Anaesthetic effect of clove basil (Ocimum gratissimum L.) essential oil on the giant river prawn (Macrobrachium rosenbergii, De Man 1879) exposed to different water pHs

The increasing concern for invertebrate welfare has led to improvements in management procedures and protective legislation; under discussion is the use of anaesthetic agents to improve the welfare of farmed prawns. In parallel, the aquatic environments present changes in their physical and chemical characteristics, like the acidification of waters. With this perspective, our study analysed the effectiveness of Ocimum gratissimum essential oil (EO-OG) as a natural anaesthetic for Macrobrachium rosenbergii at different concentrations and water pHs. Juvenile M. rosenbergii were exposed to five different EO-OG concentrations (0, 100, 200, 300, and 400 μL L⁻¹) and vehicle control (ethanol), at two water pHs (8.08 ± 0.10 and 6.42 ± 0.15). Induction time was EO-OG concentration-dependent regardless of water pH; i.e. the time required for sedation and anaesthesia decreased with the increase of EO-OG concentration. No correlation was found between recovery time and EO-OG concentration. The most effective EO-OG concentration was 400 μL L⁻¹, with prawn sedation in 2.0–2.5 min, anaesthesia in 3.5–4.1 min, and full recovery in 16 min, regardless of water pH. The different tested water pHs influenced sedation time; i.e. sedation time was higher in mild acidic water than in mild alkaline water. Anaesthesia induction and recovery times were similar at both water pHs. Survival rates were 100% in all experimental groups. In summary, EO-OG is effective as a sedative and anaesthetic for juvenile M. rosenbergii in routine procedures, at the recommended concentration of 400 μL L⁻¹, with mild alkaline water.