Abstract
The aquaculture industry has consistently grown to meet consumer demands. The rainbow trout (Oncorhynchus mykiss) sector, for instance, has expanded its reputation as a North American recreational fish to a globally cultured and consumed choice food. Despite its advancement, the rainbow trout industry, like many other farm operations, struggles with disease outbreaks. Rainbow trout are susceptible to a variety of diseases, one major bacterial pathogen being Flavobacterium psychrophilum, the causative agent of rainbow trout fry syndrome or bacterial coldwater disease. To manage such diseases, aquaculture has adopted control methods from other agricultural sectors—such as antibiotics—which, while initially effective, pose the risk of promoting antibiotic-resistant strains. Vaccination presents a promising solution, and the use of functional feeds is emerging as another viable strategy for enhancing fish health and disease resistance. Humic acids, which are organic substances derived from the microbial activity on decomposing material, show potential with reports of growth and immunity stimulation. To test their effectiveness in rainbow trout aquaculture, an 8-week feeding trial immediately followed by a 28-day F. psychrophilum challenge was conducted with juvenile rainbow trout.To assess the growth performance effects of humic acids, 750 rainbow trout were fed diets incorporating reed-sedge peat. The experimental diets consisting of 0% humic acid (Diet 1), 0.125% humic acid (Diet 2), 0.2% humic acid + butyric acid (Diet 3), and 0.25% humic acid (Diet 4), were each fed in triplicate. The rainbow trout were measured by weight and length on a biweekly basis, then evaluated based on average individual mass, tank biomass, K-value, specific growth rate, and feed conversion rate. At the end of the feeding period, intestinal tissues from each treatment group were histologically analyzed for signs of inflammation. The experimental diets were found to have no significant effect on the growth performance of the rainbow trout, nor were they associated with significantly different intestinal inflammation by the end of the feeding trial.
Beyond the feeding trial, the immunostimulatory and anti-inflammatory capabilities of the humic acid-supplemented diets were tested through an F. psychrophilum challenge using the rainbow trout from the previous feeding trial. After F. psychrophilum was injected into 60 fish per diet, they were monitored for mortality and sampled for blood, spleen, kidney, and intestinal tissue. The mortality rate, lysozyme concentration, antibody titer, histological inflammation, and immune gene expression were used to examine each diet. Among each treatment group, the cumulative percent mortality, lysozyme activity, antibody titer, and intestinal inflammation presented with no significant differences. Upregulation in pro-inflammatory cytokines IL-8 and IL-1β was observed in the humic acid-supplemented diets. This study indicates that while humic acid-supplemented diets did not significantly impact growth performance or reduce inflammation in juvenile rainbow trout, they did influence immune gene expression, notably by upregulating pro-inflammatory cytokines IL-8 and IL-1β. These findings suggest potential immunomodulatory effects worth exploring in future research.