The effects of eucalyptus <i>Eucalyptus globulus</i> and oregano <i>Origanum vulgare</i> essential oils on anaesthesia response in a Namibian marine species, silver kob <i>Argyrosomus inodorus</i>

Abstract

The anaesthetic effectiveness of separate and combined eucalyptus Eucalyptus globulus and oregano Origanum vulgare essential oils (EO) were  investigated in silver kob Argyrosomus inodorus and were compared with clove oil Eugenia caryophyllata. Argyrosomus inodorus adults (595.61 [SE  11.82] g) were exposed to three concentrations of oregano EO (OEO: 25, 50 and 100 μl l⁻¹), eucalyptus EO (EEO: 300, 500 and 600 μl l⁻¹), and their  mixtures (12.5 μl l⁻¹ OEO to 150 μl l⁻¹ EEO; 12.5 μl l⁻¹ OEO to 300 μl l⁻¹ EEO; 25 μl l⁻¹ OEO to 150 μl l⁻¹ EEO), with each fish treated as a replicate. Fish  were also exposed to 40 μl l⁻¹ of clove oil (CEO), which acted as a control. The two EOs and their mixtures proved to possess anaesthetic  capacity in A. inodorus. Anaesthesia induction time (seconds) decreased with increased concentrations, whereas the inverse was reported for the  recovery response. Only OEO qualified as an ideal anaesthetic agent for A. inodorus, especially 25 and 50 μl l⁻¹, as the anaesthesia induction times  with these concentrations were less than 180 s and recovery was within 300 s. It seems that EEO could achieve greater anaesthetic effectiveness  only at a higher concentration (600 μl l⁻¹) or when mixed with OEO (at 25 μl l⁻¹ OEO to 150 μl l⁻¹ EEO). This study introduces oregano as a promising  anaesthetic in A. inodorus compared with clove oil and EEO. However, further evaluations are deemed necessary.