Traditional flow-through systems reuse 0% of the water, whereas the most advanced RAS technologies reuse 95-99% of the water through a series of water treatment steps, making them environmentally more attractive. By regulating the culture conditions and shifting production to land, RAS can be built practically everywhere, regardless of local conditions, and can alleviate lack of available space and competition for access to water, all of which are critical in a world of limited natural resources. While being important for the fish, trace mineral (TM) levels in the water may have a negative impact on the natural environment and potentially on the fish . To reduce the environmental impact of aquatic animal feeds on (open) water quality, several countries have regulated the maximum level of mineral supplementation. The European Union, for example, set a whole feed Zn limit of 180 mg/kg for salmonids and 150 mg/kg for other fish species . As a consequence, aquafeed manufacturers need to develop methods to be more efficient with TM supplementation.
Our study investigated two factors that can affect the TM availability/uptake by the fish and the water/effluent level of TM: composition of the TM premix (inorganic vs TM -AA complexes) and the w ater refreshment level (RAS vs. flow through system), resulting in four different treatments. With this purpose, two diets were formulated to be equal in their nutrient composition with the exception of the type and level of TM used. One of the diets was supplemented with inorganic forms of TM ( TM-I; sulphate s of Fe, Zn, Cu, Mn and Se) and the other with TM-AA complexes (TM-AA; Fe-, Zn-, Cu-, Mn- and Se-AA complexes) . In the TM-AA diet, TMs were supplemented at half the level of the inorganic. The two diets were fed for 8 weeks to ±78 g initial body weight r ainbow t rout reared in two different systems differing in water refreshment rate , high (HWR; 1.5 times the system volume) and low water refreshment (LWR; ±311 L / kg feed ). At the end of the feeding period , the effect of the diet , rearing system and their interaction were evaluated in terms of growth performance, fish composition, TM retention and losses , plasma and hepatic TM content, and activity of hepatic enzymes playing a key role in the antioxidant defence (e.g. SOD, GPx, CAT). Rainbow trout grew better (SGR %/d ) in the L WR (1.98) system compared with the HWR (1.92), regardless of the diet fed . Replacement of inorganic TM with TM-AA complexes by the half has significantly improved digestibility of P , Zn, Mn and Se . Moreover, and regardless of the system type, t he supplementation with TM-AA complexes reduced significantly Fe, Cu, Mn and Se total losses while maintaining TM body retentions similar. The analysis of hepatic TM content revealed a reduction on Fe and Se contents with TM-AA vs TM-I and in the Se content of trout raised at HWR vs LWR system. However, neither the diet n or the system type affected the activity of enzymes playing a key role on antioxidant defense system . Plasma analysis showed higher levels of Mn and lower levels of Zn in trout fed TM-AA vs TM-I diet, regardless of the system type. In summary, our findings prove that replacement of inorganic with TM-AA complexes can offer an interesting strategy to reduce TM losses into the environment, while respecting EU legislation for upper limits of TM and still maintaning fish performance and health.