In recent years, the effective immunostimulating properties of β-glucans have been widely proven not only in mammals, but also in aquatic animals. The aim of this study was to determine the effect of dietary β-glucan supplements on the percentage of individual types of leukocytes and the haematocrit of the sea trout (Salmo trutta L.).

This study was performed to determine the effects of dietary supplementation with β-glucan on the growth performance and skin-mucus microbiota of sea trout, Salmo trutta L. in Latvia. The investigations were performed during an eight-month period (September 2018 – April 2019). A total of 15,000 sea trout were divided into five groups. The experimental fish were fed formulated diets enriched with 1 g kg−1 β-glucan (D2), 3 g kg−1 β-glucan (D3), 6 g kg−1 biological product BGN-2 (BGN-2) (D4), and 14 g kg−1 BGN-2 (D5). The control diet (D1) was not supplemented. Our results showed that fish fed diets D4 and D5 achieved significantly (P < 0.05) higher growth parameters compared to those fed the other diets. Pseudomonas and Aeromonas were detected as the main component of fish skin and gill microbiota. Beta-glucan did not affect the skin-mucus microbiota of the sea trout. All isolates were resistant to amoxicillin, ampicillin, cefalexin, and erythromycin and susceptible to gentamicin. The multiple antibiotic resistance index for all isolates was higher than 0.2.