To support the rapid growth in the aquaculture industry, there is a need to develop sustainable feed resources as alternatives to fish meal and plant ingredients. Insect meals such as black soldier fly larvae (BSFL) meal are promising alternatives with a low environmental footprint. Previous controlled short-term experiments suggests that BSFL meal is a suitable protein source for Atlantic salmon (1). The aim of this experiment was to investigate how insect meal inclusion impacts growth performance and gut health of Atlantic salmon during the grow-out period in seawater under production field conditions.
A total of 320,000 Atlantic salmon (Salmo salar) with an initial and final weight of 1,200 and 4,500 g, respectively, were fed diets containing either 0, 4, and 8 % defatted BSFL meal (IM0, IM4, IM8) for 5 months in a field experiment on a commercial salmon farm (Nordfjord, Norway). The experimental diets were produced in a twin-screw extruder and the technical quality was assessed for expansion, hardness, durability and sinking velocity. Samples from intestinal content were taken from distal intestine (DI) for microbiota analysis with 16s ribosomal RNA sequencing and DI tissue was histologically analysed with haemotoxylin and eosin, periodic acid-Schiff and Alcian blue staining. In addition, samples were taken from DI for RNA sequencing.
Results and discussion
The physical pellet quality of all diets was high, but expansion, durability and sinking velocity decreased significantly with 8 % BSFL inclusion, indicating that high inclusions of BSFL may reduce pellet quality. No significant differences in growth rate, feed conversion ratio or mortality were observed between the dietary groups. Histological findings were not significantly attributable to diet, but there was a tendency of increased numbers of ectopic goblet cells in the IM4 diet (p = 0.058). For gut microbiota, alpha diversity was not significantly affected by insect meal inclusion, however, beta diversity significantly increased (Fig. 1A and 1B). These findings suggest that insect meal inclusion changed the microbial composition without increasing microbial richness. Bacillaceae and Lactobaciallaceae were significantly more abundant in groups fed BSFL. This is in line with previous studies on salmon smolts  and post-smolts  fed BSFL. Many bacterial species of Bacillaceae can produce chitinase  and lactic acid bacteria are considered beneficial in the fish gut . RNA-seq of the DI revealed that IM4 compared to IM0 showed only down-regulated terms related to lipid metabolism and biosynthesis (Fig. 2A). When comparing IM8 to IM0 (Fig. 2B), two terms were up-regulated (response to estrogen and positive regulation of gene expression) and one was down-regulated (acid biosynthesis). Moreover, the comparison between IM4 and IM8 (Fig. 2C) showed twenty-one up-regulated terms related to immune signaling, cell proliferation and extracellular components, possibly corresponding to the tendency of increased prevalence of ectopic goblet cells in the DI in the IM4 group.
Replacing conventional protein sources with moderate levels of defatted BSFL meal did not compromise growth performance or health in Atlantic salmon under field conditions. These findings suggest that black soldier fly larvae meal at moderate inclusion levels is suitable as a sustainable, alternative protein source for Atlantic salmon in seawater.
 Weththasinghe P, Rocha SDC, Øyås O, Lagos L, Hansen JØ, Mydland LT, et al. Modulation of Atlantic salmon (Salmo salar) gut microbiota composition and predicted metabolic capacity by feeding diets with processed black soldier fly (Hermetia illucens) larvae meals and fractions. Animal Microbiome. 2022;4(1):9.
 Li Y, Bruni L, Jaramillo-Torres A, Gajardo K, Kortner TM, Krogdahl Å. Differential response of digesta- and mucosa-associated intestinal microbiota to dietary insect meal during the seawater phase of Atlantic salmon. Animal Microbiome. 2021;3(1):8.
 Cody RM. Distribution of chitinase and chitobiase in bacillus. Current Microbiology. 1989;19(4):201-5.
 Ringø E, Hoseinifar SH, Ghosh K, Doan HV, Beck BR, Song SK. Lactic Acid Bacteria in Finfish—An Update. Frontiers in Microbiology. 2018;9.