The effects of dietary heat-killed probiotics bacteria additives in low-fishmeal feed on growth performance, immune responses, and intestinal morphology in juvenile olive flounder Paralichthys olivaceus

•The Bacillus sp. SJ-10 and Lactobacillus plantarum were heat-killed and used as additives in a low-fishmeal diet.•HKBSJ-10 supplemented diets improved juvenile olive flounder growth and immunity.•HKBSJ-10 supplemented diets increased plasma insulin, GH, IGF-I activity in juvenile olive flounder. Fishmeal is an important source of protein in the fish diet, but its supply is unstable due to environmental pollution and overfishing of anchovy and herring. In addition, the disease outbreak of fish has been increased, and the abuse of antibiotics and vaccines has been becoming a problem for residual substances. Therefore, studies that address diets with reduced fishmeal content and additives that can increase efficiency and replace antibiotics are needed. Hence, in this study, the probiotics bacteria Bacillus sp. SJ-10 (BSJ-10) and Lactobacillus plantarum (LP) were heat-killed (HK) used as additives in a low-fishmeal diet. The two bacteria with a 45 % fishmeal diet were added at a concentration of 3.34 × 108 CFU/mL, and both of probiotics bacteria were mixed and added to the same concentration in order to produce the diet for HK BSJ-10 (HKBSJ-10), HK LP (HK-LP), and MIX (HKBSJ-10 + HK-LP) groups. We raised four replicates of juvenile olive flounder for 8 weeks. In the HKBSJ-10 group, the plasma insulin, GH, and IGF-I level were higher than the other groups (P