Oral administration effect of yacon, ginger and blueberry on the growth, body composition and plasma chemistry of juvenile olive flounder (Paralichthys olivaceus) and immunity test against Streptococcus iniae compared to a commercial probiotic, Lactobacillus fermentum

•Administration of yacon (YC), ginger (GG) and blueberry (BB) on fish and challenge test against Streptococcus iniae was compared a probiotic..•Administration of YC, GG and BB did not affect growth, feed utilization, body composition and plasma chemistry of fish.•Administration of YC, GG and BB lowered cumulative mortality of fish against S. iniae after the both 4 and 8-week feeding trials.•YC, GG and BB were effective immunostimulant rather than growth promoter of fish. Dietary inclusion effect of yacon (YC), ginger (GG) and blueberry (BB) on the growth, feed utilization, body composition and plasma chemistry of olive flounder and immunity test against Streptococcus iniae compared to a commercial probiotic (Lactobacillus fermentum) was investigated. Six hundred fish were randomly distributed into 15, 50 L flow-through tanks (40 fish per tank). Five experimental diets were prepared in triplicates. No additives were added in the control (CON) diet. The commercially available 0.5% L. fermentum and 1% each of YC, GG and BB powder were included instead of an equal amount of water and wheat flour, referred to as the LF, YCP, GGP and BBP diets, respectively. Fish were daily fed with one of the experimental diets for 8 weeks. At the end of the 4 and 8-week feeding trials, fish were artificially infected with S. iniae and the cumulative mortality was monitored for the next 8 days. No differences in growth performance, feed utilization, body composition and plasma chemistry were observed at the end of the 8-week feeding trial. However, the highest cumulative mortality was observed in fish fed the CON diet starting at 42 h until the end of 8-day post observation after the both 4 and 8-week feeding trials. The cumulative mortality of fish fed the YCP, GGP and BBP diets was comparable to the LF diet. Therefore, YC, GG and BB were effective as immunostimulant against S. iniae rather than growth promoter of olive flounder.