RAS-Designed Diets Result in Lower Accumulation of Nitrogen, Phosphorus, and Zinc in Recirculating Aquaculture System Compared with Traditional Flow-Through Designed Diets

A four-week trial was conducted to compare the effects of a conventional flow-through system diet (FTS) and an experimental RAS diet (ERAS) on fish performance, water quality and general system implication in a replicated recirculation aquaculture system (RAS). Six identical RAS, each with a total system water volume of 1500 L and cylindrical rearing tanks of 1000 L were stocked with Atlantic salmon (<i>Salmo salar</i>) post-smolts with an average weight of 199.7 ± 28 g, to an average stocking density of 30 kg/m³ and reaching approximately 49 kg/m³ at the trial end. The ERAS diet were composed to inhabit typical RAS feed characteristics compared with the FTS diet, such as a higher fecal stability and reduced protein levels (−12%), but at the same time increased fat content (+8%) to secure similar gross energy levels (22–23 MJ kg⁻¹) between the two diets. Water quality parameters were measured individually. The total accumulation of minerals and metals was analyzed in water from different parts of the system at the start and end of the trial period for both diets. No differences in growth, condition factor, feed conversion rate (FCR), or survival of fish fed the two dietary adaptations were observed. The system using the ERAS diet showed significantly higher pH (+1.2%) and alkalinity (+17%) and lower total ammonia nitrogen (TAN) (−18%) and NO₂⁻ (−46%) compared with the FTS diet. The count of drum filter activations was also significantly lower (−13%) with the ERAS diet. Compared with the FTS diet, the ERAS diet had a lower probability (−4%) of generating particles smaller than 50 μm, and that the RAS was also more effective in removing particles from the drum filter, prompting a lower daily activation of the filter of 22.1 ± 3.0 counts compared with 25.5 ± 3.5 for the FTS diet. Mineral analysis showed a significantly lower accumulation of total phosphorus (TP) (−90%) and dissolved phosphorus (DP) (−92%) in the RAS units using the ERAS diet compared with those using the FTS diet. Compared with a traditional flow-through diet, these results highlight the benefits of using an RAS-adapted diet that matches the energy requirement of flow-through diets regarding water quality, system performance, satisfactory growth, and condition.