Nutritional evaluation of partially defatted Hermetia illucens meal in rainbow trout (Oncorhynchus mykiss) diets under commercial-like conditions

This study evaluated the potential of black soldier fly (BSF; Hermetia illucens) meal as a sustainable protein source in rainbow trout (Oncorhynchus mykiss; initial average weight of 117.1 g) diets under commercial-like conditions (water temperature of 15 °C). The research focused on assessing the in vitro and in vivo digestibility of essential amino acids and overall growth performance in response to incremental dietary inclusion levels of partially defatted BSF larvae meal (2.5%, 5%, and 10%) as partial replacements for fishmeal. Experimental diets were formulated to be isonitrogenous, isolipidic, and isoenergetic. In vitro assays showed that protein hydrolysis was generally efficient but declined at higher BSF inclusion levels, likely due to the increased chitin content hindering protease activity. In vivo trials demonstrated high apparent digestibility coefficients (ADCs) for the BSF meal, with ADCs for protein and amino acids decreasing at 5% and 10% inclusion levels. A strong positive correlation (r2 = 0.987) was found between in vitro hydrolysis and in vivo protein digestibility. Growth performance, including specific growth rate, protein efficiency ratio, and somatic indices, remained within optimal physiological ranges across all treatments. However, a significant increase in feed conversion ratio was observed at 10% inclusion, potentially linked to reduced methionine availability. The findings validate the nutritional feasibility of using BSF meal up to 10% in rainbow trout diets, aligning with previous experimental-scale trials. While promising, further research is recommended to enhance nutrient bioavailability, particularly of essential amino acids, to support wider adoption in aquaculture systems.