Investigation Of Natural Feed Additives On Performance, Immunity, And Bacterial Resistance In Cultured Warm Water Fish Species

Abstract

Functional feed additives are a promising option for enhancing production performance and health of several aquaculture species. Through a series of studies, this dissertation research investigated the potential effects of certain functional additives on growth performance, immune responses, gut microbial dynamics, and disease resistance in commercially important freshwater aquaculture species under both experimental and production conditions. We evaluated humic substances (HS), protease complexes (PC), humic substance-butyrate blends (HSB), and yeast cell wall (YCW) as sustainable dietary supplements to enhance fish growth, health, and resilience against bacterial pathogens in intensive aquaculture systems. Across five independent experiments, multiple feeding and bacterial challenge studies were conducted on channel catfish (Ictalurus punctatus), Nile tilapia (Oreochromis niloticus), and largemouth bass (Micropterus salmoides) in indoor recirculating aquaculture systems (RAS) or in pond-supplied outdoor flow-through systems. The experimental methods integrated growth performance assessments; immunological assays targeting non-specific immunity (lysozyme activity, gene expression, or phagocytosis) and the gut microbiota to evaluate host responses to the dietary treatments; and, where applicable, single- and coinfection bacterial challenge models to evaluate disease resistance. In the initial study, continuous feeding of HS- or PC-based diets improved channel catfish survival, increased skin and serum lysozyme activities after 30 days, and altered gut microbial community composition after 60 days. Furthermore, the supplementation of an improved compound, i.e., HSB alone or with YCW for 45 days, enhanced resistance to Edwardsiella ictaluri and bacterial coinfections involving Flavobacterium covae and E. ictaluri in channel catfish raised in RAS, with improvements associated with the expression of antimicrobial peptide, cellular protease in the gill of fish fed the HSB- or YCW diets. In a long-term study (90 days), HSB supported increased growth performance over time, but time, rather than dietary treatment, influenced temporal gut microbiome succession in channel catfish raised in semi-production outdoor tanks, including shifts from opportunistic microbial taxa toward more commensal-associated communities. Subsequently, HSB supplementation improved the growth performance of Nile tilapia raised in raceways supplied with pond water for 50 days. While HSB, YCW, or HSB+YCW had no effect on innate immune responses, YCW enhanced tilapia survival after exposure to Streptococcus agalactiae Ib, and HSB increased resistance to S. agalactiae Ib and Flavobacterium oreochromis coinfections. Importantly, the single F. oreochromis challenge dose overwhelmed the population, resulting in disease severity higher than that of a single S. agalactiae Ib infection or coinfection. In largemouth bass, dietary HSB supported increased weight gain after 60 days in a RAS and increased the fish’s resistance to Aeromonas veronii infections in an in vivo challenge. Meanwhile, neither HSB, YCW, nor HSB+YCW impacted targeted innate immune responses or hematological parameters. Overall, the findings of these trials corroborated several previously reported studies, underscoring the impact of functional additives in fish feed on host performance in aquaculture systems. Specifically, we demonstrated the potential of HS and PC in commercial catfish production and the limitations of continuous HS application on reducing microbiota diversity. However, a blend of HS and BA (HSB) showed potential to improve growth performance, maintain baseline health, and microbiota in a commercial-like system and reduce susceptibility to multiple bacterial infections in channel catfish, Nile tilapia, and largemouth bass. The outcomes of these studies contribute to the growing understanding of interactions among nutrition, immunity, and the microbiome, the impact of polymicrobial infections, and support the development of environmentally responsible and sustainable strategies for modern aquaculture production systems.