• European eel;
  • Aqui-S vet;
  • iso-eugenol;
  • metomidate;
  • anaesthetics;
  • stress response;
  • physiology


The effects of 40 mg/L metomidate and 300 mg/L iso-eugenol (i.e. 600 mg/L Aqui-S vet.) anaesthesia on the stress response in European silver eel during a process of complete anaesthesia, recovery and long-term survival were studied. Metomidate is a hypnotic agent with no analgesia, whereas Aqui-S vet. is a true anaesthetic. There was no difference between fish exposed to Aqui-S vet. and metomidate, with respect to time to anaesthesia (mean 3.8 and 2.6 min respectively) and recovery (mean 7.6 and 6.5 min respectively). For the metomidate group, the plasma cortisol concentration increased to a peak during the recovery phase, and was significantly higher than for the Aqui-S vet. group, which showed no such increase. Fish were kept in tanks with seawater for monitoring of long-term survival (4 months) after anaesthesia treatment, and no mortality was observed in either group. The results indicate that metomidate is a potential stressor to European eels during exposure, and we do not recommend using metomidate in this species. Aqui-S vet., however, seems promising as a stress-reducing anaesthetic for European eel, and if used properly could improve animal welfare and survivability during and after common ecology-related procedures, as capture, tagging and size measuring.