Microalgae feed additives improve growth, immunity, and resistance to Vibrio anguillarum infection in juvenile rainbow trout, Oncorhynchus mykiss

The present study was conducted to evaluate the effects of three different dietary microalgae on growth, intestinal histology, immune biomarkers, and resistance to a bacterial pathogen (Vibrio anguillarum) in juvenile rainbow trout, Oncorhynchus mykiss. Four experimental diets were prepared, including a basal diet (CON) and three diets containing Chlorella sp. (CHL), Haematococcus sp. (HAE), or Schizochytrium sp. (SCH), at 0.5% for each microalga which was supplemented in the basal diet. A total of 180 juvenile rainbow trout with an initial body weight of 12.16±0.01 g (mean±SD) were randomly distributed into 12 tanks and reared by the semi-recirculation system. After six weeks of the feeding trial, the weight gain (99.4%), specific growth rate (1.92%/day), and myeloperoxidase activity (5.08), of the fish fed the HAE were significantly higher than those of fish fed the other diets (p<0.05). The intestinal villus length of the fish fed the HAE diet (1.34 µm) was significantly higher than that of those fed the CHL (1.13 µm) and CON (1.14 µm) diets. The cumulative survival rates (CSR) were recorded for 27 days after intraperitoneal injection of the bacterial pathogen V. anguillarum. The CSR of the fish fed the HAE diet (75%) was significantly higher than that of those fed the other diets. It is suggested that Haematococcus sp. (0.5% inclusion in the diet) may improve weight gain, specific growth rate, intestinal villus length, and myeloperoxidase activity as well as increase the survival rate of juvenile rainbow trout against the V. anguillarum challenge.