Introduction
The intensification of aquaculture practices has heightened the need for sustainable strategies to improve fish health and reduce dependency on antibiotics. Atlantic salmon (Salmo salar), a cornerstone species in global aquaculture, is particularly susceptible to stress and disease under intensive farming conditions. Recent research has highlighted the immunomodulatory potential of natural compounds, such as 1,3-β-glucans derived from microalgae, which may enhance host defense mechanisms and overall physiological resilience. As a functional feed additive, algal 1,3-β-glucan represents a promising approach to improve immune responses, growth performance, and survival rates in salmon aquaculture, aligning with the industry’s shift toward more sustainable and health-oriented production systems.
This study evaluated the effects of dietary algae-derived 1,3-β-glucan supplementation on growth performance, immune response, and disease resistance in Atlantic salmon (Salmo salar). A total of 360 fish from the Lake Superior State University Aquatic Research Laboratory were assigned to control and treatment groups receiving 250–1000 g/ton of algal 1,3- β-glucan over six months. Key parameters assessed included condition factor, immune activity, and resistance to Aeromonas salmonicida (Furunculosis). In addition, effect of algal 1,3-β-glucan on reducing thermal stress in rainbow trout (Onchorhynchus mykiss) have been assessed.
Materials and methods
This six-month study evaluated the effects of dietary algal 1,3-β-glucan supplementation on growth, immunity, and disease resistance in Atlantic salmon fry (6–8 inches) at the Lake Superior State University Aquatic Research Laboratory. All experiments were conducted at 10–12°C in a controlled environmental chamber.
Phase I (Days 0–14): Fish (n=360) were acclimated in twelve 30-gallon tanks (30 fish/tank), with standard feed provided twice daily. Length and weight measurements were recorded on days 0 and 14 to assess baseline condition.
Phase II (Days 15–45): Fish were divided into four treatment groups: Control (no β-glucan) , and three algal 1,3-β-glucan doses - 250, 500, and 1000 mg/kg feed. Condition factors, immune parameters, and hematological indices were assessed on days 30 and 45.
Phase III (Days 46–75): Fish were challenged with Aeromonas salmonicida (1 × 10⁵ CFU/fish) via intraperitoneal injection. Positive controls received the pathogen, while negative controls received PBS. Daily mortality was recorded for 30 days. Additional immune and hematological assessments were conducted on days 60 and 75.
Health and Immune Assessment: Growth and condition were monitored via weight gain, length:weight ratio, and condition factor. Hematological parameters including red and white blood cell counts, hematocrit, and hemoglobin were measured using hematological assays.
Thermal stress study: E fficacy of algal 1,3-β-glucan on reducing heat stress was assessed by measuring the scale cortisol level. Treatment groups involved negative control, cortisol control (CORT 500), 500 ppm fulvic acid, 250 ppm water soluble (WS) algal 1,3-β-glucan , 500 ppm fulvic acid + 250 ppm WS algal 1,3-β-glucan and 250 ppm fulvic acid + 125 ppm WS algal 1,3-β-glucan.
Results
Survival rate was linearly corelated with increasing doses of algal 1,3- β-glucan. The highest survivability was observed in the algal 1,3- β-glucan at 1000 g/MT (90%), compared to 46% in the control group.
A significant improvement in the growth was observed between control and algal 1,3-β-glucan treated groups (500g/MT and 1000g/MT). There were n o statistically significant differences observed in hematocrit, or red blood cell counts among treatment groups. These findings support the potential of algal 1,3-β-glucan as a functional feed additive in sustainable salmon aquaculture.
The scale cortisol level at the start was on average 0.056±0.040 µg/kg. After 3 weeks, the scale cortisol in CORT500 group and Fulvic acid 500 group had increased. T he variation in scale cortisol between individual fish is smaller in the groups given WS 1,3-β-glucan than in the control or CORT 500 group. This is an indication that the algal beta-glucan helps tuning the HPI axis activation during the first three weeks.
Conclusion
The study demonstrates that algal-derived 1,3-β-glucan is an effective immunostimulant and growth promoter in Atlantic salmon and thermal stress management tool in trouts . Its incorporation into commercial diets has the potential to improve fish health and performance while reducing reliance on chemotherapeutics. These findings support the broader adoption of microalgal bioactive in sustainable aquafeed formulations.