Evaluation of various dietary supplements and strategies to enhance growth and disease management of hybrid striped bass Morone chrysops x M. saxatilis
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The US hybrid striped bass (Morone chrysops x M. saxatilis) industry has been negatively impacted by infectious diseases because there are very few approved drugs and vaccines. Therefore, a series of experiments was conducted to explore the potential use of various dietary supplements including autolyzed brewers yeast, the commercial prebiotic GroBiotic?, oligonucleotides and levamisole for improvement of hybrid striped bass growth, immunity and resistance to disease caused by various pathogenic bacteria. In two trials with brewers yeast, fish fed diets supplemented with yeast at 2% generally showed enhanced weight gain and feed efficiency compared with those fed a basal diet. Brewers yeast also positively influenced resistance to S. iniae infection. In addition, results of immune response assays demonstrated that brewers yeast can be administered for relatively long periods without causing immunosuppression. GroBiotic? (Grobiotic) also resulted in significantly enhanced weight gain, innate immune responses and resistance of juvenile hybrid striped bass to S. iniae infection. An additional experiment with sub-adult fish showed significantly reduced mortality of fish fed a diet supplemented with GroBiotic? at 2% when subjected to an in-situ Mycobacterium marinum challenge. This is the first report of positive effects from dietary prebiotics for fish health management, although many fundamental questions should be pursued further. Dietary supplementation of a commercial oligonucleotide product (Ascogen P?) at 0.5% of the diet was shown to enhance resistance of hybrid striped bass against S. iniae infection and increased their neutrophil oxidative radical production. However, the effect on growth was marginal. Dietary levamisole supplementation at a low level (100 mg/kg) enhanced the growth and feed efficiency of juvenile hybrid striped bass. However, an elevated dosage (1000 mg/kg diet) strongly suppressed growth, feed intake and feed efficiency. Hypothesized beneficial influences, including antibody production and resistance to S. iniae and A. hydrophila were not substantiated. Although dietary levamisole increased fish macrophage respiratory burst, an in vitro study failed to show a direct effect on cultured macrophages. This suite of studies demonstrated the potential use of some dietary supplements to enhance hybrid striped bass production. Thus, immunonutrition represents a valuable strategy to apply in aquaculture.