Ammonia concentration in relation to ammonia toxicity during a rainbow trout rearing experiment in a closed freshwater-seawater system

Abstract

The concept and mechanism of ammonia toxicity in fish are discussed with particular reference to the role of un-ionized ammonia (NH 3). The toxicity of this component arises because of its ability to pass easily through cell membranes, whereas the ionic forms (NH 4 + and NH 4SO 4 −) are restrained in movement. Safe working conditions for fish rearing in respect of ammonia are described in terms of a toxic threshold of un-ionized ammonia. In the case of rainbow trout this is 0.3 ppm as nitrogen in the water bulk. Discussion is also given to additional safety margins which arise with particular natural waters, depending on the state of the carbonate system and the way in which it affects the pH and the un-ionized ammonia concentration in the physiologically important layer of water immediately adjacent to the gill membranes. The daily monitoring of total ammonia, un-ionized ammonia, pH and temperature during a 10-month rainbow trout rearing experiment in fresh and sea water, in which fingerlings were brought up to market size in a closed circuit system, is described. The results show that high feeding, high stock intensity conditions which favour rapid growth and economic rearing, are in fact also conditions which can, paradoxically, produce extremely low concentrations of un-ionized ammonia, and hence complete safety in respect of the ammonia toxicity hazard. Un-ionized ammonia concentration in the water bulk was consistently between 0.0001 and 0.1 ppm nitrogen, i.e. between 1 3000 and 1 3 of the toxic threshold for the water bulk. After the establishment of populations of ammonia bacteria in the sand filters, un-ionized ammonia concentrations were normally below 0.002 ppm nitrogen in fresh water and 0.0005 ppm in sea water, i.e. less than 1 150 and 1 500 of the toxic threshold.