## 4 Discussion  
The fish species used in this study, rainbow trout, has been listed as one of the most invasive species in the world [32], and it is also on the blacklists of invasive species in some European countries [33]. However, the capacity of rainbow trout to establish self-sustaining populations in Europe is quite limited despite popular stockings for recreational purposes [33]. Farming rainbow trout on land in RAS and aquonics decreases the potential risk of fish escapes as compared to, e.g., rearing in cages. From the environmental protection point of view, aquonics can also be regarded as a method complying with the best management practices [34]. In the present study, rainbow trout grown in an integrated system with spinach had higher SGR (1.95 \(\pm\) 0.12) compared to fish grown in RAS (1.67 \(\pm\) 0.08). [PERSON] et al. [2] reported an SGR of 1.58 \(\pm\) 0.03 for RAS-grown rainbow trout of a similar size and temperature, which indicates good growth of fish and thus good rearing conditions in both of our systems. Rainbow trout has already earlier been shown to be a suitable species for aquaponic systems [15]. The feed conversion ratio of rainbow trout in the aquaponic system was lower (0.85 \(\pm\) 0.08) than in RAS (1.06 \(\pm\) 0.03). As the amount of feed consumed was almost equal in both treatments, this, in turn, was seen as higher growth rates of the fish in aquonics. In experiments where feed intake is monitored, FCR for juvenile rainbow trout fed dry pellets is typically about one, and more commonly below one [2; 35; 36]. A plausible explanation for the improved production parameters in the aquonic system was the difference in water quality. For example, the onset of nitrification was faster in aquonics than in RAS, and the overall level of nitrate was lower in the aquonics treatment (12.30 \(\pm\) 0.83) than in RAS (26.98 \(\pm\) 1.04). On the other hand, [PERSON] et al. [3] did not find any difference in the final weight or FCR between rainbow trout reared in \"low\" (30 mg/L) or \"high\" (91 mg/L) nitrate in RAS. However, the average final biomass and density were significantly higher in the \"low\" nitrate treatment, affected by higher mortality in the \"high\" treatment. However, the FCR was rather high in both treatments (about 1.3) [3]. FCR-values below one are also reported, e.g., for Murray cod (_Maccullochela peelii pelli_; 0.85) [37] and Nile tilapia (_Orechromis niloticus_; 0.93) [38] reared in aquonics. Managing water quality is crucial for the growth and survival of the fish [39]. During protein catabolism, ammonia is excreted by fish through the gills into the water and it is first oxidized to nitrite and then to nitrate. Nitrite is produced as an intermediate product during the nitrification process and can be oxidized into nitrates if the biofilter is well established [1]. Long-term exposure of fish to nitrite and TAN can be lethal if they exceed the acceptable concentration, which should be less than 1 mg/L but preferably close to 0 [1], and salmonids are more sensitive to nitrite than many other species [40]. The acceptable limit for unionized ammonia nitrogen for cold-water fish is 0.025 mg/L, and the proportion