Black soldier fly (BSF) (Hermetia illucens) has been identified as a promising novel ingredient in fish feeds. The knowledge on optimal downstream processing of BSF is, however, lacking in the literature. The main objective of this thesis was to contribute to increased knowledge on this topic. This thesis presents five papers.
Paper I investigated the dose-dependent effects of two differently processed full-fat BSF larvae products in diets for Atlantic salmon (Salmo salar) on technical pellet quality, nutrient utilization, and growth performance. Atlantic salmon pre-smolts were fed full-fat BSF meal or full-fat BSF paste, added to extruded diets, replacing increasing levels of protein from fishmeal and plant protein sources (6.25, 12.5 and 25% by meal and 3.7 and 6.7% by paste). The expansion and water stability of feed pellets numerically decreased with increasing inclusion of BSF meal and paste. Replacement of up to 12.5% and 6.7% of protein with full-fat BSF meal and paste, respectively, did not compromise fish growth performance, but growth rate decreased at 25% meal replacement. Protein digestibility decreased linearly with increasing dietary level of BSF meal or paste, but the protein retention was not affected by dietary BSF inclusion.
Subsequently, Paper II investigated the dose-dependent effects of dietary full-fat BSF larvae meal and paste on gut health and systemic immune responses in Atlantic salmon. Replacement of up to 12.5% protein with meal and 6.7% with paste improved gut health by improving distal intestine histology and reducing enterocyte steatosis in pyloric caeca. Replacing 25% protein with full-fat meal did not cause inflammatory changes in distal intestine histology but increased the level of pro-inflammatory cytokine IFNγ in the distal intestine and gave mild to moderate enterocyte steatosis in pyloric caeca. The BSF meal and paste in diets caused minor effects on protein expression in skin mucus and systemic immune responses in fish.
Paper III investigated the effect of inclusion of BSF larvae meals and fractions in Atlantic salmon diets on technical pellet quality, nutrient utilization, and growth performance. Atlantic salmon pre-smolts were fed extruded diets containing meals (full-fat, de-fatted or de-chitinized meals) or fractions (oil or exoskeleton) of BSF. Meals replaced 15% protein from fishmeal and plant sources in a control diet. Lipid-rich full-fat and de-chitinized meals in the diets numerically reduced expansion and water stability of pellets. Full-fat and de-chitinized meals improved fish growth rate, while de-fatted meal, oil and exoskeleton diets supported similar growth performance as the control diet. Full-fat BSF meal also gave a higher growth rate and feed intake than other meals and fractions. Nevertheless, de-fatted meal and exoskeleton gave a better feed conversion than full-fat meal. Full-fat meal and oil did not compromise protein digestibility, but de-fatted and de-chitinized meals and exoskeleton reduced protein digestibility. Protein retention was not affected by BSF meals and fractions in the diets, except for the de-fatted meal, which increased the retention of digested protein.
Paper IV investigated how dietary BSF larvae meals and fractions affect the gut microbiota composition and their predicted metabolic capacity in Atlantic salmon. The diets that contained BSF chitin, i.e., BSF meals and exoskeleton diets, increased the abundance of chitinolytic Lactobacillales and Actinomyces in fish gut. The diets that contained BSF lipids, i.e., BSF meals and oil diets, increased the abundance of Bacillaceae. Fish fed full-fat meal had a phylogenetically diverse and unique gut microbial composition, dominated by beneficial lactic acid bacteria and Actinomyces, and showed a predicted increase in microbial mucin degradation.
Overall, differently processed BSF could partially replace protein and lipid in Atlantic salmon diets without compromising the growth performance, and fish response to dietary BSF varied with the protein replacement level and processing method. At 15% protein replacement, full-fat meal improved growth performance in salmon, and de-fatting and de-chitinization did not further improve fish growth.
Additionally, in Paper V, we conducted a meta-analysis to determine the effect of dietary BSF on nutrient utilization and growth performance in salmonids including Atlantic salmon and rainbow trout (Oncorhynchus mykiss). This analysis showed that dietary BSF did not compromise the protein utilization and growth performance in salmonids. Nevertheless, replacement of fishmeal by BSF decreased growth rate and feed intake in salmonids, but replacement of non-fishmeal sources improved growth rate and feed conversion. This stresses the importance of the protein source(s) used in the control diet when evaluating nutritional value of BSF
