bstract
Dietary supplements such as immunostimulants and prebiotics hold promise as a potential replacement of antibiotics in
maintaining fish health. A 21-week feeding trial was conducted to evaluate the commercial prebiotic GroBioticR-A, a
mixture of partially autolyzed brewers yeast, dairy ingredient components and dried fermentation products, in the diet of
hybrid striped bass exposed to chronic mycobacterial infection caused by Mycobacterium marinum, as compared to partially
autolyzed brewers yeast (BrewtechR). The basal diet was formulated to contain 40% protein, 10% lipid and an estimated
digestible energy level of 3.5 kcal/g. Supplements of 1 or 2% brewers yeast and 2% GroBioticR-A were singularly added to
the basal diet and each diet was manufactured by extrusion processing with a twin-screw extruder. Each diet was fed to three
replicate groups of small (initially averaging 64.5 g/fish) and one group of large (initially averaging 118 g/fish) hybrid striped
bass in 1187-l circular tanks operated as a recirculating system. Fish were fed twice daily to apparent satiation and growth
performance monitored for 16 weeks. An in situ infection of M. marinum became well established at week 16 such that fish
were fed once daily and mortality was monitored for a total of 21 weeks.
Enhanced growth performance was generally observed in fish fed diets supplemented with GroBioticR-A or brewers
yeast compared to fish fed the basal diet throughout the feeding trial with significantly ( P b0.05) enhanced weight gain
observed after 12 weeks of feeding. At the end of the feeding trial, fish fed 2% brewers yeast had significantly higher feed
efficiency than fish fed the other diets. The in situ mycobacterial challenge employed in this experiment resulted in overall
cumulative mortality of approximately 25%. Fish fed 2% GroBioticR-A had a significantly ( P b0.05) enhanced survival
(80%) compared to the other treatments (72–73%) at the end of 21 weeks. It is concluded that dietary supplementation of
2% GroBioticR-A showed moderate but significant ( P b0.05) protection against mycobacterial infection. Dietary