Performance and physiological consequences of completely replacing soy protein in rainbow trout (Oncorhynchus mykiss) diets with semi-defatted black soldier fly (Hermetia illucens) larval meal

In recent years, the aquaculture industry has seen an increasing substitution of fishmeal (FM) with plant-based ingredients. However, challenges inherent in the use of such ingredients drive an ongoing search for sustainable and cost-effective alternatives. To study the potential of high insect protein content in rainbow trout diets, a dose-response study was set up to assess the effects of replacing up to 100 % of soy protein concentrate (SPC) with a meal of semi-defatted black soldier fly (Hermetia illucens) larvae. In a 10-week feeding experiment, a homogeneous group of 1100 rainbow trout (initial body weight: 135.8 ± 15.3 g) were supplied with 10 different feeds: a commercial soy-based control and diets in which the SPC content was replaced with increasing proportions of black soldier fly larval meal (BSF). All diets also incorporated the same low fishmeal content (7.5 %) and were tested with (Control+, 25BSF+, 50BSF+, 75BSF+ and 100BSF+) and without (Control, 25BSF, 50BSF, 75BSF and 100BSF) a faecal binder treatment. At the end of the experiment, growth performance, feed utilisation, organosomatic indices, and fillet yields were determined alongside histological and transcriptomic analysis of the liver and intestine. Results indicated that substitution with BSF was associated with increased feed intake and a significant, although non-linear, improvement in growth, hinting at nutritional deficiencies in the commercial SPC controls. However, fish fed with insect-free diets exhibited improved protein retention and feed conversion ratio (FCR). No significant differences were apparent in intestinal or liver histology, gene expression or fillet processing yield between treatments, indicating that even a complete replacement of SPC with BSF can take place without compromising rainbow trout health or productivity. This study is a pioneer in demonstrating that a complete substitution of SPC with BSF is possible without adversely affecting the performance of rainbow trout in aquaculture, highlighting once more the potential of these insects as an alternative for feeding salmonids in the near future, as long as regulations worldwide enable more sustainable production of insects for feed purposes.