Bergen College of Engineering, Lars Hillesgate 34, 5008, Bergen, Norway

Rogaland Research Institute, PO Box 2503 Ullandhaug, 4004, Stavanger, Norway

The second part of the study reports measurements of the mass related ammonia quotient (diurnal cycles) of Atlantic salmon kept in a landbased tank farm and calculations of waterflow requirements. The total variation find of the mass related ammonia quotient values were between 0·03 and 0·08. Based on two of the most extreme diurnal cycles with respect to temperature and fish size, the specific waterflow requirement applying ammonia as a limiting factor was in the range 0·04–0·17 litres/kg min.

The third part of the study applies the measurements obtained for the ammonia quotient to the models. The results show that un-ionized ammonia may become the first limiting factor by combinations of a high difference in the dissolved oxygen concentration between the inlet water and the discharge, a high pH and a low threshold level for un-ionized ammonia. However, oxygen is usually found to be the first limiting factor when compared to un-ionized ammonia.

If the value 0·5 mg NH₄---N/litre is used as the limiting criterion for total ammonia, the difference in the dissolved oxygen concentration between the inlet water and the discharge should not exceed about 7 mg/litre. However, in seawater oxygenation is necessary to provide and maintain this difference. Therefore both models result in the same conclusion for moderate levels of oxygen addition.