Feeding of rainbow trout (Oncorhynchus mykiss) with organic ingredients replacing fish meal

Demand for organic aquaculture is rising, but its viability will depend on the availability of economically viable raw materials to formulate organic diets. In the current work, organic diets were formulated based on different alternative protein sources distinguished by their ecological origin, insect meal (IN), seabass by-products (SB) and Iberian pig by-products (IB) in rainbow trout (Oncorhynchus mykiss) and their effects on growth, efficiency, productivity, and intestinal health. Fish with an initial weight of 67.2 g were fed two times a day until apparent satiation for 150 days. The control diet containing fish meal (FM) originated the highest final weight (298 g). Results obtained in the final body weight and the specific growth rate feed conversion ratio average daily gain indicate that the SB-FM, SB-IB, and SB-IN-IB diets presented a lower performance (272 g, 257 g, and 258 g final weight respectively) and FM-IN and IN-IB diets had the lowest final weight (215 g and 183 g respectively). An improvement in growth performance and nutrient utilization was observed in the SB-FM, SB-IB, and SB-IN-IB diets concerning the FM-IN and IN-IB diets. The lowest retention efficiencies of protein, fat, and essential amino acids were found in the IN-IB diet. The highest apparent digestibility coefficients (ADCs) of protein, energy, calcium, and phosphorus are found in Control and FM-IN diets. Results of enzymes showed that both trypsin and chymotrypsin are relatively low in IN-IB and SB-IN-IB. Fish-fed FM-IN and IN-IB diets showed histological changes in the liver and intestine. Considering the intestinal microbiota composition, the three dominant phyla were Firmicutes (59–89%), Spirochaetota (5–35%), and Proteobacteria (3–16%), but no differences between diets were obtained. No significant differences were observed on the Alpha diversity Shannon index. Therefore, although differences in growth were observed, the high substitution of fishmeal did not imply an alteration of the intestinal microbiota, possibly due to the high dominance of Firmicutes. Nevertheless, from an economic point of view, SB-IB diets gave the lowest economic conversion index and the highest economic profit index. In conclusion, the substitution of fishmeal affected the growth of the animal, registering the best results in the control followed by diets containing fishmeal of marine origin, but the lowest price of animal by-products originated the best economic results.