Insect meal in fish feeds has been gaining attention over the last decades due to its high energy and protein content that can positively impact growth performance, gut microbiota, immune response and survival. Albeit insect protein is considered a valuable alternative as a partial or complete replacement of fishmeal in aquafeeds, large scale rearing for production is not yet cost-effective. However, research shows that using insect protein as an additive in fish feeds will also induce or influence the aforementioned effects. Therefore, two experiments were conducted to evaluate the specific effects of insect (black soldier fly) larval meal (Protein+, a commercial product developed by Oreka Solutions) as an additive on growth performance and immune response in juvenile rainbow trout (Oncorhynchus mykiss).
Experiment 1 tested Protein+ supplemented at a 7% in commercial diets and fed in triplicate to rainbow trout (1,800 fish, 3.0g initial weight) randomly distributed into 9, 250-L tanks for 21 weeks. Protein+ fed group showed significantly higher growth and lower feed conversion ratio (FCR) than the control group. Gene expression of immune genes tumor necrosis factor beta (TNF-β), tumor necrosis factor alpha (TNF-α), interleukin 8 (IL-8) and interleukin 1 beta (IL-1β) supported the immune enhancement effect that was hypothesized from Protein+.
Experiment 2 further investigated the role of Protein+ in immune enhancement. The first 28 days consisted of a short-term growth performance trial in which a total of 600 rainbow trout (2.0 g) were randomly distributed into 12, 189-L tanks and fed three different diets in four replicate tanks to satiation level including a 0% (control), 4% and 8% Protein+ supplemented diets. Consequently, a second 28-day pathogen challenge trial with Flavobacterium psychrophilum was conducted and fish remained on the same diets used in the growth performance component. For this part of the trial, 300 fish were randomly distributed into 12, 19-L tanks, four tanks per diet, where one of each 4 was mock-challenged. Spleen and kidney samples were collected pre- and post-challenge for gene expression analysis for the same proinflammatory cytokines from Experiment 1. Histological analysis of distal intestine was performed for both and no significant differences (p ≥ 0.05) in morphology of lamina propria, thickness of connective tissue and vacuolization were observed among diets pre- and post-challenge. Results from this study revealed that supplementation of Protein+ did not enhance the significant growth of fish during pre-challenge period and survival rate during challenge study (p ≥ 0.05). However, 4% Protein+ fed group exhibited numerically higher growth and survival rate than the control group. Results from gene expression analysis are underway and will help determine the potential role of Protein+ in immune performance, post-feeding.
Conclusively, results from both experiments revealed that Protein+ can be used as a feed additive. However, further research is warranted to investigate the long term feeding of 4% Protein+ for sustainable salmonid production.