The recirculating aquaculture systems (Fig. 1) reached steady state after 30 days acclimatization period (Figs 2 and 3) during which the rearing tanks were stocked of juvenile Atlantic salmon (size range from 190 to 200 g). Biomasses were adjusted to the respective treatment groups with initial densities of 15 kg m 3 in the rearing tanks. Feeding
quantity (80 g feed system 1 day 1) and feeding frequency (4 meal day 1) were the same in these
12 systems. Culture conditions were adjusted to thesalmon requirements and similar among groups.
Total ammonia nitrogen removal rate (before and after biofilter, + in Fig. 1) and mean nitrite concentrations (rearing tanks) were stabilized after 30 days, and remained constant at sampling places (Figs 2 and 3). After stabilization of RAS, fish were replaced by the experimental salmon