Growth performance, hepatic gene expression, and plasma biochemistry of rainbow trout fed full-fat meal, defatted meal, oil and chitin from black soldier flies

Insects, such as black soldier flies (BSF), are sustainable alternatives to fishmeal and fish oil that have been shown maintain fish growth performance, especially at low dietary inclusions. However, BSF is commonly processed to remove chitin and oil that may have additional benefits to the immune response and disease resistance of fish. The aim of this study was to investigate the hepatic gene expression and plasma biochemistry of rainbow trout (Oncorhynchus mykiss) fed BSF and BSF components. A three-month feeding trial was performed where rainbow trout (90.0 ± 4.0g; mean ± SD) were fed either a control diet or diets that contained: 10% full-fat BSF, 10% defatted BSF, 4% BSF oil, and 1% BSF chitin. Liver was analyzed for gene expression via qPCR and plasma was analyzed for 21 biochemical markers. The 4% oil and 1% chitin diets had the highest effect on growth performance, i.e. final weight (p=0.043) and feed intake (p=0.017), compared to the control diet, while no effects of these diets were found on hepatic gene expression. The 10% full-fat BSF diet affected gene expression, specifically upregulating the pro-inflammatory cytokine IL-8 (p=0.033), which may be due to the higher content of saturated fatty acids (SFA) and subsequently immune cell activity in the fish or the combination of the SFA and antimicrobial peptides in BSF that stimulate the innate immune system. No other immune and oxidative stress genes, or 21 plasma biochemical markers were found to be significantly affected (p>0.05). This research provides support to include and not remove oil and chitin from BSF since these components can improve growth performance and immune response of rainbow trout.