Evaluation of dried yeast and threonine fermentation biomass as partial fish meal replacements in the diet of red drum Sciaenops ocellatus

In response to the increasing demand and thus cost of fish meal (FM), alternatives to this ingredient have been extensively investigated in recent years. As such, research on various types of alternative protein feedstuffs, such as industrial microbial byproducts, is being pursued for FM replacement. To this end, the purpose of this study was to determine the efficiency of two single-cell protein sources, dried yeast (DY) and threonine fermentation biomass (TFB) as partial replacements for FM in the diet of red drum. A reference diet was formulated to contain 40% crude protein (CP) equally contributed from FM and soybean meal (SBM). In addition, four experimental diets were formulated with different inclusion levels of DY to supply 20, 30, 40 or 50% of dietary CP, and three additional diets had different inclusion levels of TFB to provide 10, 20 or 30% of dietary CP. Red drum juveniles were sorted by size (initially averaging 8.01 ± 0.29 g), and groups of 20 fish/aquarium were stocked into 30, 110-L aquaria. After feeding the reference diet for a 1-week conditioning period, triplicate aquaria were randomly assigned to each dietary treatment and fish were fed to apparent satiation for 8 weeks. In regard to weight gain, feed efficiency and survival, increasing inclusion levels of DY and TFB did not negatively affect these responses. Therefore, it can be concluded that, for the conditions in which this experiment was conducted, TFB and DY could replace up to 30 and 50% of the protein provided by FM/SBM (50/50%), respectively, without affecting red drum's performance. In addition, distinctly different microbial communities in the gastrointestinal tract of fish fed the various diets was evident based on a denaturing gradient gel electrophoresis dendrogram analysis. In conclusion, DY and TFB are suitable alternatives for replacing FM in the diet of red drum, although more studies should be done to determine their maximum inclusion levels.