Abstract

Functional feed ingredients with antimicrobial activities are being explored as a tool to mitigate diseases in Atlantic salmon (Salmo salar L.). Skeletonema marinoi is a marine diatom known to have antimicrobial activity. Here, two separate experiments, including a growth study (Exp. 1) and a bacterial challenge (Exp. 2), were conducted to evaluate the effects of dietary S. marinoi as a functional feed ingredient on the growth, feed utilization, stress biomarkers, antioxidant defense, and immunogenic responses of Atlantic salmon. In Exp. 1, a 16-week growth trial was conducted in a recirculating aquaculture system (RAS) stocked with ten fish (59.6 ± 1.6 g, initial weight) in each of five replicate tanks per treatment. Fish were fed twice daily with either the reference diet or the reference diet supplemented with 0.5% S. marinoi. In Exp. 2, a 27-day bacterial challenge with Vibrio anguillarum Serotype O3 (SO3) was conducted in RAS using 36 fish (100–150 g) per 150 L tank in triplicate. Fish were acclimatized to the RAS for 14 days and fed twice daily with the reference or 0.5% S. marinoi diet prior to the bacterial challenge. Blood plasma, liver, and intestine samples were collected after the growth trial, before (initial, 0 day) and post-challenge (5, 10, 27 days). The results indicated that in Exp. 1, there were no significant differences in survival, final weight, weight gain, feed intake, feed efficiency, condition factor, and hepatosomatic index between fish fed the reference diet and 0.5% S. marinoi diet (p > 0.05). In Exp. 2, before challenge, Atlantic salmon fed the diet with 0.5% S. marinoi showed significantly higher superoxide dismutase (SOD) activity in comparison to fish fed the reference diet. On day 5 post-challenge, significantly higher hepatic malondialdehyde (MDA) was measured in the 0.5% S. marinoi group compared to the reference group. Plasma immunoglobulin M (IgM) level increased (p < 0.05), and intestinal IgM gene expression was up-regulated after challenge in fish fed 0.5% S. marinoi, in contrast to those fed the reference diet at 27 days post-challenge (p < 0.05). Overall, these findings suggest that S. marinoi is more likely to be associated with immune modulation rather than adverse stress response; however, further work is required to confirm pathogen-specific immunity.

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