Evaluation of brewers yeast (Saccharomyces cerevisiae) as a feed supplement for hybrid striped bass (Morone chrysops x M-saxatilis)

Two separate feeding trials were conducted to evaluate graded levels of dried brewers yeast in the diet of hybrid striped bass. A basal diet was formulated to contain 40% protein, 10% lipid and an estimated digestible energy level of 3.5 kcal/g. In Trial 1, three incremental levels of dried brewers yeast (1%, 2% and 4% of diet) were added to the basal diet in place of cellulose. In Trial 2, the same levels of brewers yeast were added to the basal diet, but menhaden fish meal and menhaden oil were adjusted to provide isonitrogenous and isolipidic diets. Each diet was fed to three (Trial 1) or four (Trial 2) replicate groups of juvenile hybrid striped bass twice daily at rates approximating apparent satiation for 6 or 8 weeks. After the second feeding trial, a Streptococcus iniae bath challenge was executed to test the effects of diet on disease resistance. Enhanced weight gain and feed efficiency were generally observed in fish fed the diets supplemented with yeast compared to the basal diet in both trials. In the second trial, body composition of whole fish, hemocrit and serum lysozyme levels were observed to be within normal ranges and not influenced by the various dietary treatments. After 9 weeks of feeding in the second trial, exposure to S. iniae resulted in no mortality and reduced signs of disease in fish fed diets supplemented with 2% and 4% brewers yeast, while 20% mortality was observed in fish fed the control diet (P=0.1). In the second trial, blood neutrophil oxidative radical production, extracellular and intracellular superoxide anion production of head kidney macrophages and serum lysozyme were measured after 16 weeks of feeding each diet. Fish fed the diet with 2% brewers yeast were found to have significantly (P<0.01) higher blood neutrophil oxidative radical and extracellular superoxide anion production of head kidney macrophages than control fish. However, no significant differences in intracellular superoxide anion and serum lysozyme were observed among the treatments. Based on the result of this study, it is concluded that brewers yeast positively influenced growth performance and feed efficiency of hybrid striped bass as well as resistance to S. iniae infection. In addition, results of immune response assays demonstrate that brewers yeast can be administered for relatively long periods without causing immunosuppression. (C) 2003 Elsevier Science B.V. All rights reserved.