Is ammonia a growth stimulant?


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For years we've all been told that ammonia is a toxin that can lead to gill damage, disease and death, but now some new research has shown that it could actually have some beneficial effects, too.

Several different scientists have suggested that a very low chronic level of ammonia for an extended period seemed to have a positive effect on the growth rate of farmed salmonids, such as trout. However, to date, no one had been able to prove it.

A new study undertaken by CM Wood of McMaster University in Ontario, Canada, has just been published in the Journal of Experimental Biology and found that the observations may have been right after all.

Wood set up two rigorously controlled experiments using young Rainbow trout, Oncorhynchus mykiss, and exposed subsets of them to different a range of levels of ammonia but fed them the same foods.

Says Wood: "In the first series, a small-scale laboratory proof-of-principle study at 15 degrees C, there was a significant stimulation of mass gain, gross food conversion efficiency, condition factor and protein production per fish at =70 micro mol l(-1), without an increase in voluntary food consumption or change in 'in-tank' O(2) consumption or ammonia excretion rates. These growth stimulatory effects were not seen at =225 micro mol l(-1), where the fish consumed more food, and excreted more ammonia, yet achieved the same mass and protein content as the controls."

The second series of 71 day experiments was a much larger study based at an aquaculture facility. Initially, the total ammonia levels appeared to inhibit the fishes' growth, but after a while things started to improve for the fish.

The second study found that at 6.5°C growth rate, conversion efficiency and protein production per fish were all significantly stimulated at =225 micro mol l(-1). However, the same procedures didn't have an effect 70 micro mol l(-1).

Says Wood: "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.

"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."

For more information read the paper: Wood CM. (2004) - Dogmas and controversies in the handling of nitrogenous wastes: Is exogenous ammonia a growth stimulant in fish? J Exp Biol. 2004 May 15;207(Pt 12):2043-54.