FULL-FAT MEAL AND FRACTIONS OF BLACK SOLDIER FLY Hermetia illucens LARVAE IN DIETS FOR ATLANTIC SALMON Salmo salar: EFFECTS ON NUTRIENT DIGESTIBILITY, GROWTH PERFORMANCE AND GUT HEALTH
Introduction
Black soldier fly larvae (BSFL) (Hermetia illucens) have a great potential as a sustainable novel feed ingredient in fish feed . Numerous studies reported the effects of dietary inclusion of BSFL on fish species including Atlantic salmon ( Salmo salar) (Belghit et al., 2018; Weththasinghe et al., 2021). These studies have proposed that the fractions of BSFL might differently affect the fis h. However, to the best of our knowledge, the effects of different fractions of BSFL have still not been evaluated in a single study. T herefore, the present study investigated the effects of f ull-fat meal and fractions of BSFL in diets on nutrient digestibility, growth performance and gut health in Atlantic salmon pre-smolts.
Materials and methods
Six experimental diets were produced: a control diet based on fishmeal, plant protein sources and fish oil (CD); full-fat BSFL meal diet (IM), defatted BSFL meal diet (DFIM); de-chitinized BSFL meal diet (DCIM); BSFL oil diet (IO) and BSFL exoskeleton diet (EX). The full-fat, defatted and de-chitinized meals replaced 15% of the protein content of the control diet. A total of 900 Atlantic salmon pre-smolts with 28 g of mean initial weight were distributed into 18 fiberglass tanks (50 fish per tank ), and fed with one of the six experimental diets. The growth performance parameters and nutrient digestibility in fish were estimated. The histological evaluations of pyloric caeca and distal intestine of fish were done . In addition, immunological parameters (IL- 1 β and IgM) in distal intestine were evaluated by indirect ELISA . The RNA sequencing of distal intestine and 16S rRNA sequencing of gut microbiota of fish are ongoing.
Results and discussion
The full-fat and de-chitinized meals improved the growth rate of salmon, while defatted meal , oil and exoskeleton fraction supported similar growth performance in fish as the control. Furthermore, t he improvement in the growth rate of fish fed the full-fat meal diet was accompanied by higher feed intake, whilst d efatted meal gave a better feed conversion ratio than the full-fat meal. Replacement of dietary protein with defatted meal, de-chitinized insect meal or exoskeleton fraction reduced protein digestibility, whereas neither full-fat meal nor fractions affected lipid digestibility (Table 1).
In the pyloric caeca, fish fed de-chitinized insect meal diet showed low occurrence and severity of enterocyte steatosis compared to control and full-fat meal diets (Figure 1A). Evaluation of the distal intestine revealed normal and healthy morphology for most of the fish . The fish fed de-chitinized meal showed a higher level of pro-inflammatory cytokine IL-1β in the distal intestine compared to the fish fed the control diet (Figure 1B), whereas distal intestine IgM level showed no difference between the fish fed insect diets and the control diet.
Conclusions
In conclusion, t he full-fat BSFL meal improved feed intake and growth rate in salmon when replacing 15% of dietary protein, however, defatted meal gave a better feed utilization than full-fat meal. Dietary inclusion of d e-chitinized BSFL meal improved the histology of pyloric caeca of fish by reducing enterocyte steatosis and increased pro-inflammatory IL-1 β level in the distal intestine. These results contribute to establish a baseline for the study of insect-based diets that are capable of modulating the immune system of fish .
References
Belghit et al., 2018. Potential of insect-based diets for Atlantic salmon (Salmo salar ). Aquaculture. 491, 72-81.
Weththasinghe et al., 2021. Full-fat black soldier fly larvae (Hermetia illucens) meal and paste in extruded diets for Atlantic salmon (Salmo salar): Effect on physical pellet quality, nutrient digestibility, nutrient utilization and growth performances. Aquaculture. 530, 735785.
