The oil fraction and partially defatted meal of black soldier fly larvae (Hermetia illucens) affect differently growth performance, feed efficiency, nutrient deposition, blood glucose and lipid digestibility of rainbow trout (Oncorhynchus mykiss)

The effects of partially defatted black soldier fly larvae meal (BSFLM) and mechanically-extracted oil from that same insect (BSFLO) on growth, feed utilization, body composition, gut histology and blood plasma biochemistry of rainbow trout were evaluated over a three-month growth trial. Seven experimental diets containing 0 (Diet A), 6.6 (Diet B), 13.2 (Diet C) and 26.4% (Diet D) BSFLM, and 2.5 (Diet E), 5.0 (Diet F) and 10% (Diet G) BSFLO were randomly allocated to 21,140-l tanks at 30 trout (45.7 ± 1.4 g) per tank. To estimate the nutrient digestibility of these ingredients, Diet A was supplemented with 0.5% titanium dioxide and served as the Reference diet to create two test diets containing 80% of the Reference diet combined with 20% of defatted BSFLM (diet B-test) and 20% of BSFLO (C-test). Water temperature was maintained at 13.9 ± 0.3 °C in both trials. Growth performances, measured using the thermal-unit growth coefficient (TGC), were high with TGC values ≥ 0.260, regardless of BSFLM or BSFLO inclusion. Feed conversion ratios (FCR) were ≤ 0.91. However, TGC was significantly lower with Diet D (26.4% BSFLM) (P < .020) and FCR values increased significantly at each increment level of BSFLM (P < .05). There was a negative relationship between relative body protein content and dietary inclusion of BSFLM, but not BSFLO (P < .001). Hydroxyproline was the sole amino acid that increased in trout fed BSFLM or BSFLO. The inclusion of BSFLM and BSFLO had no impact on the histology of the posterior intestine, but villi in the anterior intestine of trout fed Diet D (26.4% BSFLM) was significantly shorter relative to the Control (1035 ± 146 μm vs. 1176 ± 160 μm). There was a negative relationship between blood glucose and BSFLO inclusion level, which suggested BSFLO had some antihyperglycemic effect in fasted rainbow trout. The ADCs of protein and amino acid in BSFLM varied between 87 and 93%, but the digestibility of lipid and dry matter in BSFLM was relatively low at 73 and 75%, respectively. The digestibility of hydroxyproline was significantly superior in diets containing BSFLM or BSFLO than in the Reference diet (P = .014). Based on these results, the maximum inclusion of BSFLM recommended in rainbow trout diets is 13%. The maximum inclusion of BSFLO could potentially be superior to 10%, but this hypothesis needs further study. The enhanced digestibility of hydroxyproline in diets containing BSFL products has potential benefits for fish fed low fishmeal diets.