Growth performances of rainbow trout (Oncorhynchus mykiss) juveniles fed on a diet containing processed fish as a partial substitute for fish meal

The recovery of valuable nutrients from fishery discards and seafood processing is a promising alternative to the use of fish meal and fish oil, which are considered limited resources and non-sustainable feedstuffs. Studies focused on the replacement of fish meal with alternative sources, also consider the quality traits of by-products derived from wild and farmed aquatic organisms. In the past, these waste by-products were discarded without any attempt to recover or process into low market-value products, such as pet food and fertiliser. Recently, it has been ascertained that they still contain a fair amount of nutrients that could be better employed. By-products typically include muscle cuts (15– 20% of the whole fish), skin and fins (1–3%), bones (9–15%), heads (9–12%), viscera (12–18%) and scales (5%). The use of discarded raw materials and processed by-products can reduce the pressure on fish stocks as well as diminish the waste and negative environmental impact associated with the processing of aquatic animals. The present paper aims to evaluate the growth performances of rainbow trout juveniles fed on a diet including processed meal, obtained from seafood processing, as a partial replacement (50%) of fish meal. The processed meal was prepared after having extracted oil from discarded fish, which had been grounded and cooked at 70 °C, utilising both a hydraulic press and centrifugation. The results were compared to those obtained by a control group, represented by conspecifics of the same initial mean body weight (25 ± 2 g), fed on a conventional diet (CD) composed of fish meal as the main protein source. The two feeds were isonitrogenous (45%) and isolipidic (21%). The fish were reared in duplicate tanks/group, with a recirculating aquaculture system. Growth performances and food conversion rate were evaluated after 90 days of feeding. Good productive parameters were obtained in the two groups with similar performances. No significant differences were found in the final mean body weight, which ranged from 105 g and 110 g. The feed conversion ratio was around 1.2 and the survival rate was around 98% in both groups. From an economic point of view, considering production and feed costs, the results suggest that there is an economic return in replacing 50% of the fish meal protein with processed seafood when this replacement is performed during the rainbow trout pre-growing phase.