Dietary supplementation with lemon balm, Melissa officinalis, does not improve performance of juvenile barramundi (Lates calcarifer) held under chronic high temperature

Increasing water temperatures are pushing aquaculture species to their thermal limits resulting in poor production performance. Additionally, animals in aquatic systems are often subject to concurrent stressors such as crowding, handling, and external influences such as disease and algal blooms. To ameliorate these effects, functional aquaculture feeds are a valuable area of research to pursue. Lemon balm ( Melissa officinalis L., MOF), as a dietary supplement or a bath, has sedative, antispasmodic, antibacterial, antiviral, and anti-inflammatory properties at optimal temperatures. However, the potential value of MOF in mitigating the effects of chronic thermal stress in fish is unknown. In this study, MOF was added into the diets of juvenile barramundi at concentrations of 0% (control), 0.2% (0.2% MOF), and 0.5% (0.5% MOF). Fish were acclimated to 30°C upon arrival, then underwent a thermal ramp up to 37°C where they were held for a minimum of 3 weeks prior to measurements. MOF at the experimental inclusion rates did not provide chronic stress relief (similar growth rates, feed conversion ratios, feeding rates, metabolic rates, scale cortisol content, and thermal tolerance values) in thermally challenged barramundi. In fact, fish fed MOF had significantly lower survival throughout the trial than those fed the control diet. Likewise, MOF provided no benefit to fish in responding to a secondary acute stressor (either a 2 h crowding event or a pathogen challenge with Streptococcus iniae ). Therefore, we conclude that MOF in diets of juvenile barramundi does not mitigate the adverse effects of chronic thermal stress and concurrent acute stressors.