–1, without an increase in voluntary food consumption or change in `in-tank' O₂ consumption or ammonia excretion rates. These growth stimulatory effects were not seen at[T_Amm]=225 μmol l^–1, where the fish consumed more food, and excreted more ammonia, yet achieved the same mass and protein content as the controls. In the second series, a larger study conducted in an aquaculture facility at 6.5°C, growth rate, conversion efficiency and protein production per fish over 71 days were all significantly stimulated at [T_Amm]=225 μmol l^–1, but not at 70 μmol l^–1, without any change in voluntary food consumption. These effects occurred despite an early inhibition of growth at both [T_Amm] levels. When ration was restricted, growth was reduced and there were no longer any differential effects attributable to[T_Amm]. While the effective levels of[T_Amm] differed between the two series, in both, the P_NH3 level stimulating growth was ∼23μtorr. The results are interpreted as reflecting either a stimulation of ammonia incorporation into amino acids and protein synthesis and/or a reduction in metabolic costs. The finding that low levels of exogenous ammonia can serve as a growth stimulant without altering food consumption may be important for aquacultural practice, and challenges traditional dogma that the effects of ammonia are detrimental to growth.