Assessment of Spirulina Residue Meal as a Substitute for Fish Meal in Juvenile Rainbow Trout (Oncorhynchus mykiss) Diets: Impact on Growth, Antioxidative Capacity, Carcass Composition and Hepatointestinal Health

A 7-week study was conducted to investigate the effects of replacing fish meal (FM) with Spirulina residue meal (SPRM) on the growth, feed utilization, carcass composition, antioxidant ability, liver and intestinal histology of juvenile rainbow trout (Oncorhynchus mykiss) (initial body weight 5.36 ± 0.04 g). Four isonitrogenous (42%) and isolipidic (16%) diets were formulated to replace FM protein with SPRM at 0 (SPRM0), 10% (SPRM10), 20% (SPRM20) and 30% (SPRM30), respectively. Results showed that growth, feed utilization, carcass amino acid profile, serum biochemical indices, antioxidant ability, intestinal and liver histology were not significantly affected by dietary SPRM levels. Whole-body lipid content decreased as dietary SPRM replacement levels increased, and fish fed diet SPRM30 had lower lipid content than that fish fed diet SPRM0 (p < 0.05). Fish fed diet SPRM30 had higher C16:1n-7, C20:3n-6, total saturated fatty acid (SFA) and total fatty acid (TFA) contents in muscle than those fed other diets (p < 0.05), while these fatty acids had no change when FM was substituted with 10% and 20% SPRM (p > 0.05). The muscle C22:6n-3 (DHA) content decreased, but C18:3n-6 and n-6/n-3 polyunsaturated fatty acid (PUFA) ratio increased with increasing SPRM levels, and fish fed diet SPRM30 had significantly lower DHA content and higher n-6/n-3 ratio than the group fed SPRM0 (p < 0.05). The C22:1n-9, C18:2n-6c, C20:4n-6, total n-6 PUFA, and monounsaturated fatty acid (MUFA) content in muscle observed in SPRM30 were similar to the SPRM0 group (p > 0.05), but higher than the SPRM10 and SPRM20 groups (p < 0.05). In conclusion, 30% of FM protein could be replaced by SPRM in diets of juvenile rainbow trout without having a significant negative effect on growth, feed efficiency, antioxidant ability, and structure of liver and intestine, but could reduce DHA content, increase n-6 PUFA and n-6/n-3 PUFA ratio in muscle.