Rainbow trout production is one of the most significant aquaculture segments of U.S. fish farming, with most of the industry located within Idaho and North Carolina. Bacterial pathogens, such as Flavobacterium psychrophilum, are largely responsible for mortalities within this sector. The industry has access to only three approved antibiotic treatments, and antimicrobial resistance has become a concern for fish producers. U.S. aquaculture has shown a great interest in antibiotic alternatives, such as probiotics or immunostimulatory compounds. A probiotic strain of Bacillus velezensis has been previously shown to confer protection against warm water pathogens, such as virulent Aeromonas hydrophila (channel catfish) or Streptococcus iniae (Nile tilapia). Given these health-related benefits, a study was designed to evaluate the application of this probiotic in rainbow trout fry throughout the early rearing period.
Rainbow trout eyed eggs were obtained from a commercial source (Troutlodge) and were incubated within Heath trays (12°C) before transfer into aquaria following hatch and swim-up. Newly hatched fry were fed a probiotic-amended diet (1×107 CFU g-1) or a reference diet (without probiotics) for 30 days (Fig. 1). Each dietary treatment was assigned to a separate recirculating aquaculture system with five 50L replicated aquaria stocked with 100 yolk-sac fry per aquarium. Throughout the rearing period, water quality was maintained as follows: temperature (13.4±0.2°C), dissolved oxygen (10.9±0.1 mg/L), pH (7.72 ± 0.5), total ammonia nitrogen (0.18 ± 0.3 mg/L), nitrite (0.07 ± 0.02 mg/L), nitrate (20.3 ± 17.1 mg/L), hardness (96 ± 6 mg/L), alkalinity (95 ± 3 mg/L). Every week, whole fish were sampled for biometric measurement, and samples were flash-frozen for immune gene expression. Fish are currently being measured using ImageJ for seven morphometric characteristics, including standard length, notochord length, yolk sac area, eye diameter, jaw length, myotome height, and body area. Proinflammatory cytokines (i.e., IL-1b, IL-8, and TNF-a) are now being assessed for gene expression using qPCR. Together, these data will shed light on probiotics and molecular ontogeny of their immune system during early larval development during early rearing stages for cold-water species.